position tracker with op amp

[Uncle Vernon]

no I need the pot in the motor to tell me the valve position

If you selected the actuator you selected the wrong one, you've made things a lot more complicated than they need to be.

I've opted for an mcu design and am getting better results

Assuming the mechanical hardware was not your choice, an MCU is probably your best choice.  Using a temperature sensorwould make life even easier but I understand if that isn't an option. Hint you want to slow the action as much as possible, response of the overall cooling/heating will be much slower than any actuator.

except that I'm having overshoot problems now even though I've set a "dead window" To make it near viable the dead window has to be so large that it becomes sluggish.

In execution, sluggish will be a good thing!

[Simon]

by actuator do you mean the valve ? no not my choice, it is something we already use in other stuff with a controller but now they want a stand alone one.

By sluggish I meant that it responds in too big a steps. it stops ok but then needs a large shift of the control pot to move it again and that makes it move by a large amount: I loose fine control.

I have another idea, I could do a burst of manual PWM or just a single pulse after which I recheck the positions rather than wait for the whole program to re-run the loop, or I just send one short pulse and then go back and recheck positions. I could implement it on small position changes and let it run full power until the position becomes that close, I can get 1-2 uS pulses out of setting a bit high and then low.

The guy running the overall project is a bit of an old fashioned type. Relays were high tech to him, infact our last air con system for our last military design used a bloody panel full of relays. Now that he wants to move into more automation and electronic control he will be giving me a lot of wrong information as I'm sure he will eventually figure out that the control pot should not be directly positioning the valve but should be a temperature control that will then control the valve to control the temperature.
Another plus to using a pic: I can change things later mostly in software without starting from scratch. His request is infact a bit odd, unless he really just wants crude heat power control, I mean as the engine warms less valve opening will be required, you'd think that having put electronic control in there you'd take the person out of the control loop and let them set the temperature they want and let the system do the thinking instead of having a person keep altering the control as it gets hotter as the engine warms up

[Uncle Vernon]

by actuator do you mean the valve ?

Valve is the bit that regulates the water, actuator is the motor that drives the valve. In most cases they are separable, your may be a fixed assembly.

no not my choice, it is something we already use in other stuff with a controller but now they want a stand alone one.

I'd assumed that, in this application you would usually select a spring return actuator that works much like a servo. PWM or control voltage from the pot and your done.

I could do a burst of manual PWM or just a single pulse after which I recheck the positions rather than wait for the whole program to re-run the loop or I just send one short pulse and then go back and recheck positions

That is going to be your best strategy, you need to just inch the valve, so your pulses will need to be short. Even with this you will be battling some hysteresis. You are never likely to get fine control, but you should be able to achieve a reasonable degree of positioning.

The guy running the overall project is a bit of an old fashioned type. Relays were high tech to him, infact our last air con system for our last military design used a bloody panel full of relays.

Don't totally dismiss that approach, in harsh environments KISS principle is best. It's take 30 years to get real reliability into automotive electronics.

His request is infact a bit odd, unless he really just wants crude heat power control, I mean as the engine warms less valve opening will be required, you'd think that having put electronic control in there you'd take the person out of the control loop and let them set the temperature they want and let the system do the thinking instead of having a person keep altering the control as it gets hotter as the engine warms up

Not that odd, what you are proposing is "climate control" there is a hell of a lot of vehicle operating without any form of closed loop heating control and rather than the electronics a simple bowden cable positions the valve.

[Simon]

well I was getting things better until the thing has just decided to pack up. Haven't a clue what happened. But yes even with small pulses controlling it is a problem. I think I need at least 30-50 uS pulses or it won't move, of course once it has liquid going through it things will change.

To make sure i don't burn out my mosfets I'm tying the gate of one N channel to the drain of the other one so that both sides of the bridge cannot come on together as lots of switching is causing problems with them heating up.

My comments about the person running the project relate to the fact that he is a bit of a drip. He knows the theory of air con systems but his practical implementations often have a lot of problems, our last military job was redrawn a number of times and the last of over 100 units were still going out with latest mods. He has no idea about electronics and getting him to understand things is hard

[Mechatrommer]

and getting him to understand things is hard

fire him!

[Simon]

and getting him to understand things is hard

fire him!

well he is above me and soon to retire i think, the next in line is an even bigger asshole.

He took the valve home and tried to control it with a potentiometer on it's own, what the hell he thought he was doing i don't know

[Simon]

well I have achieved some reasonable success. I'm pleased but not entirely satisfied, it all seems to lie in the detail of the pulse width duty and frequency to match the motors mechanical properties. And of course all will change when the valve door is onder 2 Bar of pressure from the coolant that will hopefully help stop some overshoot.

Now i need to nail down the ADC Vref details, the max position voltage the sensing pot sends back is 3.5V (70%) so I am using an external reference for the ADC to help increase the dynamic range of the system, I may yet fiddle with the math in the program to make use of the 0-20% portion of the control pot that is unsused as the minimum position pot output is 1V (20%) so I could mathematically scale it to help make fully closed at 0 and fully open at 100% on the control pot

[Mechatrommer]

the next in line is an even bigger asshole.
He took the valve home and tried to control it with a potentiometer on it's own, what the hell he thought he was doing i don't know

not so bad i think, he's willing diy and get his hand dirty. ask him to join mechanical forum or here

[Simon]

not so bad i think, he's willing diy and get his hand dirty. ask him to join mechanical forum or here
[/quote]

I doubt he'd even be able to use a forum actually. To be honest if you have a valve with a pot in it and the manufacture has told you (He has a nice big datasheet he probably can't even understand) that you need control circuitry and you try and control it with another pot your kind of beyond hope. I mean it was only once he realized he wanted to bite off more than he could chew that he came to me (the underling working as goods in inspector) to find a solution.

This is the guy that was going out to clients in singapore with an analogue oscilloscope in his hand that he did not even know how to use (and the settings were so messed up that it took me a half hour to even get something on the screen just before declaring it broken). When it was really needed he brought it to me and asked me how to use it, at that point I told him that no an old analogue scope was not going to be of use in capturing back EMF and that I'd bring my own digital scope in.