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Offline bw

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Servo sizing help
« on: March 19, 2011, 09:19:39 PM »
I am wondering if someone might be able to help sizing a servo motor. 

I am hooking up a 4th axis using and older Haas 5c indexer.  I am running the Ajax cnc setup for Mach and need to use a brushed DC servo.
I have 117 VDC in the cabinet already but can add another power source if necessary.

The indexer takes 72 revolutions of the drive shaft to go 360 degrees
I already have a 5:1 gearbox and can use either nema 23 or 34 frame or just make my own to fit if needed.
What I don’t know is what size motor or RPM I need to get the proper torque and speed?


Any help would be appreciated.

Offline Hood

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Re: Servo sizing help
« Reply #1 on: March 20, 2011, 05:21:45 AM »
The way I worked out what the min torque I would need  for my lathe was to mount a bar  2m long to the end of the ball screw by the middle of the bar. I then used a load  of weights and added them to one end of the bar until I got movement then weighed the weights. That way I was able to work out what Nm I needed.
 Reason I used a 2m bar was so that the bar fixed by the centre would balance itself and thus only the weights would have any influence. You could use a shorter bar and just calculate on the length. Likewise if you wanted to use Ft pounds it would be a similar process.  Instead of weights you could use a spring balance scale to pull the end of the bar, I used weights as I needed a big torque and didnt have a balance big enough but with your gearing I would imagine the torque required will be quite small.
Speed is easy enought to work out as any motor you purchase should have a V/1000rpm constant so with your 117v supply you will be able to work out what speed you will get when you take into account the reductions.
Oh and one last thing, when you are looking for a motor be carefull as often the hobby oriented venders give you a peak torque rating, you really want to use the constant rating.
Hood
« Last Edit: March 20, 2011, 05:24:40 AM by Hood »
Re: Servo sizing help
« Reply #2 on: March 20, 2011, 05:26:25 AM »
Is there any way of telling what size motor was originally fitted?

ATB
derek
You can "chop it off" but can't "chop it on"

Offline bw

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Re: Servo sizing help
« Reply #3 on: March 20, 2011, 07:09:07 AM »
Thanks
The indexer takes nothing at all to move without the motor on it, I can take the minute degree wheel connected to the worm drive and just touch it with my finger and get movement. I will also confess to not bring able to wrap my head around the power rpm calculations for the servos powers and speed.  Many of the servos I see have a max voltage of 90 VDC, many also are 24VDC, if I hook up the 117 to it I can’t see how it won’t burn it up, or do I need to reduce or add another power supply to create the proper RPM’s?

I should have also said that it originally came with a stepper motor.  A Slo-Syn M092-FD09, I was able to find some info on it.
This is all that’s on the motor.
2.5 VDC
4.8 Amps
200 steps /rev

I found this info doing a search on the motor.
Max holding torque
(Unipolar) 300 oz-in 
(Bipolar) 370 oz-in
(Rotor inertia) 0.0174 oz-in

It also had
Minimum holding torque
2 on 300 oz
1 on 180 oz

I am usually pretty good at picking up on things like this but I am having a heck of a time because I don’t full understand it all.

Offline Hood

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Re: Servo sizing help
« Reply #4 on: March 20, 2011, 07:28:24 AM »
If your power supply is 117v DC then you will need a drive and motor that is capable of handling these voltages. Normally what you put into a drive will be the max you can take out but in  reality its slightly less. If you are going for a lower volt Drive/Motor then you will need a power supply to match.
Speed of servos is directly related to the voltage supplied, for example if your motor had a rating of 10v/1000rpm then putting 5v in would result in 500rpm, 20v would be 2000rpm. It wont be exact but will be close.

Hood
Re: Servo sizing help
« Reply #5 on: March 20, 2011, 08:46:55 AM »
Just wondering why you need to use a brushed servo?

I have a 8Nm stepper on my 4th axis, it's quick and hasn't missed a beat.

ATB
Derek
You can "chop it off" but can't "chop it on"

Offline Hood

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Re: Servo sizing help
« Reply #6 on: March 20, 2011, 08:52:53 AM »
Derek, I am thinking the reason will be the Ajax control only outputs analogue signals rather than Step/Dir. Having said that I think there may be stepper drives available that take analogue input, they are likely expensive though.
Hood

Offline bw

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Re: Servo sizing help
« Reply #7 on: March 20, 2011, 09:14:02 AM »
You are correct Hood,
It is a closed loop brushed dc servo system.  If I could use the stepper I would not be having this problem. 

Offline Hood

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Re: Servo sizing help
« Reply #8 on: March 20, 2011, 09:17:39 AM »
I am thinking as the original was s stepper then the motor RPM would likely have been around 1000rpm max so if you look for a servo and drive that needs 117DC or more then you will be set as even a 160V servo running 110V will likely be faster than your stepper would have been.
Hood
Re: Servo sizing help
« Reply #9 on: March 20, 2011, 10:07:23 AM »
I see ;D

This might be some use - I was thinking of using the YAPSC when I upgraded my Anilam crusader 2 lathe control. Eventually I decided to fit  Viper 200 drives and fit new encoders to the brushed motors.

http://www.cnczone.com/forums/showthread.php?t=74532&goto=newpost
The thread is called "YAPSC:10V : The glue between Step/dir and +/-10V servo amp "

I have no idea how well it works, but would mean you could use a modern step/dir servo drive or even a stepper drive.

ATB
Derek.
You can "chop it off" but can't "chop it on"