Design of Voltage Regulating Control Device of Improved PID Algorithm for the Vehicle AC Generator Based on DSP

The performance of the voltage regulating control system will have a direct impact on the power supply quality of vehicle generator. Therefore this paper researches a design of voltage regulating control device of improved PID algorithm for the vehicle alternating current (AC) generator based on DSP. According to the analysis of the operation and the voltage regulating principle of the vehicle AC generator, using the TMS320LF240 DSP as controlling chip, the optimized AD signals conditioning circuit, PWM circuit and improved increment PID algorithm excitation controller based on digital PID algorithm are designed, which only use integral controller by the incremental algorithm, and the proportional and derivative controller by the positional algorithm. After integrated debugging and simulation, the reliability and high precision of the voltage control system are validated.


Introduction
The vehicle electrical equipment needs a stable supply voltage.But vehicle generator has a big range of revolving speed, so the voltage regulating control device is very important to guarantee the quality of generator power supply.The performance of the voltage regulating control system will have a direct impact on the power supply quality of vehicle generator.The analog excitation voltage regulator has problems, for example serious drift with temperature changing, inconvenient maintenance and low precision.This paper presents a new type of voltage generator excitation control system based on TMS320LF240 DSP which has powerful data processing ability, high-performance static CMOS technology and high-speed real-time control ability.Using the TMS320LF240 DSP as controlling chip, the optimized AD signals conditioning circuit, PWM circuit and improved increment PID algorithm excitation controller based on digital PID algorithm are designed.

Working Principle
With the engine running, the generator output voltage U can be is calculated as follows.It can be concluded from above that the output voltage U of the generator changes with the generator speed n and the load IR.To keep the stability of the output voltage generator, the field winding current intensity or direction and the regulate air gap of the magnetic field can be adjusted.

The Overall Design
Using TMS320LF240 DSP as the control chip, a kind of optimized AD conditioning circuit and an improved incremental PID algorithm are developed, and the output voltage of the vehicle AC generator is regulated by the PWM circuit.First of all, the output voltage has to be detected, as figure1, then the generator three-phase AC voltage is changed into DC voltage by AD signal conditioning circuit, and converted to digital signal by AD converting circuit, then put into the input of the DSP, DSP regulates the field current intensity or direction by PWM circuit after the improved incremental PID algorithm processing, at last, the generator's excitation is controlled through the drinving circuit to have the generator output a stable voltage.So the device must have Software of DSP and the auxiliary circuit including analog signal conditioning, AD convertion, PWM, driving and congtoll circuit.Figure 1 shows the overall scheme of the system design.
Figure 1.The overall scheme of the system design

The Overall Software Design
Generator voltage control is a fast real-time closed-loop control system, and the regulating activities have to be quickly realized on the quikly changing of the voltage, so the software system may include the system and the clock configuration module, AD conversion module, filter module, PID control algorithm module, and PWM output module, to process the real-time regulating signals.The Figure 2 shows the overall software diagram.
Figure 2. The overall software design

Improved Increment Type PID Excitation Control
The generator terminal voltage deviation works as PID algorithm input, after PID algorithm processing, results are sent into PWM controller.The quality of PID excitation control directly decides the quality of the voltage regulation system.Simply put, the correction function of PID excitation controller is as follows: the proportional part rapidly proportionally reflects the generator terminal voltage deviation signal, and once deviations formed, the controller immediately generates a control role, in order to reduce the deviation, maintain the generator voltage constant.The integral part is mainly to eliminate the steady-state error of excitation system, improve the system of indiscrimination degree.The differential part reflect the trend of the deviation signal changing, and before the error signal becomes too large, an effective correction signal is early introduced to the system, thus the operation speed of the system is speeded up, the adjusting time is reduced.
Compared with Positional PID algorithm, incremental PID algorithm requires deviation parameters of the first two moments.The integrating part of positional PID algorithm needs the large amount of calculation, and proportional and differential calculation amount is small.According to the advantages and disadvantages of the two algorithms, in this article only integral part is calculated by incremental operation, the other two are calculated by positional operation, developed a PID algorithm combining positional algorithm with incremental algorithm.The proportion, integral, differential controller can be independently added, therefore, PID excitation controller only use integral controller by the incremental algorithm, and the proportional and derivative controller by the positional algorithm.The algorithm represent as follows:

Signal Conditioning Circuit
In order to achieve accurate and effective control, signal conditioning must be carried out before sensors signals come into AD converter.Signal conditioning concludes resistor dividing the voltage, differential amplifying and filtering.TMS320LF240 DSP contains a single polarity ADC with 16 analog channels, the minimum conversion time is 375 ns, and 16 channels can be arranged in any order and converted automatically, conversion results are sequentially placed.Therefore, this design uses the ADC of DSP.
The range of the analog input voltage of the ADC is from 0 to 3.3 V, but the generator output voltage is about 14 V.So the generator output voltage signal must be processed fist by stepping down to 0-3.3 V. Figure 3 shows the AD signals conditioning circuit.
Figure 3.The AD signal conditioning circuit

PWM Circuit
In the PWM technology, power devices work in switch state, to change the average load voltage, through changing the opening and cut-off time of the driving pulse signal of the power components.This is also the basic principle of PWM regulating the speed and voltage.The Figure 4 shows the PWM circuit.

H Bridge Circuit
This design uses bipolar reversible PWM mode, its characteristic is that in a switch cycle, the armature voltage polarity is alternating between the positive and the negative to control the intensity and the direction of the current by adjusting PWM duty cycle.If the duty cycle α is more than 50%, excitation current flow forward; If the duty cycle α is less than 50%, excitation current flow back; If the duty cycle α equals 50%, excitation current is zero.Here the direction of excitation current is relative.

Experiment
The rated voltage generator is 14 V.In order to check its control and the dynamic performance, the system is tested in two different conditions, the incremental PID parameters are: KP = 1.8, KI = 0.2, KD = 0.35.(1)When the generator speed is under a constant, changing the load, observe the change of the generator output voltage, such as shown in Table 1.
(2)When the generator is in the low load, changing the speed from 1800 to 5000 r/min, observe the change of the generator output voltage, such as shown in Table 2.The results show that the output voltage of the generator after regulating can be stabled within the range of +5%, and the device has good voltage stability characteristics, so as to ensure the stability of the vehicle power supply system operation.

Epilogue
Aimed at the stable valtage ouput of vehicle AC generator, optimized AD circuit, PWM circuit are designed, using the improved incremental PID algorithm, a type of voltage regulator control is developed.After testing, it has good performance.So the regulating control device can ensure the high quality voltage to vehicle power supply, and this design has a good application prospect.

Table 1 .
The generator output voltage change with load current changing

Table 2 .
The generator output voltage change with rotating speed changing