Stepper Motor, simple RPM controller needed - Page 1
Forum section: electronics / projects

author: HighVoltage
author_rank: Super Contributor
timestamp: August 12, 2014, 10:51:25 am
content: I need to get a stepper motor to run at a fixed 500 RPM and I am looking for a very simple speed controller without any programming.What I want to use:- BIPOLAR STEPPER MOTOR- 1.5A per phase- 1.8 ° step angle- Not much torque needed, just a fixed RPMI see lots of controller on ebay but I have no experience with them.May be someone here has tried a few controllers.Thanks for any recommendations.

author: Artraze
author_rank: Contributor
timestamp: August 12, 2014, 07:15:23 pm
content: The A4988 modules (probably what you saw on ebay) are pretty solid and the price can't really be beat. They handle all the stepper control and driving for you, so all you need to provide is a step-clock and a direction signal. The step clock will dictate your RPM, so just generate a square wave at the right frequency (via 555, microcontroller, etc) and you're all set.As far as setting up the A4988, details are pretty widely available (hereis a good page), so I'll skip the general topics.For your application, I'd recommend turning the current limit down to under 1A or lower if you're getting enough torque for the motor to not miss steps. At the motor's rated 1.5A it'll run hot and the A4988 will need cooling.You'll probably want to run at 24V if possible (mind that the A4988 needs 8V - 35V). Without looking at your motor's datasheet I can't know for sure, but usually inductance starts to limit the motor current around those speeds so you'll likely need the voltage to push the current through. Don't worry if your motor has a lower voltage given; that's really just the current rating times the DC resistance. The A4988 will keep the current within spec so your motor will be fine.Lastly, I'd suggest setting microstepping to half steps to start. If you are unfamiliar, the quick version is that microstepping uses the current PWM to smoothly position the motor between full steps by stetting sin + cos of the total current limit on each phase. The A4988 uses a fixed off time PWM with a off period of <~40us, so if you microstep step too quickly (~25kHz) you'll advance the target currents before the PWM has a chance to settle on the previous value. Since you need 1.6kHz (full) stepping, if you set the microstepping too high (e.g. 16x) that could easily be a problem. Half steps should be a nice balance between smoothing out motion and keeping the pulse rate reasonable, but you'll probably want to try different settings to see what works best.If you use half steps and a 1.8° stepping motor, your step clock should be 500rpm/60 * 360/1.8 * 2 = 3333Hz.Hopefully that helps get you started.

author: sacherjj
author_rank: Frequent Contributor
timestamp: August 12, 2014, 07:44:56 pm
content: Make sure you get a controller with a heat sink, or capability to add one. Driving the A4988 over 1A will need additional cooling, but you can go up to 2A with the heatsink.

author: GeoffS
author_rank: Unknown
timestamp: August 13, 2014, 02:25:45 am
content: A better option might bethis one.It's based on the Ti DRV8825 which can handle higher current and voltage and is not as prone to overheating as the A4498.

author: HighVoltage
author_rank: Super Contributor
timestamp: August 13, 2014, 10:27:11 pm
content: Thank you all for your input.I have ordered some parts and will test a few different solutions and then report back of what worked best

author: rob77
author_rank: Super Contributor
timestamp: August 13, 2014, 10:34:49 pm
content: if you are after a very simple and cheap solution, then you can use a 555 clock generator followed by a 74194 bi-directional shift register (if you need to change direction) and a simple dual H bridge.if you need a very precise RPM then use a Xtal oscillator with a divider instead of 555.

author: SL4P
author_rank: Super Contributor
timestamp: August 14, 2014, 12:31:00 am
content: Just curious... no criticism.If you need a steady fixed RPM, why using a stepper?Wouldn't a simple DC motor (with optional feedback) be easier, cheaper and smoother?Notch it up a step, add PWM, and imnproved starting torque.... ?

author: rob77
author_rank: Super Contributor
timestamp: August 14, 2014, 01:28:43 am
content: Quote from: SL4P on August 14, 2014, 12:31:00 amdefinitelyi would suggest a brush-less DC or brushed DC motor with appropriate controller with feedback and probably with PLL to lock the RPM to a Xtal oscillator if i would be asked to design a constant RPM drive

author: HighVoltage
author_rank: Super Contributor
timestamp: August 14, 2014, 08:06:05 am
content: Quote from: SL4P on August 14, 2014, 12:31:00 amThis is for a client project and he has already the stepper motors and fitting shafts, mounting plates and gear wheels.It was his request.Yes, I agree, a brush-less DC Motor would probably be much smoother and takes a lot of less current.May be I can convince him later.BTW, I ordered all the parts on ebay from HongKong and China.It is kind of astonishing, how cheap they are, many of them with FREE shipping.