chopper doubt

[mardus]

If I have 200 steps/rev stepper and want to drive it 1000 rev/min (16 rev/sec) ...... it is about 3200 pulses/ sec. Right?
Now, I want lets say 10 microsteps and now I have 32000 pulses/sec.

My question:
What is a minimal chopper frequency to drive it? Chopper freq. has to be higher enough then step freq. (mabye I am wrong....just thinking).

Anybody willing to explain?

[pastera]

The chopping frequency needs to be above the audio range only to avoid noise for the operator.
As far as microstepping at 1000 RPM, you would want to switch to a full step mode long before that speed to increase available torque.

Aaron

[kreutz]

In micro-stepping the PWM (chopping action) will try to control the current in both coils following a sinusoidal reference (90 degrees phase angle apart for both channels)

Each requested step will change only the reference voltage for each PWM comparator.

As Pastera said, at high rpms, the sinusoidal current is no more sinusoidal although the reference continues to be one due to the L/R constant of the coil limits the rate of current change. At such speed the motor will move smoothly in full step mode, so most designers change the reference waveform to make it equal to a full step current waveform, when certain speed is reached, in order to keep the torque at maximum. Micro-stepping advantage is at low speeds because the movement is smoother compared to full or half step.

[mardus]

Pastera and Kreutz, you don’t know how these information’s is useful for me!
Thank you very much ! Now I will do some experiments at home.

Tnx again!

[James Newton]

If, and this is a big if, your motors are medium to small in terms of watts (volts * amps) then you may be better off to skip the chopper and use an old school linear driver. This avoids the need to switch modes and gives you better power over the entire speed range.

A linear driver will heat more and requires a big heat sink on the controller but will keep the motors cooler and at full power, where a chopper driver will not heat (and can therefor manage higher wattage motors) but WILL heat the motors.

The higher the PWM frequency from the chopper, the more power is lost to eddy currents, RF transmission and inductance in the motor coils. All of that means more motor heat and less power. As the motors get bigger and are rated for more power, they are more able to dissapate the heat and have torque to spare even after the losses, so choppers work better for high power applications. Linear drivers can work better for low power applications.

And yes, I may be biased, since I sell an (open source) linear driver kit.

[jeffs555]

This avoids the need to switch modes and gives you better power over the entire speed range.

There is nothing about a linear drive that "avoids the need to switch modes". Linear or chopper, if you are microstepping, you are applying a pseudo-sine wave. This makes for a smoother drive at low rpm, but does not give as much torque as full stepping with a square wave. At higher rpm's, there is not much difference in smoothness between microstepping and full stepping, but there is a significant increase in torque with full stepping. That is why higher end drives switch to full step at higher rpm's. I believe it would be possible for your firmware to do this, but don't think it currently does.

[James Newton]

Hi Jeff, so nice to hear from you again.

Because the choppers are producing a PWM the motors have to smooth that into a sine wave in thier coils. The linistepper is producing a smoother signal, not PWM'd, so the motors do not loose power while microstepping due to high frequency losses in the coils. As a result, the faster speeds have more power.

Some old linear designs don't actively regulate the current provided to the coil, so they can take time to fire up a coil at high speed and so loose steps. The linistepper activly regulates the coil current so that isn't as much of a problem.

But please, Jeff, don't take my word for it. Go hook up a small motor to a Linistepper and to a chopper that does not switch to full step and see which one drops steps first as you increase the speed. Or you could ask Posix who found that out for himself with his step mill.

[mardus]

... Very interesting approach James. Could you explain what does it mean "medium to small" watts ( by figures, or currents... ) ?
I am old, and if I build old designs, everybody will laugh to me (just kidding)! You know what? I will try it as soon as I find some time.
Now I am asking: At what full steps/sec will be clever to switch from microstepping to full step mode?

Regards to Jeff and James

[kreutz]

Over-heating stepper motor problems due to chopping frequency related losses can be virtually eliminated by borrowing a simple technique from switching power supply designs, i.e., by placing a properly designed low-pass L-C filter across the output and using this L to control the Chopper. This removes the high frequency AC chopping losses in the motor by providing it with almost pure DC current. It also confines the EMI-causing, high frequency AC components to within the driver where they are easier to handle.

Take a look at this

[bunalmis]

Originally Posted by mardus What is a minimal chopper frequency to drive it? Chopper freq. has to be higher enough then step freq. (mabye I am wrong....just thinking). Anybody willing to explain?

Chopping frequency must higher then step frequency If you dont want the distortion of coil current waveform. Current distortion produce micro position error.

http://www.cnczone.com/forums/showthread.php?t=21014

[pminmo]

Originally Posted by bunalmis Chopping frequency must higher then step frequency If you dont want the distortion of coil current waveform. Current distortion produce micro position error. http://www.cnczone.com/forums/showthread.php?t=21014

Bunalmis I think you need to reread the thread. Specifically this one post: http://www.cnczone.com/forums/showpo...80&postcount=5

The term frequency with a chopper, is a misnomer. When the microstepping drive sees a step pulse, the current event that starts is based on the new set of phase currents. Most chips are designed that if the phase is already overcurrent for the new currents, the chopper turns the driver off just as it would when it exceeds the reference current while stationary.

[bunalmis]

I understand you but you dont understand me.

If your stepping frequency over then chopping frequency driver name is not microstepper.

Try with xy pen positioner.

mikrostep = 10

Try stepping frequency << chopping frequency and move from x=0 y=0 to x=10 y=10

After try this

Stepping frequency > chopping frequency and move from x=0 y=0 to x=10 y=10

Drawing is it same?

Dont ask to me screw pitch.