How does MID-7604 measure peak current?

[Jthomas426]

What are the details about how the MID-7604 measures and regulates motor winding current?

We manufacture motion systems incorporating small step motors. One of our customers has ordered a motion system with the MID-7604. This is our first experience with the MID-7604.

When duplicating motor performance comparable to our other Step-motor Controllers, we must set the MID-7604 to 3X the motor's traditional rated peak current. For example, for a super-small MicroMo/Arsape Step motor, we traditionally limit the current to 250mA (RMS). However, with the MID-7604, we must raise the current limit to 700mA (peak) to achieve the same performance.

While we're not sure about the current regulation philosophy of our other Step-motor Controllers, the Step-motor Manufacturer's Specification Sheet limits the current to 250mA per phase with both phases energized to prevent overheating. Pumping in 700mA from the MID-7604 to achieve comparable performance just seems excessive. However, we're just playing with numbers here and not knowing what they mean. In the end, we're ignoring the numbers and instead, measuring motor torque and winding temperature in order to set the peak current switches on the MID-7604.

* How can we think of the current setting for the MID-7604?

* Is it not a PWM device that using width of voltage pulses to produce the current?

* What does "PEAK CURRENT" and "RMS CURRENT" mean?

[BullDozier]

Hey JThomas,

The peak current is the highest possible current that the drive could surge to under a given setting.  The RMS current is the quadratric mean of the current which is basically an average of the magnitude. 

One particular reason that you may have to increase the current is that if the stepper motor is a 6 wire stepper motor and you have been connecting it up such that only half of the coils are being used (parallel) but now it is being connecting such that the full coil is being used (series) then it would take exactly twice as much current to fully power the motor than before. 

If you could provide me with some additional details what kind of stepper motor these motors are (4 wire, 5 wire, 6 wire, 8 wire), then I could possibly come up with some other troubleshooting steps for us to try and figure this out.  Also, the manual for this drive can be found here, it provides many specs that might help answer some of your other questions.

Take Care,

[Jthomas426]

Evan,

Thanks for getting involved.

The "peanut" step motor is a simple 2-phase (4 wire) device (MicroMo AM1524 in current mode).

With 12.5-ohm windings, it is overly rated at 250mA (both windings energized) by MicroMo.

Over 15 year, we have built thousands of this motor into our Linear Actuators (www.diamondmotion.com). And, we've easily overheated many of them through excessive current.

We normally use our own proprietary Controller and set the two-phase current to 175mA to assure low probability of overheating. Even with this constriction, we do not permit the motor to operate for more 4.3 minutes at 6000 full-steps per second.

Because one of our customers requested the MID-7604, we're attempting to understand the differences between our tradtional Controller and the MID-7604. For example, to attain the same performance, we must increase the MID-7604's peak current up to 700mA (RMS 470mA). This emotionally scares us because we definitely know how to lose motor through overheating. However, by instrumenting the motor with a temperature probe, we know that the motor temperature at 700mA is similar to the motor temperature caused by our Controller operating at 175mA. As you can also notice, 700mA is substantially higher than 175mA!

Now, our Controller has some mysteries since it is supplied to us by an outside contractor -- who is having medical problems right now and is not accessible.

SUMMARY

1. MicroMo says 250mA (both windings energized simultaneously).

We know 175mA is a comfortable setting (using our controller).

The MID-7604 requires the 700mA setting to produce comparable results.

Do you see our concern?

We're trying to learn something.

[Jthomas426]

Message Edited by Jthomas426 on 12-19-2007 07:15 PM

[Jthomas426]

Message Edited by Jthomas426 on 12-19-2007 07:15 PM

[Jthomas426]

Message Edited by Jthomas426 on 12-19-2007 07:15 PM

[Jthomas426]

TO      Evan

FROM   Jthomas426

As an example of what we're trying to understand:

Just now (12/19/07 @ 5:30 pm), after sucessfully operating the AM1524 Step motor at 256 microsteps, 700mA Peak Current -- to learn something, we changed the step-configuration to 128 microsteps . . . and promptly destroyed the motor by overheating - poof! We didn't expect a change in the heating effect by simply reducing the number of subdivisions to a full-step.

We want to understand the configuration interrelationships so our customers don't destroy motors by overheating.

Is there a change in heating effects as a function of full-step subdivision -- microstep number?

[Jthomas426]

TO         Evan

FROM   Jthomas426

OVERVIEW   As we're getting more familiar with the MID-7604, we may think that we did one thing, while another caused the problem. In my previous posting, we thought that changing the microstepping division had fried our motor -- but, it was not the microstep setting. Instead, it was stopping the motor with a Motion Assitant "HALT" function instead of a "KILL" function. This morning, after instrumenting a test motor with thermal sensor and current measurement, we found that the HALT does not significantly reduce the motor current -- it was the non-moving rotor with high current that quickly destroyed the motor by overheating.

REMAINING CURRENT QUESTION  While we will eventually determine (through experimentation) the current settings that are safe for our Linear Actuators, we still don't understand the large difference between National Instrument's specified currents and our other controllers' current settings that closely match the performance specified by the motor's original manfacture.

[BullDozier]

Hey JThomas,

I'm sorry to hear that you smoked a stepper motor.There are a couple of reasons that your MID-7604 drive could appear to be operating strange. 

There is a series of DIP switches on the front of the MID drive that allow you to set various things.  One of those DIP switches is the current reduction switch.  This switch is default in the ON position from the factory I believe so if it has never been turned off, chances are it's ON.  When the Current Reduction switch is active, the stepper drive drops the current energizing the coils by 50% if there hasn't been a step in the last 500 ms.  If you were gauging the strength of the drive based off of holding torque, then this could be the reason you were raising the current to a higher rating than you thought was neccesary.  This settings is for customers who have an application with stepper motors that focus more on moving torque instead of holding torque.

Another possible reason for the increased current to match performance of your drives is if the bus voltage of your drives is some voltage other than 24 VDC.  If your drives that you are used to using are say 36V drives, then at a lower current setting, they would provide greater torque at higher speeds because of the higher voltage being able to energize the coils faster.

Also, the KILL feature releases the axis from being actively driven.  The HALT command simply locks the motor while leaving the coils energized to hold the position that it is currently in. 

I hope some of this has helped you out.  Let me know if I can elaborate on anything.

Thanks,