Adaptive cruise control

The problem

Adaptive cruise control is two loops wearing one name. The longitudinal loop holds a following distance and a speed; the lateral loop keeps the vehicle in its lane. Both have to run deterministically on a microcontroller, sharing a bus with everything else on the vehicle, at a rate fast enough that the vehicle dynamics never get ahead of the controller.

Approach

Lateral motion is modelled with a bicycle model — two wheels, one steer angle — coarse enough to run in real time and accurate enough at the speeds involved. Longitudinal control is a position- and velocity-based law on the measured headway. Steering torque comes from a DC motor that also renders haptic feedback back to the driver, so the loop closes through a human as well as through the vehicle.

On the bus

Motor commands, steer angle and feedback force all travel over CAN, including to other nodes on the same bus. That makes message timing and frame budget part of the control design rather than an afterthought — a controller that is correct but late is just a slower way of being wrong.