Servo motors

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Date

05-12-2008

Re: Servo motors

Emimec

05-12-2008

Posted by Tony Jeffree on May 13, 2008, 4:50 am

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wrote:

>Lester Caine wrote:

>> The reason for looking at servo motors was to compare prices, but I am
>> finding that at this size of motor the stepper does seem to be
>> preferable? Once you can monitor it's position ;)

>
>Oh well - from £400 for a similar sized servo motor and I'm not sure if that
>includes in integral drive as against £75 for the stepper motor option
>complete - per channel ...

I hate to suggest it but...

http://www.cadcamcadcam.com/index.asp?PageAction=VIEWPROD&ProdID=62

Regards,
Tony

Posted by Wayne Weedon on May 13, 2008, 5:57 am

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Lester Caine wrote:

> The reason for looking at servo motors was to compare prices, but I am
> finding that at this size of motor the stepper does seem to be
> preferable? Once you can monitor it's position ;)
>
> Now the next step is perhaps glass scales on the bed in place of the
> encoders direct to the motor?
>

Lester. Take a look at the Italian motors supplied by motion control
products in Bournemouth. For small motors they can be quite
competitive. I use some 300w motors of this type on my Hardinge.

It's not really useful to compare steppers holding torque against a
servos peak torque. In my experience with larger machinery a servo
with much less peak torque than a steppers holding torque will have the
stepper for breakfast, dinner and tea!

MCP also supply the leadshine dc servo drive which is no longer a gecko
copy. It's better in fact.

Wayne.....

Posted by Tony Jeffree on May 13, 2008, 2:22 am

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On Mon, 12 May 2008 22:48:19 +0100, "Steve"

>> On Mon, 12 May 2008 21:35:03 +0100, Mark Rand
>>

>>>wrote:
>>>
>>>>when compared with the closed loop stepper motor set-up.
>>>
>>>
>>>Just being a little bit picky here:-
>>>
>>>A closed loop stepper motor setup would be quite rare and probably only be
>>>used by people called Gareth :-0
>>>
>>>Everyone else would use them open loop.
>>>
>>>
>>>Mark Rand
>>>RTFM

>>
>> Mark -
>>
>> Actually, Taig offer a closed loop stepper setup.
>>
>> Regards,
>> Tony

>
>Now when I asked about closed loop set ups using steppers, the response was
>that steppers are upto it and you had to use servos to implement closed
>loop...
>
>So much for "Power of the crowd"!
>
>Steve

See:

http://en.wikipedia.org/wiki/Servo

Quote: "Servomechanism, or servo, a device used to provide control of
a desired operation through the use of feedback."

So the Taig system does indeed use servos. But with stepper motors.

Next question?

Regards,
Tony

Posted by moray on May 13, 2008, 7:37 am

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> On Mon, 12 May 2008 21:35:03 +0100, Mark Rand
>

>>wrote:
>>

>>>when compared with the closed loop stepper motor set-up.

>>
>>
>>Just being a little bit picky here:-
>>
>>A closed loop stepper motor setup would be quite rare and probably only be
>>used by people called Gareth :-0
>>
>>Everyone else would use them open loop.
>>
>>
>>Mark Rand
>>RTFM

>
> Mark -
>
> Actually, Taig offer a closed loop stepper setup.
>

There has also been mention of a Servo Stepper driver on the Gecko forum at
cnczone.
I think a rough time scale for availability is later this year.

Posted by Richard on May 12, 2008, 12:58 pm

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>
>You're going wrong by confusing power and torque.
>
>Power is torque times RPM.
>

>Regards,
>Tony

That's sort of right conceptually, but just in case you are tempted to
actually try to calculate a 'real' power

Power (W) = torque (Nm) * omega (speed in rads/sec or RPM*Pi/30)

Richard

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