Position control using stepper and pot

I would like to control a current source of 120A with a pot and stepper, how can i precisely vary the pot , is there any need for any memory device to save the postion ?

rohit i think its no need of any memory but plz tell me cmplt detail of ur work INSHA-***** i shall help u

- - - Updated - - -

rohit i think its no need of any memory but plz tell me cmplt detail of ur work INSHA-***** i shall help u

I want to control a current source with pc,
for that i am using a Picmicro with stepper, the stepper rotates the a pot which inturn controls the current in the current source.
The current source doesnt have a pc control on it, so i am building this ....

ok
tell me how many degrees pot can maximumly rotate.this will guide me that from 0 to 5 volt variation maximum degrees will be. i will give this value to pic adc and then calculate how much steps stepper have to move but u must know the current location of stepper for this purpose u have to save data in controller memory

its a continous rotation pot (10 free rotation)

is it giving linear resistance variation

yes , a linear pot

zia said:

i will give this value to pic adc and then.....

Click to expand...

He said he is using a PC, not a PIC. He is planning a system like this:
Computer => stepper motor => potentiometer => current source.

I think it would be easier to just do this:
Computer => current source.

Whichever way he does it there is no need for a PIC.

I suppose if you're controlling something like a rheostat with 120A (curious to see how large this device is, any photo?) then you'll need more than a
stepper motor, you'll need some sort of gearing or belt drive or something?
Also, probably some sort of feedback would be essential, to ensure that the rheostat setting that you think you've set is actually what has been set, and
that the stepper has not skipped or something.

You could do it all in the analog domain with op-amps if you had some feedback method (e.g. another (much smaller) 10-turn pot (let's call it a
"feedback pot") that gets driven by the same motor which is driving the rheostat), and then you don't need a stepper device, just a conventional motor.
Bourns makes some very nice 10-turn pots. Maybe even use two for redundancy!
Your circuit would be trying to make two voltages equal; the one that represents the position of the operator's pot, the second being the feedback pot.
Of course, everyone wants to implement with microcontrollers these days...
In that case, you could use the operator and feedback pots to control a 555 astable, and measure the pulse width with the microcontroller. Or use an ADC.
After power-cycle, I wouldn't rely on memory to know the position of the stepper. Better to actually try to determine the real position of the rheostat (using
a feedback method as described) or find a way to measure this current. And make sure the microcontroller doesn't go haywire on a power glitch
due to poor PSU or something.. And maybe have an analog-driven indicator of the actual setting, not controlled by the microcontroller - e.g. a lamp or analog
meter controlled directly by the rheostat or by the feedback pot.

@Godfrey: Sorry, missed your post while I was typing this rather long response! Agree - if direct control is possible then that's the best option. If a motorized approach is needed, then I'd really steer clear of a microcontroller based method unless I was absolutely certain I was confident it would be reliable and would be safe - I'm not convinced it would be safe.. And certainly not implement memory based stuff like some of the earlier posts were mentioning, but rather implement feedback..

rohit and reyl
if i m right then rohit wat to control a stepper with pot and pot can be read using pic and pic then control stepper as 0v its 0 degree and 5v it will be on 360 degree. the stepper has specs of 1.8 degree/step

- - - Updated - - -

if right then plz tell me i will send u cmplt project within two days as i m in busy in my office

zia said:

if i m right then rohit wat to control a stepper with pot... [snip nonsense]

Click to expand...

You're wrong. He wants to use the stepper motor to turn the pot. The pot is to control the current source.

and pot can be read using pic and pic then..........

Click to expand...

There is no PIC! Please read the question before trying to give an answer.

Zia is correct , I want to use a PIC in between stepper and PC, so that the project can be modified later ....

Rohith, what's the purpose of this? 120A is a lot of current. And what kind of voltage? Got a photo or a link to the
rheostat?

It looks like this, but my instrument is different, i will take the photo and send later

http://www.atcomaart.com/pd/484848484853658988/power-supplies/regulated-dc-power-supply.aspx

- - - Updated - - -

To use in my lab, actually , its for superconductivity experiments

- - - Updated - - -

voltage is 0 to 24V

My insturment is aplab make and i didnt found a photo for the same in their site

- - - Updated - - -

almost the bottom one , but with out a PC interface

http://www.aplab.com/product-services/power-suppliesprdouct-a-services/gpib-programmable-linear-power-supplies-a-electronic-loads/gpib-programmable-lab-regulated-dc-power-supplies-prolab-series-300w-to-1kw/185.html

The bottom one says 0.5A? Is your one 120A?
And what are you trying to adjust? The voltage setting on it? Because normally when people say "current source" they mean a
"constant current even if the load resistance is varying". Basically, more information/clarification is needed.

Rohith_elec said:

Zia is correct....

Click to expand...

You and Zia have fun then. You want to make a 120A current source and you think your biggest problem is how to control a stepper motor?! Why do you even think you need a stepper motor?

sky_123 said:

The bottom one says 0.5A? Is your one 120A?
And what are you trying to adjust? The voltage setting on it? Because normally when people say "current source" they mean a
"constant current even if the load resistance is varying". Basically, more information/clarification is needed.

Click to expand...

My instrument is having a pot for adjusting the current and another pot for adjusting the voltage , yes mine is a 120A one.

godfreyl said:

You and Zia have fun then. You want to make a 120A current source and you think your biggest problem is how to control a stepper motor?! Why do you even think you need a stepper motor?

Click to expand...

Dear godfreyl,
not to make a 120A and controlling a stepper , but i dont know how to make a feedback mechanism ...

The current adjustment merely sets a 'max current allowed' value on most PSUs.
So I'm guessing you're adjusting the 'voltage' pot. This means do we now need to disregard everything
concerning "current source" in your posts? ("Current source" means a constant current in the face of a
changing load).
Please confirm.

yes , it delivers a constant current with changing load, for that another probe is also there to detect the drop in current, but we dont have to worry about that since once the pot is set , that current only will go through it.

- - - Updated - - -

yes, its almost like the below link

http://www.aplab.com/product-services/power-suppliesprdouct-a-services/switch-mode-dcdc-converters.html

- - - Updated - - -

see the amperage, 150A
but mine is a linear one

I think I have to give up too, I still don't really understand how this PSU is delivering a constant current or how the probe works, but clearly you do... in short, I think the earlier posts should give you at least some ideas.