Microstepper resolution

[arie_l]

Posted on January 13, 2004 at 14:41

Microstepper resolution

[arie_l]

Posted on May 17, 2011 at 10:16

Greetings,

I curently designing an application of control of laser beam with microstepper mottor. Required resolution for this app is 204800 microsteps per revolution (1024 usteps in 1.8 degree step). I consider of using L6208, but I dont find any approval in application notes or datasheet about that kind of resolution. I wonder if L6208 is suitable for such application? If not are there any replacement that does? Thank you Best regards

[gaetano]

Posted on May 17, 2011 at 10:16

Hello,

with the L6208 you aren't able to drive a stepper motor in microstepping mode. Neverthless, you could use an additional microcontoller with almost two 11 bit (or more) PWM timers to provide two sinusoidal profiles at the inputs of the L6208. This approach allows to drive the microstepper with the wanted resolution you asked for. Let me know if you are interested to this methodology. Regards, Gaetano

[arie_l]

Posted on May 17, 2011 at 10:16

Thank you for your reply

I understand that the driver must be controlled with extern logic (in case of this project it performed by 2channel 12 bit D2A controlled by FPGA, that does other things as well), I just wanted to be sure that L6208 itself does not limit the resolution for some reason (like S/N of internal comporators or anything like that). Thank you for your help [ This message was edited by: AryeLL on 12-01-2004 13:25 ]

[gaetano]

Posted on May 17, 2011 at 10:16

Well,

now all is more clear! What I suggest you to do, is to give me some specifics or needs that the driver has to satisfy to be compliant with your app. In this way I can contact the right people inside ST to have all the info that can help you. Regards, Tanio

[arie_l]

Posted on May 17, 2011 at 10:16

Greetings,

The application we designing is industrial, the driver suppose to control motor that drives laser beam. The motor used for this application is 2SQ-022-BA34 from DYNASYN. Voltage supply is 12V. This motor used in previous design with same resolution and custom build driver (using 2 PA26 amp and D2A). Current consumption per each phase is approx 0.6 A. Mechanical load is neglectable. Theoretically 10 bit D2A would be enougth, but I dont trust his accuracy so I will use 12 or 14 bit D2A. I tried to receive the approval from local ST representatives, but they are not very cooperative here. Thank you very much.

[gaetano]

Posted on May 17, 2011 at 10:16

Hello,

I contacted an application engineer that works on this driver. As soon as possible, he will reply to you specifiyng all the limits that you could face with your high resolution needs. By the way, he explained me that for your resolution needs some limitations could be present, due to the strategy used by the driver to realize the 'constant current control' regulation. Regards, Tanio

[vincenzo1]

Posted on May 17, 2011 at 10:16

Microstepping technique consists in positioning the stepper motor rotor in an intermediate positions between two ''full step'' positions.

The ability of this positioning is given by providing two different current intensities to the two windings of the motor. 1) L6208 and L6228 are able to separately control the two currents in the two windings; the current regulation is made through fixed-off time PWM controllers. Such controllers are able to control the PEAK value of the current and not (directly) its r.m.s. value (that is what affects rotor position): the winding current is a triangular shaped waveform with its peak value set by the external reference voltage (i.e. provided by a DAC). The current r.m.s. value is given by Irms=Ipk*(Ipk-di)+di*di/3, where Ipk is the peak value set by the L62X8 PWM controller and di is the current ripple. So varying with SINEWAVE profile the reference voltage of the L62X8 (microstepping generally requires sine-wave currents) we obtain a rms current which is NOT sinusoidal. The rms value distortion effect obviously depends on the current ripple... Especially with high resolution microstepping (such in this case, of course) this phenomenon is not negligible. 2) The comparator used by the PWM circuitry has an internal offset. This causes errors especially at low current values. 3) PWM circuitry has a MINIMUM on time. This may introduce errors trying to regulate low current values in particular with high L/R motor windings (see attached AN1451). Conclusion: L62X8 introduces errors on the rms current value. These errors are negligible with low resolution microstepping but, for high resolution control, the microcontroller (or FPGA) must take into account the current profile distortion and provide to the IC adequate voltage levels. Regards Vincenzo Marano

[arie_l]

Posted on May 17, 2011 at 10:16

Thank you for your advice

We probably gonna stick to L6 Thank you very much. You've been great help [ This message was edited by: AryeLL on 14-01-2004 14:21 ]