incPID Routine - Help Needed

[HenrikOlsson]

Hi,
You seem to be approcahing the project in all the correct ways, what you describe is most certainly doable and I honestly can't see why the output would swing between the extreme ends like you explain, especially with a gain of only $0100.

Can you replace the feedback voltage with another pot? Then, with a gain of $0100, you should be able to adjust the output with EITHER the setpoint pot or the feedback pot. Dissconnect the motor and just observe the output untill you get that working. I'd also try to get the variables output etiher to a LCD like you say or over a serial line to the PC.

One thing I do see though is that you have the pid_Out_Clamp set to 511 but as you say the HPWM command only accepts a dutycycle of 8 bits (0-255) so I'd set the pid_Out_Clamp variable to 255. You could always try copying pid_Out to a BYTE-sized variable and use that varaible in the HPWM statement but I don't think that is the real problem.

Hang in there!

/Henrik.

[Aussie Barry]

Hi Henrik,

I have had a high degree of success with this project today.
I disconnected the motor and replaced the feedback voltage with another pot as you suggested. I was able to get some very nice PWM signals showing on the scope. I think a lot of my problems last night were caused by a dodgy MOSFET. It failed under load this morning and the whole system fired up nicely once it was replaced.

To tune the system I added two more pots; one for Kp and one for Kd. I also connected the LCD which helped me greatly in seeing what was happening. The motor I am using doesn't have alot of inertia so Kp and Kd terms are relatively small for a tightly tuned system response. So far, so good.

I am a little confused with your velocity feedforward section. pid_velocity_cmd is initially set to zero and your program comment states that this variable is "set by the main program." From what I can find, this variable is only used in the calculation for pid_Vel_ff and pid_Acc_FF. If this is the case, it will always remain zero (as will pid_Vel_FF, pid_Acc_FF and pid_LastVelocity_Cmd). Am I missing something?

For my trials I ended up setting pid_Vel_FF manually just so the output could overcome what little insertia the system does have.

There is still a fair bit left to do on this project but before I do much more I need to transfer this circuit from my breadboard to a PCB. I am sure a lot of the instability I am seeing on my scope can be attributed to the breadboard (switching currents, ground loops etc.)

Your return comments on the feedforward situation would be greatly appreciated.

Cheers
Barry

[HenrikOlsson]

Hi Barry,
Excellent news, I'm glad you're moving forward!

If you're going to use the feedforward terms you need to set pid_Velocity_cmd in your program before you call the pid filter routine, in the same way that you must set pid_Error. In your particular case you'd probably just do something like

pid_Error = Setpoint - ADValue
pid_Velocity_Cmd = Setpoint
GOSUB PID

pid_Velocity_Cmd is the "input variable" for the feedforward terms. pid_Vel_FF, pid_Acc_FF and pid_LastVelocity_Cmd are "internal" variables and you should not need to worry about those.

In my own use of the filter the setpoint isn't velocity but position so I needed to have a separate input variable for the feedforward. My program then calculates the velocity based on the difference in commanded position between each concecutive loop.

I hope that makes sense, otherwise let me know and I'll try again.

/Henrik

[Ioannis]

Hi Barry.
I was looking your schematic and if I may suggest, replacing Q1, Q2, R16 and R17 with a Mosfet Driver chip, like microchip's, would give you much better drive to the mosfet Q3.

If you have a scope, look at node of the two emitters. If it shows that the two transistors are both conducting for a short time you may be waisting energy and heat the drivers. Also the edge that drives the Mosfet may not be very steep.

Ioannis

[Aussie Barry]

Hi Ioannis,

Thanks for your input, much appreciated.
As previously stated, this is the first stage of a much larger project - to re-power my lathe with a 180Vdc 2HP treadmill motor. Knowing that the final design will involve isolation between control and motor drive stages, I decided to replace my transistor gate drive circuit with an opto-isolated MOSFET driver (FOD3180). Also, I was experiencing lots of noise on my breadboard layout so keeping powr supplies isolated in my prototype development PCB can only be a benefit.

Attached is a copy of my new circuit diagram:

incPID_16F684_V4.pdf

JP1 & JP2 allow me to select one of two feedback sources; the analog output from the LM2907 (0-5V) or a pulse train from the shaft encoder (0-5.5kHz 50% duty cycle). My preference will be to use the pulse train and merely count pulses in my program to determine rotation speed but it was easy enough to fit both into my PCB design.

I have used multi-turn pots (VR2-VR6) to adjust the gain for the five different PID terms so, theoretically, I should be able to tune the system "on the fly". I will be able to display variables on the 4x20 LCD which will help me "see" what is happening.

The PCB (165mm x 115mm) has been exposed and etched and is ready for drilling and populating - hopefullt this will be completed tonight. I will keep you updated with more news soon.

Cheers
Barry

[Ioannis]

Just an observation. The shaft encoder is ready made or are you using an opto interrupter with a disc etc?

If the latter is the case be sure that it can detect pulses of 5.5KHz.

I think it is better to use directly the train pulse and feed a timer, TMR1 for example.

Better accuracy, less parts.

Ioannis

[Aussie Barry]

The shaft encoder is a photo interrupter with a disc - 100 steps per revolution.
I made the disc myself so I can adjust the number of steps quite easily.
I did a sanity check on the shaft encoder but counting the number of steps per second using a simple COUNT routine and displaying the result on the LCD. I then compared the result with a non-contact hand-held digital tacho. The two results were near identical - I was very happy with the accuracy of my system.

I have just finished assembling the PCB (see attached) and hope to do some testing on it tonight.




Cheers
Barry