Re: [CAD_CAM_EDM_DRO] New to servos, please help/advise?

volitan712003 wrote:

>Hello group my name is David and I've lurked here for quite some time.
>Thanks to all of you that post answers and ideas - you have been a
>great resource for all of us newbies.
>
>Can someone help me with a few things (power supply/encoder)?
>
>My mill is pictured in the "Volitan" folder in the "Photos" section.
>I'm guessing the table weighs around 150 lbs (I'll know for sure soon)
>. I have 3 of the following servos:
>
>---------------------------
>Reliance Electric DC Brushed Servo Motors, Model E712
>37.51 Oz-In/A
>25.0-30.5 V/KRpm
>Motor Terminal Resistance .75-1.02 Ohms @ 4A.
>Maximum Terminal Voltage 60V
>Maximum Continuous Speed 2100 RPM (No Load)
>Maximum Peak Torque 1500 Oz-In.
>Maximum Peak Current 44.5A
>Maximum Continuous Stall Torque At Max Motor Temp 300 Oz-In.
>Maximum Continuous Current 8.9A
>---------------------------------
>
>I have 5 TPI ballscrews, and plan on using Gecko 320's and encoders
>from us Digital. Rapid rates are not a concern as it's a Hobby
>machine. My thought is to use the third servo to drive the knee, and
>later on get a smaller one for the quill so I can use either/or
>depending on the job.
>
>Power supply amperage? I'm confused.
>
>3 geckos @ 20 amps max each = 60 amps. I've never seen anyone here
>recommend a 60 amp supply.
>
>

You don't need it. Unregulated supplies are capable of delivering greater
than the rated current of the transformer for short load peaks. The
capacitors
supply part of that peak current, and the transformer can handle momentary
overloads. Also, the Gecko drive, as a switching device, will only draw
20 A
when the motor is demanding BOTH maximum current simultaneously
with maximum voltage. This condition would only occur when the motor
was required to deliver (20 A worth of) peak torque at full speed.
At lower speeds, the voltage at the motor is less, and the Gecko is
essentially
a power in = power out device. If it needs 20 A at 10 V, and the power
supply
is 50 V, then it will demand an average current of only 4 Amps!

>3 servoes at max continuous - 26.7 amps. So I suppose it should be
>somewhere in between but I'm guessing because I don't really know what
>I'm talking about yet.
>
>

No. it is unlikely that you will ever run 8.9 A through the motors at
stall. That would be putting enormous strain on the motors, screws, etc.
A more reasonable average current might be 4 - 5 A, but because you
generally will not be applying heavy forces at high speeds, and it
should not
take much current to do the rapid traverses, the power supply current would
be more like 2 A per axis. You might double that for the knee, though.
So, an honest real-world average load on the power supply might be
8 A. If you get a 10 A unregulated power supply with sufficient filtering
capacitance, you should be fine. I tend to vastly overspecify filter
capacitors
for this reason, to handle the load peaks. Something like 100,000 uF is a
good goal to shoot for, but you will probably find half that will work very
well. Note that the system COULD draw a peak of 60 A for a few milliseconds
every once in a while.

>Power suply voltage?
>This will affect RPM's and not torque, correct?
>
>

With these motors, I'd go for 50 V minimum, and I think you could get
by with up to 72 V, despite the stated limit on the motor. There will be
some voltage drop in the Gecko drives, and that limit is probably a
long-term rating point, not a momentary limit. Unless these motors
are run near 2000 RPM, they will never see a voltage above 60.

>
>Encoder resolution. What are the pros/cons to teh different
>resolutions? I'd like the ability to thread mill if that means
>anything there.
>
>

More is generally better to get smooth interpolation. If you direct drive
the screws, you definitely want high resolution encoders. If you use
a belt reduction drive, then it is less of a problem. I have an encoder
resolution of 20,000 counts/inch in X and Y, and 40,000 in Z on my
mill. This is with 1000 cycle/rev encoders directly on the leadscrews,
5 TPI for X and Y, 10 TPI for Z. (1000 cycle/rev encoders deliver 4000
quadrature counts per rev.) An old rule of thumb for CNC was you needed
10 times the desired or displayed resolution from the encoders to keep
motion smooth. Thread milling is not very demanding on resolution, but
you want low backlash. Boring out holes with an end mill by orbiting the
X and Y axes is the place you really see poor resolution and backlash.
The backlash produces 4 "bumps" at the points where the axes reverse.
Low resolution can show up as a "stair-stepped" pattern on the walls of the
hole.

Jon