next topic »

[on:]

hi total newbie here:

i just got a phidgets stepper motor with the expectation that i can do something similar to this:



but when i went to the product page (http://www.phidgets.com/products.php?product_id=3319_0) it says 

warning: connecting the motor directly to a power supply will destroy the motor and void the warranty. If you want to check your motor make sure it is connected to a constant current / chopper drive controller.

and a quick google of "constant current/chopper drive controller" turned up something like this:



slightly overwhelming for someone new to electronics. :/

my question is, what exactly is the significance of a constant current/chopper drive controller? do i really need one?  can i limit the current with PWM or just a very big power resistor and just ignore the "constant current" thing?  (i don't need a whole lot of torque, the motor will be driving the fine control knob of a microscope)

also what exactly is a bipolar hybrid motor? do i drive it the same way i drive a bipolar motor?

[Reply #1 on:]

The point of a current controlled driver is to get enough torque at a wide(r) range of motor speeds without frying the motor. The supply voltage has to be high enough for this to work. For small steppers simple controllers aren't too expensive.

http://www.sparkfun.com/products/10267

You can control the average phase current with PWM, but you also have to measure it. That at least requires a sensing resistor and an opamp + a PWM generator. Some of that can be done in code, but is it really worth the trouble?

[Reply #2 on:]

The point of a current controlled driver is to get enough torque at a wide(r) range of motor speeds without frying the motor. The supply voltage has to be high enough for this to work. For small steppers simple controllers aren't too expensive.

http://www.sparkfun.com/products/10267

You can control the average phase current with PWM, but you also have to measure it. That at least requires a sensing resistor and an opamp + a PWM generator. Some of that can be done in code, but is it really worth the trouble?

thanks for the reply madworm

so basically i don't have to have current controlled driver if i don't intend to have the motor spin at uber fast speed right? 

btw  it's worth the trouble for me since i live in a poor remote 3rd world country where shipping is extremely expensive if i only order a small part here and there.

[Reply #3 on:]

Shipping costs bite us everywhere.

You don't have to use a special driver chip, you can use a normal dual H-bridge chip as well. But you don't get any form of motor protection that way and you need a power supply with less than 2.78V for that motor. Torque won't be optimal for anything other than very low speeds.

If you want to implement current control with PWM, you trade money vs. time and reliability. Essentially you have to implement a buck (step-down) regulator with constant current control. I don't know if the analog comparator of the avr chip is fast enough for that purpose. If that were the case you'd still need a current sensing resistor + opamp per motor phase. Also don't forget that the code for this purpose (analog comparator interrupt) would have to be 100% reliable. Any hiccup in there and you get smoke and need a new motor.

[Reply #4 on:]

A good example of a chopper controller is Polulo A4988 board http://www.pololu.com/catalog/product/1182

Runs from 8V to 35V at upto 2A/winding.