Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work.

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Old September 4th 07, 02:41 PM posted to rec.crafts.metalworking
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Default servo motor control ??

I posted this note on comp.robotics.misc but of course it soon got
lost in all the crap that someone is posting to various groups.

So I thought I would give it a try here.


I am looking for a way to control vehicles somewhat larger than
typical hobby stuff. Say the size of a small lawn tractor.

This would be for steering so I am thinking about using something like
a more muscular servo motor.

I'm aware of hobby style giant servos. They might work but not sure if
they would be beefy enough.

Industrial servo motors seem to be quite different from hobby stuff. I
have one that's rated 128 VDC, has 5 wires running to the motor and a
bunch more off the attached encoder. No real idea what's going on
here.

Searches on servo motor control bring up mostly hobby information
which I am familiar with.

There is the open servo project which might provide part of a
solution. If I can beef up the control so that it can handle something
like a cordless drill motor.

I recenly built a remote control tricycle. I handled the steering
using a regular DC motor controller (Victor 883) and got around the
limit switch problem by building a small spring loaded clutch. (Some
details on my web site under "builders log".)

But I don't really like this solution as there is no automatic
centering feature.

Any other ideas? What do those guys like Mythbusters do for remote
control full size cars?

Thanks for any help.

DOC

Have robots. Will travel. http://www.robot-one.com


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Old September 4th 07, 03:10 PM posted to rec.crafts.metalworking
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Default servo motor control ??

For the motor itself, look at converting a DC motor to servo.
http://truetex.com/servomod.htm You can select any size motor you need
then. For your application this will work great.

lots of servo control packages out there. One option is to use Advanced
Motion Control analog servo amps and a Galil computer on a PCI slot. You'd
need a way to mount a PC inside your machine to go this route. (Only the
motherboard, galil card, and hard disk while machine runs - plug in keyboard
mouse and monitor to program) This would also give you a bunch of I/O
capability in addition. I'd shop eBay for these items.

Karl


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Old September 4th 07, 03:32 PM posted to rec.crafts.metalworking
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Posts: 43
Default servo motor control ??

On Sep 4, 8:41 am, wrote:
I posted this note on comp.robotics.misc but of course it soon got
lost in all the crap that someone is posting to various groups.

So I thought I would give it a try here.

I am looking for a way to control vehicles somewhat larger than
typical hobby stuff. Say the size of a small lawn tractor.

This would be for steering so I am thinking about using something like
a more muscular servo motor.

I'm aware of hobby style giant servos. They might work but not sure if
they would be beefy enough.

Industrial servo motors seem to be quite different from hobby stuff. I
have one that's rated 128 VDC, has 5 wires running to the motor and a
bunch more off the attached encoder. No real idea what's going on
here.

Searches on servo motor control bring up mostly hobby information
which I am familiar with.

There is the open servo project which might provide part of a
solution. If I can beef up the control so that it can handle something
like a cordless drill motor.

I recenly built a remote control tricycle. I handled the steering
using a regular DC motor controller (Victor 883) and got around the
limit switch problem by building a small spring loaded clutch. (Some
details on my web site under "builders log".)

But I don't really like this solution as there is no automatic
centering feature.

Any other ideas? What do those guys like Mythbusters do for remote
control full size cars?

Thanks for any help.

DOC

Have robots. Will travel. http://www.robot-one.com


I remember one Mythbusters thing that used a very simple system, that
I think would work fine IF the vehicle will not be going very fast.
The servo merely moved fairly high current switches hooked to a
reversible DC motor hooked to the steering shaft. The motor was a
little slow, so steering response was not "sports car steering", but
for a vehicle at walking speeds or so, it should work fine.

Many years ago at work we were building a demo for a "moon rover"
project that used a similar thing, but that was an unstable vehicle
and it was a REAL handful, even at walking speeds, trying to drive it
up ANY hill, 'cause it would spin around and head downhill, but as
long as it is at least a three-wheel vehicle, it should be okay (that
moon rover as a unicycle!).

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Old September 4th 07, 04:19 PM posted to rec.crafts.metalworking
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Posts: 1,620
Default servo motor control ??

wrote:
I posted this note on comp.robotics.misc but of course it soon got
lost in all the crap that someone is posting to various groups.

So I thought I would give it a try here.


I am looking for a way to control vehicles somewhat larger than
typical hobby stuff. Say the size of a small lawn tractor.

This would be for steering so I am thinking about using something like
a more muscular servo motor.

I'm aware of hobby style giant servos. They might work but not sure if
they would be beefy enough.

Industrial servo motors seem to be quite different from hobby stuff. I
have one that's rated 128 VDC, has 5 wires running to the motor and a
bunch more off the attached encoder. No real idea what's going on
here.

Searches on servo motor control bring up mostly hobby information
which I am familiar with.

There is the open servo project which might provide part of a
solution. If I can beef up the control so that it can handle something
like a cordless drill motor.

I recenly built a remote control tricycle. I handled the steering
using a regular DC motor controller (Victor 883) and got around the
limit switch problem by building a small spring loaded clutch. (Some
details on my web site under "builders log".)

But I don't really like this solution as there is no automatic
centering feature.

Any other ideas? What do those guys like Mythbusters do for remote
control full size cars?

Thanks for any help.

DOC

Have robots. Will travel.
http://www.robot-one.com

Searching on "servo drive" may help. There seem to be a lot of
stand-alone servo drives that run a brushless motor with an encoder, and
take step and direction inputs just like a stepper driver would. They
wrap the motor with a PID loop so you have nice position control of the
motor. To get the steering centered you can put a flag of some sort on
your steering for a center reference, then once you find center you can
just count pulses off of that.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html


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Old September 4th 07, 04:28 PM posted to rec.crafts.metalworking
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Default servo motor control ??

Tim Wescott fired this volley in
:

Where does one find a "power steering valve" -- is this a stock
hydraulic supply house item?


Northern Tools, among others, sells the whole rig-up, valve, cylinders,
etc. I think I've seen them in the JC Whitney catalog, too.

LLoyd
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Old September 5th 07, 02:44 AM posted to rec.crafts.metalworking
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Posts: 2,600
Default servo motor control ??

According to :
I posted this note on comp.robotics.misc but of course it soon got
lost in all the crap that someone is posting to various groups.

So I thought I would give it a try here.


[ ... ]

I'm aware of hobby style giant servos. They might work but not sure if
they would be beefy enough.

Industrial servo motors seem to be quite different from hobby stuff. I
have one that's rated 128 VDC, has 5 wires running to the motor and a
bunch more off the attached encoder. No real idea what's going on
here.


O.K. There are two major classes of industrial servo motors.
The DC ones (which I believe that you have), and the AC ones. I'll skip
the AC ones, because I don't really know enough about them.

The DC ones, however, are typically permanent magnet field, with
a pair of wires going to the commutator to get power into what is
typically a basket of wires woven in the shape of a rotor without iron.
There may be stationary iron inside that, but it does not rotate, so the
inertial mass of the rotor is at a minimum.

In addition, there is either a separate set of similar (though
perhaps lighter gauge) windings connected through a second commutator to
a second pair of wires, or (sometimes) simply a second set of brushes at
90 degrees to the pair which bring power into the motor.

Those extra wires are the tachometer (tach) feedback. The motor
generates a voltage proportional to its speed and feeds out those extra
wires.

The motor is driven through a high-power amplifier, with a DC
voltage generated by the amplifier to drive the motor, while the output
of the tach generator is summed with an input command voltage. As long
as the motor's speed (and thus the output of the tach) is below the
input speed command voltage, the amplifier outputs quite a bit of power.
It produces less as the two get closer together in value, and when they
are identical it produces just enough output to maintain the speed.

If the command voltage is very low (for example 0.0001V when the
amplifier is configured to give maximum RPM at 10V input), the motor
will be turning so slowly that you won't even notice it -- unless you
put some tape on the motor's shaft as a flag -- then you can see it
moving very slowly.

Note that if the command voltage is below zero, the motor will
turn the other direction.

So -- you can select whatever speed you want.

The encoder can help tell a computer what the shaft angle
happens to be at the moment -- and there is often an index encoder which
produces a signal only once per revolution.

[ ... ]

But I don't really like this solution as there is no automatic
centering feature.


For this -- you need some kind of position encoder -- either
limit switches, or a potentiometer whose output is proportional to the
angle of the steering. These servo motors are intended to rotate
leadscrews at precise speeds -- not to return to a zero center as model
aircraft servos do.

Any other ideas? What do those guys like Mythbusters do for remote
control full size cars?


The serious industrial servo motors, with a computer to control
them, I suspect.

Enjoy,
DoN.
--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---


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