Ron Beaufort
Lifetime Supporting Member
Greetings to all,
this series of posts is in response to a request from jvalle in this post in which he asked for a basic explanation of the PID’s Derivative action ... I’ll do my best to tackle that subject in this and in the next several posts which follow ...
first things first ... different PLC manufacturers perform the Derivative action in different manners ... as if that weren’t confusing enough, even the same manufacturer (for example: Allen-Bradley) may perform the Derivative action in different ways even within the same processor (example: compare the units used for the Derivative with the PLC-5’s “ISA – Dependent Gains” equation and the units used with the same processor’s “AB – Independent Gains” equation ... going even further, within either of the equations mentioned just above, the Derivative action can be based on either the “Error” or on the “Process Variable” ...
now the purpose of mentioning all of this at the beginning is not to just confuse you ... but instead only to point out that all of what I’m about to say should be considered a “getting started” lesson for the Derivative action ... I’ll try to be as specific as possible but please keep in mind that there will be exceptions to all of the “rules” which I am about to cover ... if you have specific questions about any part of this, then please post with as much detail as you can manage about whatever you find to be most confusing ... I’ll do what I can to explain the subject in greater detail ... with all of that having been said, let’s move along ...
I’ll base all of my examples (unless otherwise noted) on the following setup:
an Allen-Bradley PLC-5 processor ...
using an N-type (integer) control block ...
using the “ISA – Dependent Gains” equation ...
the Error calculation is set for “E = SP-PV” ...
the Derivative action is based on the “Process Variable” ...
the minimum data input to the PID is “0” ...
the maximum data input to the PID is “4095” ...
moving right along ... I’ll assume that you already know that the PID’s final output (the CV – Control Variable) is made up of the sum of three individual components ...
(1) the Proportional action ...
(2) the Integral action ... and
(3) the Derivative action ...
of these three components, the Derivative that we’re talking about today is the one that is probably the least understood and the hardest to use ... in fact, many experienced programmers will simply tell you to: “just turn the Derivative off – and then leave it off – it’s more trouble than it’s worth” ... I’m not going to enter into that particular debate ... my purpose in writing this is to take a shot at explaining how the Derivative action works and how it is measured ... whether to use it or not remains totally up to you ...
now since this series of posts is (after all) about the PID’s Derivative action, I’ll assume (gosh I hate that word) that you’re already familiar with how the PID calculates its Proportional action and its Integral action ... as we move further through this subject of the Derivative action, we’ll eventually talk about how the PID’s Proportional setting (Kc) also influences the amount of response that we’ll get from both the Integral action and the Derivative action ... and incidentally that is why the particular equation that we’re using for our examples is called the “Dependent Gains” equation ... specifically, the amount of Integral response and Derivative response both “DEPEND” on the PID’s Proportional setting ...
on to the next post ...
this series of posts is in response to a request from jvalle in this post in which he asked for a basic explanation of the PID’s Derivative action ... I’ll do my best to tackle that subject in this and in the next several posts which follow ...
first things first ... different PLC manufacturers perform the Derivative action in different manners ... as if that weren’t confusing enough, even the same manufacturer (for example: Allen-Bradley) may perform the Derivative action in different ways even within the same processor (example: compare the units used for the Derivative with the PLC-5’s “ISA – Dependent Gains” equation and the units used with the same processor’s “AB – Independent Gains” equation ... going even further, within either of the equations mentioned just above, the Derivative action can be based on either the “Error” or on the “Process Variable” ...
now the purpose of mentioning all of this at the beginning is not to just confuse you ... but instead only to point out that all of what I’m about to say should be considered a “getting started” lesson for the Derivative action ... I’ll try to be as specific as possible but please keep in mind that there will be exceptions to all of the “rules” which I am about to cover ... if you have specific questions about any part of this, then please post with as much detail as you can manage about whatever you find to be most confusing ... I’ll do what I can to explain the subject in greater detail ... with all of that having been said, let’s move along ...
I’ll base all of my examples (unless otherwise noted) on the following setup:
an Allen-Bradley PLC-5 processor ...
using an N-type (integer) control block ...
using the “ISA – Dependent Gains” equation ...
the Error calculation is set for “E = SP-PV” ...
the Derivative action is based on the “Process Variable” ...
the minimum data input to the PID is “0” ...
the maximum data input to the PID is “4095” ...
note to SLC and MicroLogix users: the maximum data input to the PID in your systems will usually be “16383” ... this is roughly four times the value used for the examples in this series of posts ... even so, you should have no problem following this discussion of Derivative (even though it is based on the PLC-5 platform) just as long as you keep the difference between the data formats in mind ...
moving right along ... I’ll assume that you already know that the PID’s final output (the CV – Control Variable) is made up of the sum of three individual components ...
(1) the Proportional action ...
(2) the Integral action ... and
(3) the Derivative action ...
of these three components, the Derivative that we’re talking about today is the one that is probably the least understood and the hardest to use ... in fact, many experienced programmers will simply tell you to: “just turn the Derivative off – and then leave it off – it’s more trouble than it’s worth” ... I’m not going to enter into that particular debate ... my purpose in writing this is to take a shot at explaining how the Derivative action works and how it is measured ... whether to use it or not remains totally up to you ...
now since this series of posts is (after all) about the PID’s Derivative action, I’ll assume (gosh I hate that word) that you’re already familiar with how the PID calculates its Proportional action and its Integral action ... as we move further through this subject of the Derivative action, we’ll eventually talk about how the PID’s Proportional setting (Kc) also influences the amount of response that we’ll get from both the Integral action and the Derivative action ... and incidentally that is why the particular equation that we’re using for our examples is called the “Dependent Gains” equation ... specifically, the amount of Integral response and Derivative response both “DEPEND” on the PID’s Proportional setting ...
on to the next post ...