L6470: Several problems evaluating a Stepper-Motor Application

eGDY2CVw4jwV · ‎2012-11-08

Posted on November 09, 2012 at 03:39

Hello,

We are currently evaluating the L6470 to drive analytic high-pressure pumps for Mass spectrometry.

The final goal is to drive a stepper-motor in high precision micro-stepping mode at around 1.7 Nm at a maximum Speed of around 100 rpm.

For evaluation we used a small bipolar stepper-motor from a previous test with the following parameters:

Model: Trinamic QSH4218-51-10-049 (Short Specs attached)
Rated Phase Current: 1 A
Phase Current: 5 ?
Phase Inductance: 8 mH
Measured K_e: 0.02041 (attached file: BMF Parameters)

As motor driver we used the EVAL6470H board attached to a STEVAL-PCC009V1 USB-SPI Interface in combination with the dSPIN Evaluation Tool 1.7.

Further Parameters:

Supply Voltage: 30 V

At our tests we tried to do some simple spin-up and down tasks on the motor with a blank axis and getting some experiences with the motor-driver.

But we faced several problems at this point:

The Motor accelerated at different rates (we tested 2000, 1000, 500, 100 Steps/s�) to different speeds.
We tried to get it to as high as about 2000 Steps/s but it always stalled without any external force at about 1500 Steps/s (1/128 micro-stepping).
The ''Command Error'' Flag very often was set after setting parameters with the Evaluation Tool.

Here our Questions:

What could be the reasons the motor stalls at this stepprate and how could we avoid it in a ''real world'' application?
Why was the ''Command Error'' set? Is there any possibility of diagnosis? (Cables where really short, the one that was shipped with the STEVAL-PCC009V1, about 10 cm)

We really looking forward for your answers and thank you for your efforts in advance!

Sincerely yours

#stepper-motor-driver #stepper-motor #l6470

Enrico Poli · ‎2012-11-14

Posted on November 14, 2012 at 17:08

Hello,

The stall issue is probably related to a resonance condition. When the motor is unloded the mid-point resonance are heavily stimulated, in many cases adding a small load at the motor shaft dramatically reduce the resonances allowing the motor to run correctly.

You can also check the effectiveness of the BEMF compensation setup. You should consider that the BEMF compensation tool is based on an approximated model of the stepper motor, so it could not returns the optimal configuration.

You can read the following AN for further details about the optimization of the voltage mode driving algorithm:

http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/APPLICATION_NOTE/DM00061093.pdf

The communication error issue should be related to an attempt to write some configuration registers when it is not allowed (motor running or bridges enabled). Please consider that the configuration tool always write all the configuration register, so in order to avoid the communication error you need to switch the device in high impedance status.

Regards

Enrico