I bought a dozen types of bi-polar stepper motors (from different vendors) with rated voltages ranging from 4v to 12v and currents from 2A to 0.3A. I hooked them up to Pololu A4988 driver according to the minimal wiring diagram, and I am unable to get anywhere near the rated torque on all motors except those with rated voltages 10v and higher. My setup:
Power supply: Agilent E3610A set to output 12V (current limited to 2A)
MCU: Teensy 3.1
Stepper Driver: Pololu A4988
All is wired up together using a breadboard. (Could this be a problem?)
Now, for steppers like this one http://www.amazon.com/dp/B00O5AHAMC/?tag=stackoverfl08-20 with rated voltage of 4.2v and current 2A - no matter what I do with current limiting pot - I simply cant get the motors draw more than a fraction of their rated currents. Its a single turn pot - so I fall off the adjustment space before current (measured at power source AND motor coil) reaches any appreciable amount (usually it maxes out around 0.1-0.2A). Torque too is very low, naturally. I tried voltages 8-17V (the range limited by the driver board and my power supply) - but still, cant get the motors produce more torque.
Interestingly, it is not a problem with steppers rated 10-12v, for instance CanaKit stepper (http://www.amazon.com/CanaKit-Stepper-Motor-with-Cable/dp/B004G51AZ4/ref=sr_1_1?ie=UTF8&qid=1435458950&sr=8-1&keywords=canakit+stepper) rated at 0.33A and 12v - I can get them draw more current, although even in that case - there is a huge mismatch between the current they ought to draw based on VREF pin reading, and what they actually draw. For instance, measuring the VREF (~1v)and applying the formula - I should have gotten 2.5A (or slightly less due to 70% energization) - but I really measure only 0.29A on the coils.
Everywhere I read, it looks like I should not have this problem. What am I missing? Should I try a different driver, lower voltage, perhaps?
You can’t necessarily believe your measurements. Because the motor driver acts like a switching power supply, the current is constantly changing. Consequently, you cannot get an easily interpretable measurement of the current using a multimeter.
You need a noninvasive method of measuring the current. Otherwise, carefully follow the detailed instructions provided by the Pololu engineers for setting the current limit, here: youtu.be/89BHS9hfSUk
Finally, for the highest possible torque, use a high voltage power supply (30 V).
You can’t necessarily believe your measurements.
I measure the current using the AMP probe on the scope (the one that clamps around the wire w/o having to breaking the circuit). I set the motor to low speed (1 micro-step per second in 1/16th stepping mode) - and can see a very clear picture of the usual sine-wave (somewhat broken in 8 places) with 30 seconds between maximum and minimum current values (around any one of the coils). For the life of me - I cant force the 2.3 ohm Mineba motor draw more than 0.6A per winding at its peak (note, the supply at 12v measures only 0.1A). I verified the probe I’m using with known currents - and it appears to work fine.
Is this a proper way of measuring winding current, or do I also have to take into account the PWM duty cycle? I.e. plotting voltage on one of the leads of that that 2.3ohm motor with a scope (against supply ground) I see 45-60kHz PWM, but the “busyest” cycle is only 50% complete - I dont see the duty cycle slowly going from 25% to 75% like I do with one of my other motors (2.8 ohm, 10v - which also exhibits great torque)
BTW, and a bit on a tangent - when I reach the ‘singularity’ point at which the pot completes the circle and just before I turn it a bit more - things get totally weird - motor starts making irregular rattling noises, power supply goes into current over-protection mode (set at 2A), and its voltage drops to like 8 or 9v. While no smoke is observed, I try quickly getting out of this position either by continuing to turn in the same direction or turning back until things settle down. Also, merely touching the pot with a metal screwdriver makes it scream. What is happening there?
I don’t know what you mean by an “AMP” probe, but you may be on the right track for noninvasive measurements. Are you certain that the probe is correctly calibrated and works for DC current? (Many people try to use a multimeter on the current setting, and that doesn’t necessarily work as expected.)
When the current limit is set to 1.0 amps, the full step current per winding (when the motor is NOT stepping) should average about 0.7 amps, as per the A4988 data sheet, but it undergoes continuous sawtooth oscillations.
The pot resistance element is not continuous so you are risking damage to the motor and the driver by turning the wiper outside its range.
The probe I use is one of those: keysight.com/en/pd-2329609-p … =US&lc=eng it attaches to a scope, which lets me measure and plot the currents amon other things. Its hard to mess up the metering process with the scope. To be sure, I also verified it against known current sources.
I watched the current limiting video and read just about everything I could find on the web. Still a bit of a mystery to me as to why the current is not being drawn. My VRef is reading nearly 1V, which means the current is limited to about 2A - but, as I said, the 2.8 ohm motor is drawing at most 0.6A
I have everythingt wired up through a breadboard … Do you think this might be a problem? (I.e. breadboard not letting more than 0.6A through, but then at least I’d expect breadboard to get toast pretty quick - but it aint).
Regarding the pot - thanks for the heads up. I never turn the pot outside its range ‘intentionally’ - its just its hard to know where the limit is until electronics go hysterical.
[quote]My VRef is reading nearly 1V, which means the current is limited to about 2A[/quote]The internal thermal current limiting of the A4988 comes on pretty quickly, so you are unlikely to get more than about 1 amp in any case. Motor controller data sheets have a reputation for being overoptimistic. The DRV8825 will probably give you 1.5 amps.
Breadboards are not intended to carry more than a few hundred mA, and the contacts can burn out. I suggest to use direct wiring and solder all the connections.
If you really need the full rated torque of a 2.8 ohm motor, you may have to use an industrial motor driver. The Gecko G251X is a good value. geckodrive.com/geckodrive-st … g251x.html
