Modelling and performance of a pneumatic brake actuator

Abstract

Heavy vehicles have sluggish pneumatic brake actuators that limit the control bandwidth of their anti-lock braking systems. In order to implement more effective braking controllers, it is proposed that high-bandwidth, binary-actuated valves are directly placed on the brake chambers. This article details investigations made into modelling and controlling heavy-vehicle pneumatic brake actuators with a view towards implementing the novel brake actuator design. One-dimensional flow theory is combined with simple thermodynamic arguments for polytropic systems to describe the charging and discharging of a brake chamber. Particular attention is paid to the simulation of perceptible vibrations caused by the piston’s motion at relatively low charging pressures, using a hysteresis model. The resulting equations are linearized and used to design a closed-loop pressure controller for the actuator. Finally, the non-linear performance limits of the valves, caused by dead-zones and time delays, are investigated using a describing function analysis.

An investigation of the effects of pneumatic actuator design on slip control for heavy vehicles

“…It was assumed that the valves have mutually exclusive charge and discharge states and controlled flow through the chamber via a linear, proportional pressure controller that responded to electronically Downloaded by [University of Saskatchewan Library] at 01:05 15 October 2012 communicated demand signals. Both the proportional pressure controller and the validity of modelling the valve dynamics as a pure time delay are discussed in detail in [12].…”

“…The pressure controller gain was selected for individual actuators to maximise the pressure control loop's performance, as described in [12]. δ s loosely corresponds to the tolerable slip error, and could be specified as a single value equivalent to a 5-10% error for all road conditions.…”

“…Furthermore, the subscripts 'u' and 'd' indicate conditions upstream and downstream of the valve, while the subscripts 'i', 'o', and 'c' denote conditions entering, leaving, and within the chamber, respectively. The volume of the chamber was obtained from a pressure-volume relationship that was measured from a physical chamber, which is reported in [12].…”

“…It was assumed that pulse-width-modulated valves were placed directly on the brake chamber, in a configuration proposed in [10][11][12]. Brake chamber charging and discharging dynamics were described using one-dimensional flow theory and the thermodynamic relations for unsteady flow through an open system, as derived and validated in [12].…”

“…It has been shown in [10][11][12] that actuation delays in heavy vehicles could be reduced by an order of magnitude from the nominal 40 ms by placing fast pneumatic valves directly on brake Downloaded by [University of Saskatchewan Library] at 01:05 15 October 2012 chambers. Such reductions in actuation delay would allow advanced braking control methods to be used on pneumatically braked vehicles.…”