Newbie- Permanent Magnet DC Motors as Servos?

[gregestov]

Hi All,

Over the last 5 years or so, I have made a number of small 3 Axis routers using stepper motors, but I'm now in the process of designing a larger machine that I'd like to run with servos. At the moment, I know almost nothing about servos and servo systems, but I'm enjoying the learning process!

Recently I found a Taiwanese website selling permanent magnet DC motors, and I am wondering if these would be suitable for use as servos? I understand that an encoder would need to be attached to the motors (which the manufacturers say can be done), but would the motors still be unsuitable? Here is a link to the motors I've been looking at, and any expert opinions would be greatly appreciated: http://www.ginre.com.tw/Page/Page2-1_1E.htm

Thanks all, and thanks also for the great information I have found already in other threads!

Greg

[Al_The_Man]

Those may perform well as servo's, the quality difference between a general purpose DC brushed motor and a DC servo motor is in the construction, a DC servo has a higher pole count, has a skewed rotor laminations, can have from 4 to 8 brushes and high grade permanent magnet field material.
Most of the above characteristics are important for direct drive situations.
More information on the specifications of the motors indicated would be necessary to do a comparison.
Al.

[gregestov]

Thanks Al, you seem very well versed with servo motors/systems. I have got a great deal of information from some of your other posts already. I will try to get some more information from these guys and report back.

I have had trouble finding a good source of servo motors, especially being in Australia. I know Keling do servos, but it's much cheaper for us Aussies to ship things from Asia rather than the US (being practically an unofficial part of Asia ourselves).

Apart from Keling, I have also had a look at the Baldor range of servos, and a number of other brands well out of my price range. Do you, or anyone else reading, know of any other companies I may look into for cheapish/good quality servos and planetary reduction gearboxes? I'm sure this information is in these forums multiple times, but a direct question to those who know what they're talking about doesn't hurt sometimes!

[Al_The_Man]

There are some coming out of the ROC now, but I don't know what the quality is like.
Some of the companies are advertising here in the New Products forums.
Al.

[epineh]

Hi Greg, yes it is a pain to get servo's here in oz, at least without having to sell the house.

I am currently retrofitting a lathe that I will need large servo's for, I will be trying to convert a C40 generator/starter, they are still available for agricultural use, mainly tractors I guess. They have nice large commutators with small segments and are about the $100.00 dollar mark. I don't know of their suitability but I have a guinea pig that I am in the process of trying out.

They may require some work as they use a seperate field winding instead of permanent magnets, I am going to try using different field strengths at first to find an optimum setting (a simple PWM generator should do the trick)

I will let you know how it goes, or have a look for my lathe thread to keep updated.

Otherwise, I see you are in Sydney, check out any metal recyclers, the ones that specialise in machinery, you would be surprised what people throw out, worth a shot.

Cheers.

Russell.

[Al_The_Man]

Originally Posted by epineh They may require some work as they use a seperate field winding instead of permanent magnets, I am going to try using different field strengths at first to find an optimum setting (a simple PWM generator should do the trick) .

Wound field DC motors were used initially for servo's and were replaced by the more popular PM type, generally they have a larger frame size to accommodate the field windings.
The lower the field current, the higher the rpm possible at the cost of lower torque.
One thing to be aware of is if you get field loss when running, the motor runs away.
Drives for these are usually fitted with field loss detection.
Al.

[epineh]

Hi Al, that will be something to look out for, though wouldn't the controller pick it up through the encoder as well ? I guess it is a matter of what damage it does before the controller shuts it down.

I was thinking if I ever get time in my life that I could monitor the servo rpm and adjust the field strength to suit with a micro, completely independant of the controller, this way I could have good torque at low rpm and fast rapids when required, but would this make tuning impossible ?

All theorietical of course, I don't get a great deal of spare time

Russell.

[Al_The_Man]

For servo's it would be a little hairy, maybe impractical.
This type of motor was continued to be used in spindle applications, and field weakening was typically used to obtain the upper end rpm.
Al.

[epineh]

Fair enough, I may just do a set and forget for the field strength.

Russell.