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Boost Control: How To Tune A PID Control System

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How To Tune A PID Control System

05.24

00:00 - By now you should have a basic understanding of the theory behind PID control, so we're going to look at how we can go about tuning a PID control system.
00:10 PID control is a well established strategy used in many forms of industry and as such, there is a mathematical method for calculating the required gains called the Ziegler Nichols method.
00:23 In my experience, this isn't a method that's used for tuning PID in the tuning industry and, as such, we won't be dealing with it further.
00:32 If you want to research this method there are a number of resources on the internet dealing with it.
00:38 In the tuning industry, the technique we use to calibrate PID control is the trial and error method, which I'll explain in this module.
00:47 Before we move on, I want to discuss some limitations of any closed loop control system.
00:53 It's important to understand that the result of any closed loop control system will only be as good as the physical system we're trying to control.
01:03 If there are inherent problems then there is only so much the PID control can do to fix this.
01:10 An example of this might be a variable cam control system where the oil pressure is insufficient to establish good control, or a boost control system where the waste gate installation results in a boost spike before it can open.
01:25 In these situations the result you will get is going to be flawed, and if you don't understand this, you'll waste a lot of time and energy trying to fix the mechanical system with your PID control.
01:38 Often, this may prove impossible.
01:41 The PID control alone can't perform miracles and the better the system works in open loop the better your results will be once closed loop is established.
01:53 With this in mind, the first part of tuning a PID control system is to establish effective open loop control with the PID components all set to zero.
02:06 For example, in a boost control system, this would entail tuning the base duty cycle table to achieve stable boost control that is close to your target boost.
02:16 This will do two things.
02:18 Firstly, it'll prove that the system is mechanically sound and that it's possible to achieve stable control.
02:25 Secondly, it will give the closed loop system a stable place to start from, meaning it has less work to do.
02:32 Once we have the system controlling well in open loop we can start adding our PID components.
02:40 As described in the last module, we'd start by adding a small amount of proportional gain of perhaps 0.1 to 0.5, and test the results.
02:51 What I recommend doing is testing the control system to find out what sort of values lead to the system becoming unstable and oscillating.
03:00 Once you know this range, you'll have a better understanding of where your gains need to be set.
03:08 When adjusting any of the gains I recommend initially making changes by doubling the values.
03:14 This will let you quickly assess the effect of the change.
03:19 Once you go too far and the system becomes unstable you can then come back to a usable value by halving your gain.
03:27 This is a coarse way of finding the sort of values that will work for your system and you can then fine tune and refine the values.
03:37 Once we have the proportional gain close we can add a small amount of derivative gain and then, finally, a small amount of integral gain.
03:46 At each stage you can compare your results to the graphs we looked at in the last module before deciding on your next change.
03:55 Once the system is stable and tracking your target position accurately, we can test the system's response by requesting a change in target and watching how well the system tracks.
04:08 For example, we can hold the engine on the dyno and steady stage and open and close the throttle to watch how well the boost checks our target as the turbo spools up.
04:18 It's all about requesting a change in aim position and then watching how well the system responds to meet the new aim value.
04:27 Again, we can compare the response to the diagrams shown in the last module to decide what change to make next.
04:37 Setting up any PID control system relies heavily on data logging to evaluate how well the system is tracking your target as well as validating the effective changes you make to the P, I, and D components.
04:52 To do this, you'll be constantly reviewing the performance of the system using the logging features of your ECU.
05:00 It'll be worthwhile familiarising yourself with the data logging features of your particular ECU, as well as how to adjust the display parameters to show you the information important to the system you're configuring.