On 10/18/20 10:07 AM, John Dammeyer wrote:
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
These issues were resolved long time ago. Philips office computers from
1960s, and 1970s used DC motors and clutches for moving folded paper for
printing and hard paper with magnetic stripe edges for data RW. There
was no current in the motors when at standstill. Relays either turned it
on or off.
I remember replacing worn out clutches as part of regular maintenance.
Imagine cleaning dust from the clutches etc. All electronics was made
with discrete components, no ICs. Logically, if I remember correctly,
there was a simple position encoder with incandescent bulb that needed
to be replaced on schedule.
It's much easier to design modern replacement for similar mechanical
movements or situations. As already stated in this thread, it has been
done so why reinvent it? CNC software should absolutely not need to
bother with this kind of details IMO. Product design overkill kills it.
By the time you reinvent the wheel, you could make money to pay for the
components bought from fine advertisers in Digital Machinist magazine or
such.
--
Rafael Skodlar
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