Newbie- Power supply switch

[ellasaro]

Hi,

I've run some tests con my stepper with a L297 and 298 and it seems to work just fine except for some considerations.

a) If I make the stepper turn with a hi voltage it seems to make too much noise...un less I turn down de supply voltage or turn up the speed. The signal I read is also pretty noisy when the frequency and the voltage don't "get along".

b) I will be using 3 steppers so does this mean I will be aplying the supply voltage to them at all times? even when I don't actually need all 3 running? Can I use a transistor as a switch to cut the supply voltage when needed?

Thankx

[doorknob]

Originally Posted by ellasaro b) I will be using 3 steppers so does this mean I will be aplying the supply voltage to them at all times? even when I don't actually need all 3 running? Can I use a transistor as a switch to cut the supply voltage when needed?

Instead of cutting the supply voltage to a motor, wouldn't it be better to set the ENABLE pin of the L297 low to disable the motor?

[ellasaro]

If I set the ENABLE pin low will the stepper stop "squealing". I'll try it and get back to you.

I saw a stepper driver that had a MOSPEC right next to the L298 but couldn't figure out what it did.

Is there any usual rol for a MOSPEC in a stepper driver?

[wildwestpat]

It is probable that you are experiencing stepper motor resonance. You must have a load connected to the motor's shaft or the load is connected via a very springy coupling. Stepper motors will require a current from the drive electronics when not being driven step by step to lock the shaft. There is a case for fitting mechanical dampers but all other sources of poor running excessive noise should be attended to first. Stepper motors make a noise by virtue of the way they work.


Hope this helps - Regards - Pat

PS the important thing is to make sure you have enough torque available to accelerate the mechanism so that you do not loose steps due to the motor stalling.

[ellasaro]

How do I know if the noise is excesive?

Any ideas on the MOSPEC?

[doorknob]

Originally Posted by ellasaro How do I know if the noise is excesive? Any ideas on the MOSPEC?

Maybe take a video recording with sound and upload it to YouTube, then post the link here, and people can tell you if they think the noise is excessive.

I do not know precisely what you mean by "MOSPEC".

When I look up "MOSPEC" on google it shows that MOSPEC is the name of a semiconductor company, however it looks like they manufacture a variety of semiconductor devices, including rectifiers and bridges and power transistors of various types. So I don't think that is enough information to identify the function of the device that you saw on the board.

I know that some stepper drivers have built-in idle motor detection and current reduction, however I am not familiar with drivers that actually shut down the power to the motors when the motors are idle. But then I only have limited experience with stepper drivers, so I do not know whether or not other drivers may do that.

Did you get any results by disabling the ENABLE signal of the L297?

[ellasaro]

The part is MOSPEC TIP 137, COMPLEMENTARY SILICON POWER
DARLINGTON TRANSISTOR.

I haven't tried disabling the ENABLE yet. I soon as I do it I'll post my results.

In a few days I'll try and post the video with the steppers not moving but being connected to 30V anyway.

[wildwestpat]

The driver electronics for a stepper motor is usually arranged to keep the windings energised as this keeps the spindle locked at that step position. The more sophisticated drivers reduce the current in the windings when no step instruction is received after a pre-set period. This is to reduce the temperature rise in the windings whilst still keeping the rotor in position last set. In any servo system it is necessary to ensure the position is kept under control. Turning off the power to the windings will be the equivalent of starting up from cold when the power is next applied as you may have lost register - this is particularly important for an open loop servo system as there is no compensation for drift as the position is set by the number of steps sent NOT by the actual position achieved. Turning off the power would enable the motor to be turned away from the last known step and hence step count when resumed may not be correct.

It is almost certain the noise is due to resonance particularly if you are trying to bench test those motors without connecting them to a mechanical load. Use a large flywheel with a friction brake if you can not use the actual machine.

The noise of the motor when running normally is a uniform buzz related to rpm which changes to an angry sounding buzz cum rattle this is probably your squealing noise as it is quite unpleasant. The motors also buzz when stationary and may require a load to allow them to move if not accelerated gently from rest. The most significant thing is the rotor is not keeping step with the demanded positions. Starting with a low voltage the rotor should advance step for step demanded in single step mode. The frequency of the steps can then be increased and the rotor should keep pace step for step. Eventually the rotor will fail to track the demanded step rate and the motor stalls and makes a very unhappy noise. The higher the voltage the sharper the increase in current into the winding and the snappier the impulse applied to the rotor by the magnetism in the windings / coils. It is this magnetic impulse being applied to the rotor which is responsible for the buzz as the magnetic circuit is the equivalent of a spring. This is why adding inertia to the rotor i.e. connecting a load that includes both friction and inertia will calm down the impulse nature of the drive and permit it to step in a controlled fashion.

The reason for using higher voltages than the voltage rating for the motor is because the electronics is used to chop the voltage applied to the winding on and off. The switching semiconductors turn OFF when the preset current is reached and turn back on again under the infulence of a digital circuit. The function of the digital circuit is to shape the waveform applied to the motor to be smooth and as near to a sine wave as possible. The digital switching is all done in the chips and an external current sensing resistor is used to set the maximum current rating of the winding so that the finding is not used as a rather expensive fuse! The higher voltage supply used just means the faster the current builds to the maximum in the winding during each digitally controlled pulse and hence the rotor power is increased as more pulses can be fired for each step command into the winding. The extra pulses also raise the resonant frequency in the magnetic circuit which is good as it helps push the resonance up to a higher speed (rpm)

When you have the stepper motors connected to the load you will be limited to the maximum speed and acceleration that can be achieved. Then you might need to look at the use of devices to control / dampen any remaining resonance.

Hope this explains what you are seeing and hearing. Regards - Pat