Archives June 2022

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.