Our Driverless Vehicle, M19-D, needs a way of reacting to the requests sent by the motion control algorithm. Without a driver in the car, we rely on an electric motor to control the steering system, as well as a pneumatically powered linear actuator for the brakes.
M19-D’s steering system is controlled by a servo motor through a 49:1 reducing gearhead, providing near human performance in torque and speed. This is connected to the steering column through a belt drive, which can be easily removed in case the car needs to be manually steered such as for manually driven brake tests at competition.
The motion control algorithm requests a target steering angle from our MoTeC M150 Engine Control Unit (ECU). The ECU compares the request with the actual steering angle deduced from the steering rack position sensor, and calculates the number of times the servo motor must rotate to turn the wheels by the desired amount.
The motor’s internal PID controller will then try to reach this target, while sticking to the given acceleration and velocity limits set for the motor. There are also several checks in place to make sure the motor doesn’t try to turn the wheels at a greater angle than the steering rack can handle.
The braking system is the most safety critical component on our Driverless Vehicle. We must be able to activate it remotely at any time, no matter what the car is doing and even if the car has lost power. The Emergency Braking System (EBS) has two redundant air pressure reservoirs that are always providing pressure that tries to engage the brakes. In the car’s default state (and when the power is off), the brakes are fully engaged.
In order to release the brakes we use a controllable air pressure regulator, supplied by SMC, to oppose the force applied by the EBS. This pressure regulator acts as the normal “service” brake and allows us to disengage, partially engage or fully engage the brakes, depending on the commands sent from the ECU and the motion control algorithm. The service brake has its own supply of air (independent of the EBS) from a high pressure tank on the back of the car. Upon pressing the emergency kill switch on the Remote Emergency Stop (RES), power is cut to the car and the pressure regulator releases all pressure. This allows the EBS to fully engage, bringing the car to a stop from 40km/h in under 10 metres.
The safety, reliability, and tunability of the actuators on M19-D is critical to the successful completion of all dynamic events at competition, and allows the car to reach the high level of performance it was previously able to achieve as a human-driven Electric Vehicle.
By Matthew Lane and Jack Coleman