Have I over-complicated this?

[throbscottle]

Motor controller for a very small motor which will move a slider up and down a rod, no great accuracy required. Just a block diagram for now. The window comparator is to prevent hunting if the linear pot voltage is between step values of the digital one. PWM will be a 555. Amp is just straight opamp, no PID involved. Even so, I feel like I'm making this too complicated. What does anyone think?

Thanks in advance

 

[JimB]

A few rambling thoughts:

Why do you have PWM? do you need to vary the speed of the motor? or just move it and stop?

Why is "signal conditioning" feeding a digi pot? What does the digi pot do? You just need to condition to an analogue signal which represents the position of the slider.

What is the OPAMP doing? Your window comparator should be here, conparing the position feed back from the signal conditioning with the setpoint from the pot in the foot pedal. The comparator should have two outputs, one to turn the motor one way, one to turn the motor the other way.

If you are just stopping and starting the motor, you probably don't need an "H bridge controller".
You only need it if you have variable speed and PWM.

JimB

 

[throbscottle]

Hi JimB, thanks for the feedback.
Yes, the motor needs to have variable speed to track the input, though now you mention it, not necessarily in both directions - it could be control down, zip back quickly, though that might be more complicated. I put in PWM because I don't want to lose power at low speed - though again I start to feel like I'm missing an important point here...

The feedback sensor is from an optical interrupter, hence the digi-pot. (Arrived at thanks to help in another thread) If I make it a quadrature encoder, signal conditioning then comprises a buffer, a pair of D type flip flops and creating an up/down control signal to the digi-pot. The reason for using an optical encoder + digital pot is because it's quite a small motor and I don't want to load it with a linear potentiometer.

I KNEW I was putting in some extra complication! I started out with an opamp only design, then added the window comparator after I realised the digi-pot step problem. I'll take the opamp out now - thanks But then I need to create limited gain window comparator, don't I? Amp with dead zone.

It's actually going to be for the up/down control of a pcb drill which uses carbide bits. I want both hands free to position the board, and precise movement to minimise risk of breaking the bits, so I'm using a cd-rom drive head mechanism, hence the very small motor. The weight of the drill motor will be semi-balanced with a counterweight. Also on a very tight budget (well, non existent, really), so I have to work around using free or very cheap parts.

Big thanks for the pointers

 

[JimB]

OK on the digital pot, that makes sense when you describe it like that.
I was thinking that the signal conditioning could be a pic which includes the digi-pot functionality.

JimB

 

[throbscottle]

Yeah, I briefly thought about using a pic for the whole thing, but I already have most of the parts to do it this way, and it seems a waste of a pic to just use it for the encoder. More thought required...