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Before the power battery energy management system handles the fault, it first analyzes the fault to identify what kind of fault it is and the fault level that occurs. For fault identification, fault condition thresholds must be designed in advance. Generally, the state range of parameters or components within a certain period of time is specified, and the failure level is set according to the threat of the failure to components and personnel. If the parameters are outside the threshold range within the specified time, for example, if the voltage of the Tycorun Lithium Battery cell is more than 4.3V for more than 500ms, the BMS determines that the fault is that the cell voltage is higher than the upper limit value, and immediately disconnects the main relay and reports a fault warning. According to the severity of the fault, the fault level is defined as follows:

①Fault level 0: No fault.

②Fault level 1: Power reduction mode, the maximum charging power available for the battery is halved, and the discharging power is not affected.

③Fault level 2: Immediately disconnect the charging circuit relay, and do not specifically limit the charging and discharging power.

④Failure level 3: Serious failure mode, the maximum charging power is 0kW; if the vehicle speed is higher than 10km/h, the maximum discharge power is 0kW; if the vehicle speed is lower than 10km/h, the maximum discharge power is set to 10kW travel power.

⑤Failure level 4: Serious failure mode, disconnect the main relay immediately.

⑥Fault level 5: Dangerous fault mode, but do not automatically power off (if there are other level 4 faults at the same time, it should first report a level 4 fault for 1s to trigger a high-voltage power failure, and then continue to report a level 5 fault).

The power battery energy management system has safety protection functions, and corresponding measures should be taken in case of failure:

(1) Overvoltage protection. When it is detected that the highest cell voltage of the battery pack exceeds the level 2 fault threshold, the maximum available charging power of the battery drops to 0 within 2s; when the highest cell voltage exceeds the level 3 fault threshold, the BMS requests the VCU to disconnect the high-voltage relay. Get a timely response, and after 2s, the BMS will cut off the high-voltage relay by itself, and the high-voltage will be powered off.

(2) Low voltage protection. When it is detected that the highest cell voltage of the battery pack is lower than the level 2 fault threshold, the maximum available discharge power of the battery drops to 5kW within 2s; when the lowest cell voltage exceeds the level 3 fault, the BMS requests the VCU to disconnect the high-voltage relay. Respond in time. After 2s, the BMS automatically cuts off the high-voltage relay and powers off the high-voltage.

(3) Discharge overcurrent protection. When it is detected that the discharge current of the battery pack is greater than the level 2 fault threshold, the maximum available discharge power of the battery is reduced to 5kW within 2s.

(4) Charge overcurrent protection. When it is detected that the charging current of the battery pack is greater than the level 2 fault threshold, the available maximum charging power of the battery drops to 0kW within 2s.

(5) High temperature protection. When it is detected that the temperature of the power battery pack is higher than the level 2 fault threshold, the maximum available discharge power of the battery is reduced to 5kW within 2s; when it is detected that the temperature of the battery pack is higher than the level 2 fault threshold, the BMS requests the VCU to disconnect the high-voltage relay. Get a timely response, and after 2s, the BMS will cut off the high-voltage relay by itself, and the high-voltage will be powered off.

(6) Emergency power off. When the VCU sends a power-off command or the insulation resistance is low, the BMS sends a level 4 fault to the VCU. When the vehicle speed is not zero, the relay continues to be closed, and the main relay cannot be closed until the key off/on is again.

(7) High Voltage Interlock (HVIL). The HVIL of all high voltage connectors, MSD or cover switch (if any) must be connected in series to the vehicle HVIL circuit. The battery system BMS should be able to monitor the HVIL state, and when the HVIL state is abnormal, the battery system should be able to immediately disconnect the high-voltage circuit.

(8) Collision protection. When a collision occurs, the BMS can detect the collision signal, and should cut off the high-voltage output in time to ensure the safety of personnel.

There is a capacitive load in the high-voltage circuit of the electric system. If there is no pre-charging design during power-on, the main positive relay is directly closed, and the instantaneous capacitive load is closed, which is equivalent to an instantaneous short circuit. This kind of high-voltage shock will cause damage to high-voltage electrical equipment and may bring danger. In order to avoid the transient impact damage to the high-voltage electrical equipment when the high-voltage is powered on, a pre-charging process should be designed for the high-voltage circuit system before the main positive relay of the high-voltage power-on is closed.

1. Precharge principle and precharge model

1.1 The principle of precharging
As shown in Figure 1, the high-voltage circuit system precharge circuit.
In Figure 1, K1 and R1 are the pre-charging relay and pre-charging resistor respectively, K2 and K3 are the main positive relay and the main negative relay, respectively, R2 and C are the equivalent resistance and equivalent capacitance of the high-voltage system load, with Vb and Rb is the voltage and internal resistance of the power battery.

In the high-voltage circuit system, the high-voltage electrical equipment can be equivalently represented by a resistor and a capacitor, while the power battery can be simply represented by voltage and internal resistance. If there is no pre-charging design, the high-voltage main relay is directly closed when the high-voltage is powered on, which means that the high-voltage electrical equipment is directly connected through the capacitor, and the high-voltage components are also short-circuited. , it will generate a large transient current, and the transient current will cause transient impact on high-voltage electrical equipment, which will cause dangerous situations.

If the pre-charging process is introduced before the high-voltage power-on, that is, the pre-charging relay is closed before the main relay is closed, and the pre-charging resistor is connected, it will have a protective effect on the high-voltage electrical equipment. When the load high voltage reaches a certain precharge threshold, for example, the load high voltage is equal to or exceeds 90% of the total battery voltage during the precharge process as the conditional threshold for judging the success of the precharge. After the precharge is successful, the main relay is closed and the precharge is disconnected. Relay, the high voltage is powered on normally, and the high voltage electrical equipment works normally. Under normal circumstances, the high voltage on the main positive relay will be closed only when the pre-charging is successful.

2. Precharge Model

The pre-charging circuit is simplified below, a pre-charging model is established, and then the equivalent load resistance and capacitance are calculated and deduced. In the precharge circuit, compared with the resistance value of the precharge resistor and the resistance value of the high voltage load, the internal resistance of the power battery is very small and can be ignored; The high-voltage load resistance is extremely large, and the high-voltage load resistance is negligible relative to the precharge resistance.

After simplification, the precharge model as shown in Figure 2 can be obtained.
In Figure 2, Vb is the power battery voltage, K1 and Rb are the pre-charging relay and pre-charging resistor, C is the high-voltage load equivalent capacitor, Vr is the voltage across the pre-charging resistor, Vc is the high-voltage load equivalent capacitor The voltage across the capacitor, i is the current of the high voltage loop. According to Kirchhoff’s law, we can get

3. Selection of precharge resistors
   The precharge resistance is derived from the precharge model:

In the actual calculation, C is determined by the charging capacitor of the motor controller, and t is determined by the completion time of the pre-charging requirement in the power-on process. Usually, the bus voltage Vc outside the load reaches 90% of the total voltage Vb of the power battery as the condition for judging the successful pre-charging. . The selection of the pre-charge resistor is to select the appropriate pre-charge resistor according to the power-on time requirements of the pre-charge, the specification of the charging capacitor of the motor controller and the voltage requirements. The result obtained by deriving the load outside bus voltage:

The precharge process is shown in Figure 3.

For example, the charging capacitor at the motor controller end is 1100μF, and the total precharging time does not exceed 500ms. After removing the influence of factors such as the self-check time of the BMS and the action time of the relay, remove 200ms and leave the precharging process time no more than 300ms. The bus voltage reaches 90% of the total voltage of the power battery as the condition for judging the success of pre-charging, r=RC=55ms. Relay outside bus voltage:

The overload multiple of the precharge resistor is selected according to experience and cost, as long as the designed rated calorific value under the designed peak power is greater than the theoretically calculated calorific value.
If U=0.9Ubattery is used as the condition for judging successful precharge, then the precharge time t≈2.69, r=147.95ms; if U=0.8Ubattery is used as the condition for judging successful precharge, there is precharge time t≈1.98, r=108.9ms.
Considering the ability of the precharge resistor to pass the peak current, lmax=Ubattery/R=6A, and the peak power of the precharge resistor Pmax=I²maxR=1.8kW (the calorific value at the peak power is 9000J), so 50Ω100W (peak power delivery) is selected. Heat 15000J) metal precharge resistors, as shown in Figure 4.

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1. Normal high voltage power-on sequence

The normal high-voltage power-on requirement starts from the key on/Crank and is completed within 1s. The DC/DC should be required to enter the working mode (buck) after the high-voltage power-on is completed, and the integrated power unit (IPU) should be required to enter the working mode (TorqControl) after the high-voltage power-on is completed. Table 1 shows the requirements for each time point of high-voltage power-on, and Figure 2 shows the normal high-voltage power-on sequence.

2. Normal high voltage power-off sequence

The normal high-voltage power-off process starts from the key off, and each node completes the high-voltage power-off process within a few seconds. Table 3 and Table 4 shows the requirements for each time point, and Figure 5 shows the normal high-voltage power-off sequence.

3. Emergency high voltage power-off sequence

Table 6 shows the requirements for each time point of emergency high-voltage power-off, and Figure 7 shows the emergency high-voltage power-off sequence.

It should be noted that, in the T6 time period, when the VCU requests the IPU to enter the emergency discharge mode, if the motor speed is less than 100rpm, the IPU enters the emergency discharge mode within 50ms; if the motor speed is not lower than 100rpm, when the motor speed drops to After 100rmp or less, the IPU enters the emergency discharge mode within 50ms.

1. General principles of high-voltage power-on and power-off response of each control node

The high-voltage power-on and power-off responses of each control node include a normal situation response and an abnormal situation response. The response action of each control node to the high-voltage power-on and off demand shall conform to the following general principles:
(1) Undesirable acceleration, deceleration, reversing and steering of the vehicle should be avoided or prevented.
(2) Personal casualties, equipment damage or environmental damage caused by high-voltage faults should be avoided or prevented.
(3) Meet the high-voltage power-on and power-off performance requirements.
(4) The performance requirements should be met on the premise of meeting the safety requirements, that is, the principle response priority of a and b should be higher than that of c;

2. Principles of Response to Abnormal Situations

For any abnormal situation, there should be corresponding handling measures:
(1) If each control node has a response function corresponding to this abnormal situation and the function is normal, the response shall be performed according to the established function.
(2) If each control node has a response function corresponding to this abnormal situation but this function fails, it should consider adding the corresponding fail-safe function and respond according to the fail-safe procedure.
(3) If each control node has no feasible response plan for this abnormal situation, it shall take identification and explanatory measures for the residual danger.
(4) In the event of a vehicle accident, emergency high-voltage power-off should be performed.
(5) If Category I and Category II undesired personnel actions and vehicle high voltage faults occur at the same time, priority shall be given to dealing with vehicle high voltage faults. Unexpected personnel actions of Category III should be treated with emergency high-voltage power-off.

3. Principles of Response to Undesired Human Actions

(1) For category I undesired human actions, if the vehicle has no fault, respond according to the Quick Key Cycle processing logic table shown in Table 1. Description: The vehicle speed condition is to consider that after the high-voltage power-on is completed, if the vehicle is still in a stationary state, it is considered that the current driver’s “driving” intention is still unclear, and the system should be in the original count state of Quick Key Cycle; if the vehicle starts to run, then Considering that the current driver’s “driving” intention is clear, the system should jump out of the original counting state of Quick Key Cycle, and the counter should re-count.

(2) For category II undesired human actions, because the actual driving intention cannot be judged, it is assumed that the driver has taken an abnormal operation under special circumstances, so each control node should respond according to the normal high-voltage power-off sequence.

(3) Emergency response to high-voltage power-off should be made for the undesired actions of Class III personnel.

4. Response principles for vehicle high voltage fault conditions

When a high-voltage fault occurs in the vehicle, the high-voltage power-on and power-off response of each control node must meet the following principles:
(1) A high-voltage fault occurs in the vehicle during the normal high-voltage power-on process. If the fault is serious but the emergency high-voltage power-off conditions are not met, the power-on behavior should be stopped immediately and the motor should not be able to output (torque, electric energy), or DC/DC. There is power output, and the power-off is completed according to the normal power-off sequence.

(2) If a high-voltage fault occurs during the normal high-voltage power-off process, only the fault code is allowed to be recorded, and no response is made to the fault, and the power-off is completed according to the normal high-voltage power-off sequence.

(3) If the vehicle has a high voltage fault in any state (except the state that has entered or completed the high voltage power-off process), if the severity of the fault meets the conditions for emergency high-voltage power-off, each control node shall respond to emergency high-voltage power-off.

(4) For a fault that triggers an emergency high-voltage power-off, if the severity of the fault meets the condition that high-voltage power-on is not allowed, the vehicle is not allowed to be powered on again until the fault is manually eliminated.

Read more: How Lithium-Ion Power Batteries Work

High-voltage power-on and power-off technology is an important part of the power battery energy management system for electric vehicles. This section puts forward the principle requirements for the power-on and power-off behavior of each high-voltage control node of the electric vehicle electric system, and makes time mandatory requirements for the response behavior of each control node under the general principle. Each control node refers to the node on the HCAN, including the vehicle controller (VCU), integrated power unit (IPU), inverter (DC/DC) and battery management system (BMS). The contents of high-voltage power-on and power-off of electric vehicles include the normal high-voltage power-on sequence, the normal high-voltage power-off sequence, and the emergency high-voltage power-off sequence. These three power-on and power-off sequences are the core content of the high-voltage control of the electric vehicle power battery energy management system.

The following describes the related terms of high-voltage power-on and power-off technology:
High-voltage power-on means that each control node responds to the power-on demand. After the high-voltage battery pre-charges the load through the pre-charge circuit, the voltage difference across the high-voltage relay is less than the set threshold Vth, and then the total positive relay is closed. . The high-voltage power-on completion sign is that the total positive relay is closed. The voltage difference is equal to the total voltage value of the high-voltage battery minus the load voltage value at both ends of the DC bus. During the high voltage power-on process, the voltage difference setting threshold Vth should be set according to the impulse withstand voltage condition of the inductive load.

If the rated voltage is 360V, the voltage difference threshold of high-voltage power-on and power-off technology is Vth=360V×(1-90%)=36V[91]. High-voltage power-off is a process in which each control node responds to the power-off demand, disconnects the high-voltage battery, discharges the remaining power, and finally enters the sleep state according to the user’s power-off demand. The high-voltage power-off completion flag indicates that the VCU enters the sleep state. The power-off requirement can be the user’s requirement to “cut off the power”, which corresponds to the user’s “key off/Acc” action; it can also be the requirement of the electric system to automatically “cut off the power” in a dangerous situation.

The residual power discharge is to discharge the residual high electric energy in the high-voltage load after the high-voltage battery is disconnected, so that the DC bus voltage drops below the dangerous voltage or the electric energy drops below the dangerous energy level, so as to avoid the damage caused by the residual electric energy. a protective measure. Dangerous voltage refers to a voltage with a DC voltage greater than 60V or a peak AC voltage greater than 42.4V. Hazardous energy level means that the stored energy level is equal to or greater than 20J, or the continuous power level that can be given is equal to or greater than 240VA when the voltage is equal to or greater than 2V.

The abnormal situation of high-voltage power-on and power-off refers to the situation in which unexpected personnel actions occur, high-voltage faults occur in the vehicle, or an accident occurs in the vehicle. There are three types of undesired human actions:
(1) Category I Unexpected Person Action: The vehicle is in a stationary state, which occurs many times. When the high-voltage normal power-on is not completed, the user suddenly makes a “key off/Acc” power-off action, or when the high-voltage is normal When the power-off is not completed, the user suddenly performs the power-on action “key on/Crank”.

(2) Category II Unexpected Person Action: The vehicle is in a normal driving state, and the user suddenly makes a power-off action “key off/Crank”.

(3) Category III unintended personnel actions: when the DC bus carries dangerous voltage or dangerous energy, dismantling or removing the cover of the high-voltage device, the door of the box, or inserting and unplugging the high-voltage connector.

For the “multiple occurrences” referred to in Category I undesired human actions, the threshold of this number of times must be determined after comprehensive consideration. On the one hand, it must be considered to respect the intention of the user and respond accordingly to the limited number of rapid key-turning actions; The number of times its quickly turned the key. Therefore, considering the overall consideration, it is recommended that the threshold value of the pairing times be set to 3. Emergency high-voltage power-off means that the control node responds to the demand of the electric system to automatically “cut off the power supply”, emergency stops the motor output, emergency disconnects the high-voltage battery, and completes the residual power discharge within 2s according to GB4944-2001 [92], and finally according to the use of The power-off of personnel requires all control nodes to enter the sleep state. Stop the motor output, including stopping the power output in the motor generator mode and the torque output in the motor mode.

Based on the fuzzy logic energy management system, the use efficiency of the power battery of the pure electric vehicle with a single energy source is effectively improved, and the economic performance and dynamic performance of the whole vehicle are improved on the premise that the total energy of the power battery remains unchanged. On the basis of this control strategy, an optimization unit is added, which is guided by dynamic programming theory, extracts valuable control rules, and automatically adjusts the fuzzy controller according to the actual situation of the vehicle, so that the sum of the energy loss of the vehicle reaches minimum. Experiments show that:

Under the same driving conditions, the optimization method can further improve the economic performance of the whole vehicle.
Because the conventional fuzzy controller does not have the self-adjustment ability of rules and parameters, it cannot automatically adjust its parameters to adapt to the change of the object. When the robustness of the system needs to be analyzed, if the method of fuzzy control is adopted, the system cannot complete the analysis faster than using the control theory commonly used in the past. The system can also fluctuate if the quantifiers are poorly chosen, or if the control rules are poorly structured or covered.

Secondly, due to the limitations of the design itself, the fuzzy control designed by using experience is easily affected by subjective influence. Therefore, when applying fuzzy control strategy to solve complex multi-stage control problems, it is often easy to fall into local optimum. The dynamic programming can decompose the more complex problems, and then obtain the optimal solution of the whole problem by solving each problem one by one. For some difficult optimization problems, it can often reflect its superiority, especially for some discrete optimization problems. In this section, from the perspective of modifying and improving the fuzzy control principle, the optimization problem of the energy management system of pure electric vehicles is expressed as a dynamic programming theoretical optimization problem with the goal of minimizing the sum of the energy loss of the whole vehicle. The control effect of dynamic programming theory is verified by calculating the optimal control solution under specific cycle conditions.

1. Description of the dynamic programming optimization problem

Let the allocable power of the energy management system at any time be Pe. The power relationship is as follows (1):

Due to the loss of the drive motor and the power devices of the thermal management system, the power provided by the energy management system cannot be used for effective work, that is, the efficiency of the two is not 1, and the relationship is as follows (2):

Among them, is the function of the thermal management system components on the ambient temperature T, and is the function of the motor speed N. The cycle conditions of the whole vehicle are divided into n stages, k∈n, and the following assumptions are made:
(1) State variable xk: represents the power allocated by the energy system to the thermal management system from the kth time period to the nth time period.
(2) Decision variable uk: represents the power allocated by the energy management system to the thermal management system in the kth time period.
Under the action of the decision uk, the state variable xk changes, and the state transition equation is (3)

Indicates the power allocated by the energy management system to the thermal management system from the k+1th time period to the nth time period. When the energy management system distributes according to the decision variable uk, the power flows through the drive motor and the thermal management system respectively, which will generate corresponding incentives for the two, so that the characteristic states of the two will change. Since the efficiency of the drive motor is a function of the motor speed, and the efficiency of the thermal management system is related to the ambient temperature, the energy Jsys lost by the system at time k is a function of the state variable xk and the decision variable uk. Let it be the dynamic optimization stage index function vk(xk, uk), then (4):

represents the system loss caused by the power allocation of decision uk in the kth time period.
In this way, according to the power demand of the whole vehicle, starting from time 0, different control rates U will generate different new states (state variables) x, and at the same time face the problem of selecting a new control rate (decision variable) U until the whole cycle works. situation is over. Since the initial conditions of the vehicle simulation will be given, the power optimization control problem of the pure battery vehicle energy management system can be summarized as: the initial state x(0)=x0 given, the terminal x(n)=0 optimal control The problem is shown in Figure (5):

In the process of vehicle driving, if the distribution of each control rate in all cycles is optimal, the sum of energy losses generated by the system at each moment can be minimized. In the case of the same total amount of energy system, that is It can be understood that the efficiency of the energy system in the cycle is the largest, and the economic performance of the vehicle is the best. From this, the optimization objective shown in Equation (6) can be set:

fk(xk, Wk) represents the sum of the system losses caused by the power allocation of the decision uk from the kth time period to the nth time period. In the process of vehicle cycle conditions, the power of the drive system and thermal management system and the energy of the power battery system must meet the following constraints (7):

In this way, the output of the energy management system can be adjusted by solving the optimal control rate under dynamic programming, and the energy management system can be further optimized. According to the principle of Bellman optimality, to solve the dynamic programming recursive equation with constraints, the steps are as follows:
①Using the boundary condition fN(xN, uN)=0, obtain the optimal control un in the Nth stage when the state is xn.
②According to the values ​​of the formula xk+1=xk-uk and fk(xk, uk), find the optimal control uk-1 and fk-1 (xk-1, uk- 1).
③ Let k=k-1, if 0≤k≤N-1 is satisfied, go back to step ②; if not, go to step ④. ④ Obtain the optimal control u0 and f0 (x0, u0) when the state is x0 at the initial moment, then the sequence {u0, 1…, uN-1} is the optimal control strategy, and f0 (x0, u0) is the optimal control strategy. The optimal performance index corresponding to the optimal control strategy.

2. Validation of dynamic programming optimization method

Because the dynamic programming takes the minimum energy loss of the whole vehicle as the optimization goal, the energy consumption rate of the whole cycle process is lower than that of the fuzzy control strategy, and there is no under-power situation in the later cycle of the cycle. The control amount allocated by the optimization algorithm is smaller at the beginning of the cycle, because the initial temperature of the system is lower and the efficiency loss of the thermal management system is larger. As the cycle progresses, the system temperature gradually increases, the efficiency loss of the thermal management system becomes smaller, and the control amount allocated by the algorithm gradually increases. In addition, when the motor speed is low, the system energy loss is relatively large, and the algorithm will appropriately reduce the allocated control amount; when the motor speed increases to the high-efficiency working area, its guiding significance for the real vehicle is as follows:
(1) When the initial temperature is low, appropriately reduce the allocated control amount.
(2) When the temperature in the mid-term is high, increase the allocated control amount appropriately.
(3) When the motor speed is low, reduce the allocated control amount appropriately.
(4) The motor enters the high-efficiency area, and the allocated control amount is appropriately increased.

1. Energy management system based on threshold control method

The control strategy of the energy management system usually adopts the threshold value control method, that is, a threshold value of the remaining energy is set:
If the remaining energy is higher than this threshold, the thermal management system is turned on; if the remaining energy is below this threshold, the thermal management system is turned off. Now introduce an energy management system power distribution coefficient KT-M, which represents the weight of the maximum power allocated to the thermal management system, and its definition domain is [0, 1]. The maximum power allocated to the thermal management system is then shown in Figure 1:

where PTM-MAx(t) is the peak power of the high-voltage components of the thermal management system. The maximum power allocated to the drive system is therefore Fig: 2:

When PT-M(t)=0, it is the way to reduce the energy usage rate: allocate the limited battery power to the drive system to the greatest extent.
In the energy management system threshold control method, KT-w(t) is the variation, and the control strategy is as shown in Figure 3:

Among them, SOCTH is the threshold value.
From the above mathematical description, it can be seen that the threshold value control process of the energy management system is shown in Figure 4:

The driver steps on the accelerator pedal, understands the driver’s driving intention through the accelerator pedal opening θ curve, and obtains the motor demand power PMR; the energy management system can allocate the power Pe, and distribute the power to the thermal management system and the drive system according to the threshold control strategy . When the system completes the work, the energy of the power battery becomes Ee’, the maximum distributable power becomes Pe’, and the temperature of the power battery becomes T’ and the state of charge becomes SOC’.

The optimization purpose of the energy management system is to reduce the energy consumption rate of the whole vehicle when the total rated energy of the power battery remains unchanged, and to minimize the ECR by distributing the power of the energy management system between the drive system and the thermal management system. Generally, the economic performance of pure electric vehicles is evaluated by energy consumption rate.

Among them, the mileage of the whole vehicle is S, the unit is km; E is the battery energy consumed by the whole vehicle when it travels, the unit is J; k is the unit conversion factor.
The optimization of the power battery energy management system is to add an optimization unit to the control strategy of the conventional energy management system, by dynamically adjusting KT-w(t) in the interval [0, 1] to minimize the ECR. In addition, during the driving time t of the vehicle, Pmotor(t) should be as large as possible larger than the required power PMR(t) of the drive motor. From the above analysis, the mathematical model of the optimization problem of the energy management system of pure electric vehicles can be established as shown in Figure 6:

It can be seen that this problem belongs to a typical constrained nonlinear optimization problem.

2. Energy management system based on fuzzy logic

The main parameters of the control strategy design that affect the power distribution of the pure electric vehicle energy management system are:
SOC(t), battery SOC at time t;
dθ/dt, the rate of change of the accelerator pedal opening at time t;
△T, the temperature difference between the battery temperature and the battery optimal temperature working range at time t. Figure 7:

Among them, Topt-max is the upper limit of the optimal operating temperature range of the battery; Topt-min is the lower limit of the optimal operating temperature range of the battery; Tbat is the battery temperature. Due to the inconsistency of the temperature of the power battery body, it is taken in the actual calculation. average value.

Therefore, on the basis of the original control strategy, an optimized control process as shown in Figure 8 is established, and a fuzzy controller for power distribution in the energy management system is added, with SOC(t), dθ/dt and ΔT as the input parameters, and the output control parameter KT-M (t), after the thermal management system and the drive system do work according to the power distribution coefficient of KT-w(t), SOC(t) and ΔT change, and then feedback to the fuzzy controller as the next input to form a closed loop.

The working process of the fuzzy control of the pure electric vehicle energy management system is described as follows:
①When the vehicle starts, if the operating temperature Tbat of the power battery is low, PT-M is given priority.
②When the vehicle starts, if the working temperature of the power battery is normal, Pmotor is given priority.
③ When the car accelerates rapidly or runs on a hill, that is, dθ/dt is greater than a certain set threshold, the Pamr is started first to meet the high power demand of the drive motor, and then the PT-M is started.

④ When the car is running at a normal speed, that is, dθ/dt is less than a certain threshold, the priority is to meet the PT-w to improve the charging and discharging efficiency, and then meet the Pmotor to ensure the ordinary power demand of the drive motor.
⑤ If the battery SOC is low, Pmotor should be given priority.
⑥ If the battery SOC is high, PT-M should be given priority.

According to the above working process, the following basic principles are followed when formulating fuzzy control rules:
(1) When the remaining battery power SOC(t) is low, if the temperature difference ΔT is relatively small, and the accelerator pedal opening change dθ/dt is relatively large, the power allocated to the thermal management system is relatively small, namely KT-M(t) smaller.
(2) When the remaining battery power SOC(t) is high, if the temperature difference ΔT is relatively large, and the accelerator pedal opening change dθ/dt is relatively small, the power allocated to the thermal management system is relatively large, namely KT-M(t) bigger.

Read more: How Lithium-Ion Power Batteries Work

The power battery energy management system is one of the key technologies of electric vehicles. At present, the research on power battery energy management system mainly focuses on the energy management strategies of hybrid electric vehicles and pure electric vehicles. Due to the complex powertrain system of hybrid electric vehicles, there are many control strategies and a large space for development. For example, the commonly used control strategies for series hybrid electric vehicles include thermostat strategy, power tracking strategy and basic rule strategy; the commonly used control strategies for parallel hybrid electric vehicles include static logic threshold strategy, instantaneous optimal energy management strategy, and fuzzy logic control. strategy and global optimal energy management strategy, etc.; the commonly used control strategies for hybrid hybrid vehicles include engine constant operating point strategy, engine optimal working curve strategy, etc.

Pure electric vehicles can be divided into multiple energy source systems and single energy source systems according to the number of energy sources. The multi-energy source is mainly a dual-energy source system composed of a battery and a supercapacitor. The main feature of the supercapacitor’s large charge and discharge rate is used to make up for the shortcomings of the power battery by cutting peaks and filling valleys. At the same time, due to the existence of supercapacitors, which increases the complexity of the powertrain system, the available control strategies are also much more than that of single energy sources, such as threshold control strategies and fuzzy logic control strategies. For pure electric vehicles with a single energy source, because the powertrain system is simpler than that of hybrid and multi-energy source systems, there is less room for control strategies.

Combining the energy management and control strategies of pure electric vehicles of Chinese and foreign OEMs, the main control strategies are as follows: one is to reduce the energy usage rate of the entire vehicle, and only retain the high-voltage load necessary for the vehicle to travel, so as to minimize the energy consumption of the entire vehicle. The power consumption of the high-voltage system is to allocate all the limited power to the drive system; the second is to improve the efficiency of battery use. Through the use of the thermal management system, the battery has been controlled in the high-efficiency range. Although the first method can save the power consumption during driving to the greatest extent, due to the lack of the battery thermal management system, the long-term high temperature operation will accelerate the aging of the battery, which will sacrifice the economic performance of the vehicle; the second method is the most commonly used threshold. Although the control strategy is simple and stable, it cannot optimally solve the power distribution problem due to the fixed control rules, thus affecting the dynamic performance of the vehicle.

1. Energy system structure and power flow analysis of pure electric vehicle

The high-voltage system components of pure electric vehicles are mainly divided into air-conditioning system components, drive system components, low-voltage power supply system components and charging system components. Among them, the low-voltage power supply system components convert high-voltage electricity into low-voltage electricity to support the entire vehicle electronic components , the operation of low-voltage equipment such as power steering, water pumps and fans. Charging system components replenish energy from the grid through chargers or other charging equipment. The air conditioning system components are mainly used to improve driver comfort and thermal management of the power battery. The high-voltage components are the electric heater (PTC) for heating and the air compressor (ACP) for cooling, and the drive system components are used to drive the motor to the outside. Doing work, you can also brake to recover part of the energy.

From the above-mentioned components of the power battery system and the high-voltage system, a schematic diagram of the power flow of the vehicle energy management system as shown in FIG. 1 can be obtained. It describes the input-output relationship of power flow between the power battery system and the high-voltage system of the vehicle.

In the figure, Pbat represents the maximum output power of the power battery system. PA-c represents the maximum power allocated by the energy management system to the air conditioning system. Since the comfort system and the thermal management system share power devices, in order to describe their functions intuitively, the air conditioning system is equivalent to a thermal management system, which is called PT-M. Pmotor represents the maximum power that the energy management system can allocate to the drive system. PL-p represents the required power of the low-voltage power supply system. Pm represents the maximum rechargeable power of the charging system.
During the driving process of the vehicle: Pcha(t)=0; PL-p(t) is a fixed value, which is equal to the DC/DC rated power in value and must be allocated; PT-M(t) and Pmotor(t) The size can be determined according to the power required by the power bus under different working conditions, and it is an amount that can be adjusted and allocated. Let the power that can be allocated by the vehicle energy management system be Pe(t), then:

Obviously, the purpose of the vehicle energy management system is to reasonably allocate Pe(t) to PT-M(t) and Pmotor(t), so as to maximize the energy utilization efficiency of the vehicle.

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1. System framework and overview

The battery management system mainly implements the collection and reporting of voltage and temperature signals on a single battery module from the board, and performs balancing operations on the cells on the module when the balancing function is executed.
Multiple battery modules are used in the pack system design, and each battery module uses a slave board. The slave board is mainly used for the collection, reporting and equalization functions of battery cell voltage and temperature. The slave board hardware generally includes a dedicated battery acquisition chip, an isolation chip, a single-chip microcomputer, and a communication circuit. The block diagram of the slave board system is shown in Figure 1.

The MCU main control unit is placed on the high-voltage battery side, only the CAN module is placed on the low-voltage side of the slave board, and a power chip is placed on the high-voltage side and the low-voltage side, which can reduce the isolation of one power supply and reduce the EMC problem of the entire board. The functional requirements of the slave board system are as follows:
(1) Slave board channel: Each slave board has 4 voltage acquisition channels and is compatible with 5-channel mode, and 5 temperature acquisition channels. Only 4 channels can be selected as the acquisition interface.
(2) Acquisition accuracy: single voltage accuracy ± 5mV, measurement range 0~5V, temperature measurement range -40°C~85°C, required accuracy at -30°C~60°C ≤±1°C, 60°C~85°C required Accuracy≤±1.5℃.
(3) Acquisition time: the single voltage reporting period is 50ms, and the temperature reporting period is 50ms.
(4) Communication mode: The communication between the slave board and the main board adopts high-speed fault-tolerant CAN, with a rate of 500kbit/s.
(5) Acquisition method: special acquisition chips are required to simplify system design and ensure scalability.
(6) Power supply mode: the power supply current of the high-voltage side does not exceed 50mA, and the power supply current of the low-voltage side does not exceed 30mA.

2. Processor and chip and power supply

2.1. Processor
The slave board processor is used to monitor, process and report the battery voltage and temperature signals, and the functions can be realized by using the single chip microcomputer. The processor uses the freescale chip MC9S08DZ60. The processor must include at least ROM, RAM, and flash storage space, of which the EEPROM requirement is not less than 1K, the RAM requirement is not less than 2K, and the flash requirement is not less than 32K. Hardware watchdog function: The processor power-on completion time is required to be within 1s, and it can wake up and sleep through hard wires; the processor has the CAN interface function to communicate with the motherboard, and the SPI or I2C interface function to communicate with the internal chip.

2.2. Acquisition chip and isolation chip
The acquisition chip is used to manage the acquisition function of battery voltage and temperature. The function can be realized by using a special acquisition IC. The LTC6804HG-1 acquisition chip of Linear Technology is selected, which can provide the following resources: the single voltage acquisition channel is 12channel, 5 GPI0 ports (can be multiplexed into 5 temperature acquisition channels); multiple acquisition chips are allowed to be used in parallel, and a daisy-chain structure can be provided. The acquisition resolution of the voltage channel is 16bit, the total voltage acquisition accuracy (including analog front-end and back-end processing) meets -2.8~2.8mV, the acquisition time of the acquisition chip is 130us, the withstand voltage requirement of the acquisition chip is not less than 60V, and the voltage acquisition channel The measurement range is 0~5V, and the acquisition chip with hardware diagnosis function channel can perform hardware diagnosis on the undervoltage and overvoltage of the battery cell voltage, and report the fault status. The acquisition chip has the SPI interface function to communicate with the processor, and the chip temperature range is -40℃~125℃. The isolation chip is required to meet the electrical isolation of the battery side and the low-voltage side, and the electrical isolation meets the requirements of insulation and safety regulations. The electrical design defines the RMS voltage of 400V, and the minimum clearance and creepage distance of 4.00mm, which are mainly considered in the PCB layout. The isolation chip uses ADI’s I Coupler digital isolation chip ADuM12011.

2.3. Power supply
The slave board is powered by the low-voltage system and the high-voltage system, and the power supply system is designed as follows: the low-voltage power supply comes from the low-voltage battery of the whole vehicle (about the knowledge of the battery, I accidentally found an article before, and found that the author knows the knowledge of the battery Very thorough, if you are also interested, you can visit Tycorun Battery to read)
), the normal working voltage is 12V, the voltage range is 6~16V, and the working current does not exceed 50mA. Use the power management chip for power supply control, provide internal 5V or 3.3V, and supply power for CAN and isolation chips. The high-voltage power supply comes from the module of the high-voltage battery, and the voltage range is 8~25V. The high-voltage module directly provides power for the acquisition chip, and converts it into 5V through DC-DC or LDO to power the MCU and other chips on the high-voltage side. Power management can support power-on and power-off management of hard-wired Enable. Enable high level wakes up the power management chip and performs power-on initialization. It is required to complete initialization and start measurement within 120ms, and send a normal CAN signal; after Enable low level, the power management has a self-locking function, which supports the processor after the power-off management is completed. , go to sleep again.

3. Interface definition and CAN communication

3.1. Interface Definition
The slave board is respectively connected to the battery terminal and the main board. The design requirements for the interface are as follows: the voltage input interface channel of the battery terminal is 4 channels, the temperature input interface channel is 4 channels, the connection terminals are designed separately, and the on-board connection terminals are used. The power supply terminal and the main board communication terminal have Redundant design to ensure the cascading of multiple acquisition sub-boards. The cascading method is shown in Figure 2.

3.2.CAN communication
The slave board has communication functions such as CAN. The communication follows the following requirements: the external port of the slave board needs to provide at least one high-speed CAN communication with the main board, the communication rate is 500kbit/s, and the CAN2.0 communication protocol. The acquisition chip and processor need to have communication functions such as SPI or I2C for internal communication; CAN needs to reserve a terminal resistance, and the CAN network ID can be configured independently. The voltage and temperature signals are collected from the board and reported to the main board through the CAN signal. At the same time, the equalization command of the main board is sent to the slave board through the CAN signal to realize the equalization function.

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As the core controller of the power battery system, the hardware main board of the battery energy management system mainly realizes the BMS strategy operation and execution functions, including information collection of total voltage / total current, real-time SOC estimation, fault diagnosis and storage, real-time control of the main relay, etc.

1. System framework and overview

The hardware circuit block diagram of the designed motherboard system is shown in Figure 1, including power supply circuit, digital / analog input, high / low-end drive output, PWM input / output, can communication, etc.

The designed microprocessor adopts Infineon’s tricore32bit series tc1728, the main frequency is 133MHz, and the storage space is 1.5MB flash; Abundant peripheral IO resources can meet the design requirements.

1.1. working voltage

The working voltage range of the main board is 6 ~ 16V, and the static current of the main board shall not exceed 1mA.

1.2. Self programming and diagnosis ability

The motherboard can support self programming in the offline stage through UDS protocol. The mainboard is required to have hardware diagnosis functions, such as detecting the open circuit and open circuit status of the output IO port (including short circuit to ground and short circuit to power), and turning off the output function in time according to the fault status. The requirements for shutdown time are as follows: high voltage relay drives IO, acquisition board / high voltage board enable signal, and the response time after fault detection is 10ms; The rest output IO, and the response time after fault detection is 100ms.

1.3. Electrical interface and can communication interface

1.3.1 power interface

The power interface block diagram is shown in Figure 3, and the pin definition is shown in Table 4.

The designed maximum withstand voltage of the power supply is 65V and the maximum current is 5A; The designed maximum current resistance of the power supply ground is 25A, and the offset voltage to the ground shall not exceed ± 0.5V.

2. Digital input and analog input

The high-level interlocking detection is designed to be effective at high level. It is required to pull down 1.2k Ω resistance to the ground, and the filtering time constant shall not exceed 1ms. The emergency diagnosis line is designed to be effective at high level. It is required to pull down 1.2k Ω resistance to the ground, and the filtering time constant shall not exceed 1ms. The fast charging wake-up line is designed to be effective at high level. It is required to pull down 3.3k Ω resistance to the ground, and the filtering time constant shall not exceed 1ms. The slow charging wake-up line is designed to be effective at high level. It is required to pull down 3.3k Ω resistance to the ground, and the filtering time constant shall not exceed 1ms.

Fast charge signal detection input port, the internal resistance is 4.64k Ω, pulled up to 5V, the connected effective signal is lower than 1V, and the non connected invalid signal is 5V; The temperature sensor adopts NTC, the typical value is 10K Ω, and the designed pull-up resistance is 3K Ω to 5V.

2.1. Digital output

Main board digital output pin pin, main positive relay control, main negative relay control and fast charging relay control. The design is equipped with small relay drive. The relay model is acb33401, the interface is active at low level, and the driving current does not exceed 300mA. Precharge relay control, slow charge relay control, design direct drive relay evr10-12s, the interface is low-level effective, and the drive current does not exceed 300mA. Accessory relay control, designed to directly drive relay acb33401, the interface is effective at low level, and the driving current does not exceed 300mA.

2.3. Communication interface, storage space and mechanical interface

Pin pin of the main board communication interface. The main board uses two-way can, in which hcan is used as the whole vehicle can to communicate with the whole vehicle controller; As an internal can, Ican communicates with acquisition slave board and high voltage board; Both channels of can adopt high-speed can, and the communication rate is 500kbit / s.

The flash storage space of the main board is 1.5MB, which is divided into application software area, calibration area, test area, production data retention area and boot loader area according to function. The external EEPROM storage space is 8KB, which is used to store vehicle data. The connector adopts Tyco series automotive connector, which is divided into a / B two parts, with a total of 121pins, as shown in Figure 6.

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