> From: Jon Elson [mailto:el...@pico-systems.com] > On 10/18/2020 03:57 AM, Chris Albertson wrote: > > The above is not the way it works. No one would design > > anything like that. But let's say you did. What would > > happen? The motor would vibrate and slowly rotate and have > > close to zero torque > The Geckodrives G-320 series, for one, does this. A number > of other PWM servo drives from the well-known manufacturers > such as Servo Dynamics, Copley, AMC and others do the same. > The advantage is there's no dead zone around the null > point. The disadvantage is a lot of power dissipation in > the transistors and motor. Many of these drives require a > series inductor in the motor wires to control the > triangle-wave current. > > Yes, the motor has zero torque at the null, that is by > design. As soon as the following error is non-zero, the PWM > duty cycle shifts from 50-50 to an asymmetric ratio, and the > torque increases. That's the whole point of a servo system. > > Jon
I thought all servo motors did it that way. I can see they could also remove the PWM from the windings completely for 0 power but if there's even the slightest bit of load on the motor the closed loop would cause a correction to hold it in place resulting in essentially the same behavior? Also, the servo systems measure the current through the windings. If the motor isn't turning then the back EMF from the current changing in the winding is pretty small and the motor reaches full current pretty quickly. Then the circuits generally start chopping the waveform based on either a hard clock signal or just based on the LRC of the windings. If the motor is rated at 3A for example, and the applied voltage is 100V the current will build up to 3A pretty quickly. Hence the need for that extra inductance to just slow it down a tad so the measuring circuit can see the current and the circuit can remove the drive voltage. Now the current has to decay in the windings and if the say CW drive signal is still asserted it will then re-apply the 100V holding the current at 3A. This will continue throughout the CW time of the normal PWM drive rate (say 15kHz to 25 kHz). Then if the motor is stopped, when the PWM signal to the drivers has reached the 50% point it has a small dead band time for the devices to switch off, and then enables the CCW drivers. Now once again, the 100V (in effect -100V) is applied by the other half of the H-Bridge to the motor and the same chopping action occurs. Although the HP_UHU driver doesn't do this there is nothing to prevent a processor controlling all this to change the current detection from 3A down to 0.5A if it hasn't see a request for motion for a period of time. Or not even switch to a different comparator. Just change to a different 50% duty cycle. For example, CW for 5%, OFF for 45%, CCW for 5%, OFF for 45%. Rinse and repeat. It's still a 20kHz PWM which is a period of 50uS. So CW for 2.5uS, OFF for 22.5uS, CCW for 2.5uS, OFF for 22.5uS. The current limiting feature still comes into effect if the current increases past the example 3A but the overall average current for the entire 50uS period has dropped to 10%. Now the motor doesn't heat up as much but is still locked. I don't know if the HP_UHU with Henrik's module does that. I'll ask him. John Dammeyer _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
