Developing automated vehicles is challenging task to say the least. Fundamentally, you have perception, decision and control. Each of these carries with it a host of challenging tasks while the end result is a near perfect implementation of a safe and comfortable experience. The purpose of this write up is to address what is necessary on the control side of things to deliver safety and comfort.
- By-wire control systems are necessary to provide a base platform than can accommodate automated driving. But even if by-wire is available, we still need to come up with a control algorithm that can deliver a safe and comfortable ride.
- Adjusting and calibrating a PID control algorithm is a timely task because much depends on the physics of the vehicle and understanding all the calibration measures between the computer output and the control systems input.
We are going to start with the basic and most fundamental element of automated driving and that is “control” using by-wire signalling necessary for longitudinal and lateral control of the vehicle. But how are we going to transfer our control signals from the computer to the activators?
Furthermore, for control to work smoothly and safely you will need a control algorithm that is most likely PID or some variation of it. PID Theory is all about adjusting the gains necessary to deliver safe and smooth performance.
For automated vehicle functions it is imperative to have by-wire control of throttle, steering and braking. In other words, you need digital signal control and servo actuation that require no human input at all. Off road applications may retrofit manned vehicles with steering column actuators or pedal actuators but in the passenger car segment this is not practical for anything beyond very early stage development activities. (such as early DARPA challenges).
Most modern production cars already have by-wire control of engine management. This started more than 15 years ago and it is an extension of EFI (electronic fuel injection). On the other hand, steer-by-wire technologies are rare on production vehicles. And while many vehicles have a power assistance feature applied to steering that uses a traditional power steering system consisting of a hydraulic steering rack fed by an engine driven hydraulic power steering pump.
Electric Power Assisted Steering (EPAS) was a key development in the move to steer-by-wire. EPAS eliminates the hydraulic pump and replaces it with an electronic motor. In theory, a ESAP system could replace the steering column completely, but steering columns are a necessary safety element in the event of a failure of the EPAS system. Often, but not always, a car’s ESAP system has enough torque to handle all steering activities in applications such as automatic parking or active lane keeping technologies. It is in these systems where L2+ automated driving applications could be applied.
Meanwhile, brake-by-wire is the more complex by-wire application. The traditional brake system has a direct mechanical link between the brake pedal and the brake master cylinder. Often a vacuum powered booster is employed to help force the hydraulic fluid from the master cylinder to each brake caliper or wheel cylinder to stop the vehicle. A brake-by-wire system may pressurize the brakes system with a hydraulic pump or entirely do away with the hydraulic system and operate each wheel's brake caliper electrically.
Most modern production cars don’t have brake-by-wire systems, however, hybrid and plug-in electric vehicles do. This is one reason hybrid or EVs are often the chosen base platform for automated vehicle development. Alternatively, you can add a brake-by-wire system found on some hybrid vehicles. The actuators from these hybrid vehicles can be added in-line to a car with a traditional brake system to control brake pressure.
Controlling the Vehicle: Calibration & Signal Management
On the control side of automated vehicles, you have a host of challenges associated with delivering the proper commands for safe and smooth operation. Fundamental to control is PID Theory.
A proportional–integral–derivative controller (PID controller or three term controller) is a control loop feedback mechanism (controller) widely used in industrial control systems and a variety of other applications requiring continuously modulated control. A PID controller continuously calculates an error value as the difference between a desired set point and a measured process variable and applies a correction based on proportional, integral, and derivative terms (sometimes denoted P, I, and D respectively) which give their name to the controller type.
In practical terms, PID automatically applies accurate and responsive correction to control functions to maintain proper and smooth operation. Without PID you would have a very jerky ride!
Once the PID is set up, there is a constant multiplier that acts on each of the PID gains. Then, to create smooth and responsive control, we adjust these multipliers… this is called PID tuning. We start by increasing the P gain and leaving the I and D at 0, then after that we carefully increase the I gain and then the D gain. Once tuning is complete we have responsive, fast, and accurate, smooth control.
Here is a really good video on understanding PID control for automated vehicles.
Calibration: a big part of calibration is PID tuning
Another part of calibration is knowing what value the computer uses and matching that with an applied (real-life) value. For example, if our computer sends a steering angle of 300, we need to know what angle this will be in degrees on the actual vehicle.
Another example, if the computer is sending a throttle value of 15,000 to the throttle actuator, we need to know about what vehicle speed this will bring us to. Also, we need to find minimum and maximum values for each control, to make sure we do not send values could damage or fault the actuators, but also for safety reasons we need to make sure we do not slam on the brakes too hard, or torque the steering wheel too hard.
Developing automated vehicles is challenging task to say the least. There are many tough problems that need to be solved before a safe and comfortable ride can be delivered.
By-wire control systems are necessary to provide a base platform than can accommodate automated driving. But even if by-wire is available you still need to come up with a control algorithm than can deliver a safe and comfortable ride. Adjusting and calibrating the PID control algorithm is a timely task because much depends on the physics of the vehicle and understanding all the calibration measures between the computer output and control systems input.
Some elements of by-wire control is more challenging than others. For example, just because a car has electronic power steering does necessarily mean it is suitable for self-driving cars. Furthermore, take out the steering wheel altogether and you will need a redundant EPAS system that can cover for a failure in the primary unit.
If you are interested in following the VSI build-out of automation you might be interested in VSI Pro, this is our newest subscription service which documents the total build in detail along with sample code bases, fixes to problems, challenges and so on. Furthermore, throughout the build we will document the functional performance of all our enabling partners. This includes sensor packages, ECUs, localization assets and more. VSI Pro is a service designed for R&D departments working on autonomous vehicles. VSI Pro delivers practical advice and reviews of common enabling technologies, how well they performed, and where the gaps are.