PID motor control: Can it be done this way?

[tomal]

I'm working for a PID motor control (AC/Single phase/220V/50Hz) using CCS C.
In the first step I made it in open loop mode (regular controller) as you see in the picture that the RPM sensor connected to CCP1 with a dotted line. Zero cross, speed variation and rpm count everything works perfect. Now I'm in the way to work for PID. I'm thinking it in the proposed way (pls see the picture).

1. RPM signal is converted using RC filter and connected to comparator 1 (A0) input
2. Cvref (A2) used as feedback for PID error detection (error=set_speed-current_speed)
3. Comparator 1 out is digital tach and connected to CCP1 for RPM count



I would like to know, can it be done in this way...can I configure internal comparator to work in this way?
Or, is there any better way than I'm thinking?

[Mike Walne]

You've had no response in a week.
Probable that nobody else can understand what you are trying to do either.
I certainly can't. It all looks rather complex to me.

Why not work on the KISS principal?

All you need to get RPM is one digital input.
You appear to be doing both analogue & digital.
Where do you get set speed from?
The rest, up to motor drive, is software.

Mike

[tomal]

Thanks Mike. At least you cared to reply.

I did it finally. Just one question though...how much time PID usually takes to stabilize the speed from no load to load contition?

[Mike Walne]

How long is a piece of string?

Seriously, can't tell without lots more information:-

Motor size, power, torque at no load & full load etc.
Is overshoot allowed?
List goes on and on ............

Suggest you do some testing.

Investigate whether you have set up optimal conditions.

Mike

[asmboy]

your circuit makes no sense, on several counts.

** why are RA2,3 tied and with a cap to ground ?

**what is the ra4 c2 connection to the freq opto interruptor?

**why the unknown TC of the RC circuit into A0

** you don't even SHOW an output to spin UP the motor
and close the PID loop !!!
what controls motor excitation anyway ??

HINT: its not in the schematic

** and lastly with no values for parts and no code,, what do you expect ??

[tomal]

@Mike:
The controller I'm experimenting is for 1hp motor, rated for 35k rpm. My desired rpm is 5k-25k. It is actually the router (motor) for CNC router. Since it is an AC/50Hz motor and I have to compensate firing angle to maintain the speed.

This is the first time I worked with PID so some of the points are not cleared to me. Speed stabilization is one of them. i.e. I set the speed for 5k in no-load condition and then starts cutting. But speed drops immediately to 3.5k whenever tip touches the surface. Then slowly stabilizes to the set point (5k). I noticed that the stabilization takes 5-6 seconds. It is not a problem for lower cutting speed but may break the cutter (cutting bit) at higher cutting speed (I haven't experimented higher speed yet). Hope you understand my concern now.

@asmboy:
[EDIT] This is not the circuit I worked with and not the complete one. Here is just the feedback part related to my question.

I was initially not sure to compute the error. I thought I'll feed the RPM as analog (C1, R2 & R3) and (re)generate the pulse using internal comparator. CVref will be used as feedback (instead of digital tacho). However, (regenerated) tacho at RA4 connected to CCP to count and display the RPM. Forget about dotted line. I should'nt have drawn it.

It was a complex concept initially developed in my mind and I left it later. I finally did it in a more simpler way.

[Mike Walne]

Hi Tomal,

Try to avoid ageist comments, you risk offending the forum members most likely to offer significant help.

Asmboy took the trouble to analyse your offering, I didn't care to.

You now say you've not used most of the 'schematic' shown.

So, in total, you are giving us nothing to work on, either hardware or software.

As for you current problem:-

I'm assuming you have constants Kp,Ki & Kd for each of the P,I & D terms.
Do tests under no-load and full load conditions.
Investigate the effect(s) of varying each of the above constants in a logical and systematic manner.
You will learn how PID works.
You will discover how far away you are from optimal operation.

With little else to guide me, that's my best suggestion.

Mike

[asmboy]

[Mike Walne]

Again with little else to go on.

How quickly are you measuring rotational speed?
It's crucial.
If your measurement is too slow, then nothing else can happen reasonably quickly.

Just a wild guess.

Mike

EDIT Have you done any kind of analysis?

[gpsmikey]

I would think an important input to your control algorithm would be load current so you could measure the load on the motor. I would expect the load current to change faster than the rpm of the motor when it starts (or stops) cutting.

mikey
_________________
mikey
-- you can't have too many gadgets or too much disk space !
old engineering saying: 1+1 = 3 for sufficiently large values of 1 or small values of 3

[temtronic]

mikey's right about load current monitoring ! You don't say what material the CNC will will used on,but from first hand experience 'wood' is interesting, MDF is hohum and aluminum is well, just boring (no pun intended)
You should have at least 3 identical runs into whatever material you're going to use and record ALL the data.The more numbers ,the better the guess you an make about the PID constants. If you have access to MATLAB,just import and compute...MATLAB will quickly spit out the numbers for you.

hth
jay

[tomal]

Thanks guys for your expert opinion.

My feedback is rotation and I'm not monitoring load current at the moment. Target materials are wood, HDPE (plastic) and some aluminium. But I'm using HDPE in current development phase.

I did some fine tune for the constants and reduced the stabilization time from 5-6 sec to 1-2 sec. But sudden drop in load condition has no improvement at all. Is this normal?

[temtronic]

Now you know why we've said you must monitor the current. Speed alone won't tell you everything.
Also, we don't know how you're controlling the router's speed,that bit of electronics will need to be 'tweaked' as well.
And depending on the material being used, the 'travel speed' of the router will need to be adjusted in order to maintain proper cutting.You have to go slow when removing 1" of MDF but can go fast if only 1/8" aluminum.

hth
jay

[tomal]

Thanks jay. Seems I need to look on current monitoring.
But isn't it the same?

Since it's a CNC application and the subject is load or no-load. Load currrent will change only if the cutter touches the surface. I think PID algo never know when it has the load and will immediately compansate whenever noticed the difference. I think same thing happens with the rpm feedback.

By the way, I have a strange thing happened today. I was out of stock for crystal so I worked with the internal osc. My crystal's arrived today and I changed the fuse but PIC runs vey slow with the crystal clock. Is my PIC18F2420 bad?

Internal clock: