Control of Pneumatic Actuator for Automated Mechanical Transmission Dry Friction Clutch Base on the Pulse Width Modulation Signal

To ensure quality of dry friction clutch engagement in automated mechanical transmission during vehicle starting-up and maneuvering the control range of clutch actuator has to be maximum wide. It depends on the consistency of the clutch actuator geometric parameters with the electrical characteristics of the used solenoid valves, the output stage of the controller and the PWM control signal frequency. In addition to precision electronic control the driver must be able to “manually” operate the dry friction clutch in emergency. That is why friction clutch must have two independent control circuits. The original automated drive with a duplicate pneumohydraulic circuit for the friction clutch is presented in the paper, as well as the research results of the PWM frequency influence on operating range of the clutch pneumatic actuator. The research was based on the analysis of semi-natural experiment for assessing the functional performance of the designed automated mechatronic control system for the truck mechanical transmission. Ecomat R360 controllers were used as a hardware base of the test bench information control system. The developed software for the controller with one-parameter feedback on the clutch release lever movement allows to provide the PWM signal of varying duty ratio to the proportional solenoid valve of the automated drive. A graphical representation of the research results was performed with visualization possibilities of CoDeSys V2.3. During the semi-natural experiment, the polynomial dependence between variation of the clutch actuator control range and the generated PWM signal frequency in the range up to 400 Hz was revealed as well as practical recommendations on the choice of the optimum PWM signal frequency are also given in activity. The research results can be used in an adaptive control algorithm for automated mechanical transmission of trucks and road trains to ensure precise control of the clutch actuator in the starting-up and maneuvering processes.

Considering the above, as well as using the experience of foreign companies, specialists of the BNTU Automobile Department have developed an original duplex drive [6] of dry friction clutch, which is integrated into powertrain control mechatronic system (Fig. 1) and has two independent control loops.
Automated drive, as the primary circuit, includes a pneumatic actuator 14 and the proportional solenoid valve 13 installed on the gearbox (Fig. 1). Duplicate hydraulic drive consists of a pedal 8 with a hydraulic cylinder 7, pipeline and pneumo-hydraulic booster 6 ( Fig. 1, 2). Clutch release node is a lever mechanism, which consists of two levers 16 and 17 mounted on the bush of clutch fork shaft. One of the levers is connected with a power actuator of the clutch automated drive. Another one comes into the duplicate hydraulic drive.
Calculation of the actuator active diameter is based on the maximum clutch disengagement force, the drive ratio and system's nominal pressure. When designing automated dry clutch for serial production, the actuator active diameter is tended to choose from a standard series of pneumatic cylinders. As known well about actuator on cylindrical type, the dead zone and friction between the piston and the cylinder surface are changed during operation of the vehicle, that influence on the control quality of the clutch transition process. At the test stand of the BNTU, a brake chamber (type 16) with diaphragm type was used as a clutch actuator to improving the control quality by reducing the dead zone and friction.
At the clutch engagement in the automatic mode, for example during starting or maneuvering, in order to control proportional solenoid valve 13, the strategy of the Direct Semi-Active Control [7] is offered to use. The meaning of the strategy is applied to the solenoid coil PWM signal with a subsequent change of its duty ratio according to a predetermined algorithm. Наука техника. Т. 20, № 1 (2021) и Science and Technique. V. 20, No 1 (2021) Fig. 1. Schematic diagram of powertrain control mechatronic system with clutch duplex drive: 1 -brake pedal; 2 -mode selector; 3 -position sensor; 4 -accelerator pedal; 5 -display; 6 -pneumo-hydraulic booster; 7 -hydraulic cylinder; 8 -clutch pedal; 9 -air supply; 10 -gearbox actuator; 11 -contact sensors; 12 -speed sensors; 13, 21 -proportional solenoid valves; 14 -clutch actuator; 15, 22 -displacement sensors; 16, 17 -clutch release levers; 18 -splitter actuator; 19 -auxiliary brake valve; 20 -pressure sensors; 23 -engine actuator; 24 -pneumatic cylinder for engine turn off; 25 -engine brake actuator At all transient processes electronic system has to provide such a clutch engagement tempo that would to avoid dynamic overloads in the transmission on the one hand and not to exceed the criterial limits for energy loading of friction clutch, evaluated by specific work and power of slipping [8], on the other.
If the mode selector 2 is subsequently set by the driver (Fig. 1) to the "D" position (drive) and the accelerator pedal is pressed, the transmission ECU sends a signal to the clutch control proportional solenoid valve 13 and in parallel by CAN the information about the current position mode selector and accelerator pedal 4 is transmitted to the engine ECU. As soon as the clutch is disengaged, the transmission ECU generates sequential control signals to the solenoid valve block и of gearbox and splitter actuators for the first selecting the rod, and then enabling the optimum gear step by actual driving conditions as: actual vehicle speed and mass, the rotation speeds of the engine crankshaft and driving wheels, the number of the previous gear steps. After that, the signal on the solenoid valve 13 is modified and the clutch is engaged on the set rate. Depending on the degree to which the clutch is engaged, the fuel supply gradually increases in proportion to the position of the accelerator pedal set by the driver in the initial phase.
AMT start-up adaptive algorithm [9] in the automatic mode of the power unit is the following sequence of operations: transmission mode selector control -full clutch disengagement -main and additional gearbox control -engine control -full clutch engagement.
After moving the mode selector 2 to the "D" (drive) position (Fig. 1), the driver presses on the accelerator pedal 4 for vehicle starting-up. The proportionality between the pedal rotation angle (by position sensor 3) and the movement of the fuel injection pump regulator lever (by displacement sensor 22) is provided by the engine ECU by pulse-width modulation (PWM) signal to a proportional solenoid valve 21 with using feedback on the movement of the fuel injection pump regulator lever and the engine crankshaft rotation frequency [10]. The required rate of vehicle star-t-up is determined by the "gas" pedal press speed, on the basis of which the transmission ECU calculates the pro-required clutch engagement rate and the enable gear number. In parallel, while receiving signals from the mode selector limit switch, the engine speed sensor, the fuel supply position sensor and the accelerator pedal position sensor, the transmission ECU generates a control signal to open the proportional solenoid valve 13. The valve is activated and the compressed air comes from the receiver of the supply part 9 into the clutch actuator 14 (pneumatic chamber), as the result of which the clutch is disengaged. After working out the signal from the clutch lever sensor 15, the transmission ECU sequentially generates signals to the solenoid valve block of the gearbox actuator, as well as the air-control valve for shifting the splitter. After shifting on the required gear (the respective gearshift limit switches are closed), the transmission ECU sends a PWM-signal to the proportional solenoid valve 13, increasing the duty cycle of the signal by a certain amount depending on the "gas" pedal press speed. The compressed air from the clutch actuator -pneumatic chamber 14 through the proportional solenoid valve 13 is gradually discharged into the atmosphere, providing the required clutch engagement rate. To avoid "overloading" of the clutch, feedback is introduced into the control circuit according to the increment by time of difference between angular velocities of the driving and driven clutch parts (d/dt). The feedback is also entered into the engine control circuit to avoid ICE stalling by the reduction in the crankshaft angular velocity ω 1 while the clutch is engaging.
Control algorithm adaptation occurs when the mechanical and (or) electrical characteristics of the mechatronic system components vary, as well as the driving behavior or the vehicle weight state change.
The functionality of the developed AMT mechatronic system (Fig. 1) and the operational integrity of the vehicle start-up adaptive algorithm are confirmed by the result of a semi-real experiment on test complex BNTU.
Clearly, that the control quality of dry friction clutch will depend on the width of the PWM signal range. The range value is influenced by the consistency of the actuator geometrical parameters, solenoid valve operating characteristics and the controller output stage. At constant parameters of the last dominant influence on control range has generated the PWM signal frequency.
For debugging powertrain automated mechatronic system a special test bench was created in the laboratory of the BNTU Automobile Department. The test bench consists of diesel engine, dry friction clutch, mechanical gearbox, inertial mass and a load device. Ecomat R360 controllers of CR2500 series were used as a hardware basis of the designed control system.
To investigate the influence of PWM signal frequency to control range of clutch actuator, a special software has developed. It allows to initialize the signal generator and to provide the PWM signal of varying duty ratio (but a certain frequency) in automatic mode to the proportional solenoid valve VEP 3121-1 that controls the clutch actuator. During the experiments, PWM frequency was varied from 150 to 250 Hz in steps of 10 Hz. The clutch lever position, fixed by the potentiometric sensor MY-615A of angular displacements, was used as a feedback. A graphical representation of the seminatural experiment results was performed with visualization possibilities of CoDeSys V2.3. Some records of engagement/disengagement of dry friction clutch are shown at Fig. 3 Fig. 3. Oscillogram of clutch engagement/disengagement process at PWM frequency of 150 Hz: first screen -digitizing the signal from the clutch lever position sensor, bits; second screen -current to the coils of the solenoid valve, mA; third screen -PWM reload value, bit As shows the results of experiments the PWM signal range was the greatest at a frequency of 150 Hz. Its value was 40.7 %. There is a steady trend towards a narrowing of the control range with frequency increasing. Already at 200 Hz control range decreased by 26.04 % and was 30.1 %. At 250 Hz the range decreased by 34.15 % and was 26.8 %. The above described control range dependence of the PWM frequency is illustrated in Fig. 4. During the vehicle operation, the friction plates of the clutch are wore and resulting of which is a change in the control range of actuator. To improve the quality of the clutch transition process when starting-up vehicle, adaptation of the control algorithm is required. Adaptation to control range of clutch actuator can be implemented at the software level by the dependence of the control range variation on the PWM signal frequency in an analytical form. The dependence of the control range variation on the frequency of the control PWM signal can be expressed by an approximating polynomial whose coefficients are obtained by processing experimental data using the Polyfit function [11, p. 379] in the МatLab package.
Justification of the polynomial degree choice was carried out on the basis of the smallest modules criterion MAPE (Mean absolute percentage error) [12, p. 1104 where D PWM -control rate of clutch actuator, %;  -frequency of control PWM signal, Hz. The variation of PWM frequency does not influence on the current range. During the clutch lever movement the magnitude of the current to the solenoid valve coils varies from 0.272 to 0.614 A (Fig. 3), i. e. operating current range is 0.342 A. Taking into account operating characteristic of the used solenoid valve and its zone of stable operation it is 50.3 %, which is a satisfactory result. Clutch actuator hysteresis is shown in Fig. 5.
Clutch actuator hysteresis should be integrated in AMT adaptive control algorithm of vehicle starting-up process. The semi-real experience results of start-up process when debugging of AMT adaptive control algorithm with PWM frequency 150 Hz on test complex BNTU are shown in Fig. 6.