Re: [CAD_CAM_EDM_DRO] Bridgett update

Jon,

Thanks for the idea to try out on the encoders. I will put a pull on them
and see what happens. I will be careful to not use too low a value as the
Gecko have a limited current capacity for the encoders (50 ma if I remember
correctly).

Actually the reduction in resolution is a good thing. I ordered the 250 line
units on purpose. I could have gotten 500 line units at the same cost, but
choose not to. At 500 line (2000 count) per rev I was very to need to go
with a G340 and use the pulse multiplier to get the rapid rate I wanted.
With the 500 line encoder on the X and Y of Bridgett my calculated step
increment is .00005" which is well beyond the resolution the iron is capable
of. With the 250 line encoders the step distance is now .0001 which is a
little more realistic and cut the required pulse train in half. It actually
worked out well on the X axis as the servo with the factory encoder was too
long to fit on the machine. I would have to mount the motor from the other
direction and make up spacers and use long bolts to hang the motor on and
then have to make a custom belt cover. With the US Digital encoder I
shortend the motor by 2-1/4" and got the clearance I needed to mount the
motor in the stock manner.

Tim
[Denver, CO]


-----Original Message-----

>
>> First a warning, this is about big iron and fast rapids so if that is
>> offensive to you or generally intimidating please skip reading this post.
>>
>> I just bolted a big old e-bay special NEMA 42 brushed servo motor onto

the X

>> axis of Bridgett (Bridgeport series II). Connected it to a Gecko G320 and
>> used a 62 VDC power supply. Rapids are rock solid at 150 ipm!! I know,

many

>> of you are tired of hearing about the Gecko drives, but every time I hook
>> one of the step and direction servo units to a machine I am blown away

with

>> the performance improvement from a stepper. All I can say is try it,

you'll

>> like it.
>>
>> An interesting story on the E-bay servo motors. Then came with some real
>> nice 500 line encoders with ball bearing shafts and one of those real

slick

>> factory mounts to the servos using a helical coupler. When I hooked them

to

>> a scope they seemed to be working fine. I then hooked the motors to the

G320

>> and fired it up and the servos would stabilize and then jump suddenly

then

>> stabilize again. Motion was the same way, smooth, then some jumps and
>> lurches and then smooth. I pulled the encoders and used some duct tape to
>> attach one of the Electronic Goldmine mini-servo motors with 1000 line
>> encoder to the shaft where the original encoder was. Wired it up and

turned

>> it on. The servo stabilized just fine (considering how loose the

connection

>> was to the encoder) and motion was very smooth. Diagnosis was that the
>> encoders that looked good on the scope have a loose connection somewhere

in

>> the electronics. I then ordered a set of US Digital E2 encoders with a

250

>> line resolution and they bolted on very nicely with just a tiny bit of
>> handiwork on the holes of the mounting ears. Fired up the drive and it is
>> stable as can be and performing great. The encoders cost me as much as

the

>> servo motors, but for $90 including the encoder I don't know where I

could

>> have gotten a servo suitable for a series II any cheaper (or get another

one

>> for that matter).
>
>More likely it is an electrical interference problem, from the PWM output

pulses

>of the Gecko drive getting into the encoder inputs. Probably a carefully

selected

>pullup resistor from +5 V to the encoder signals will clean this up. But,

you want

>to make sure you really have it cleaned up, as you will have the same sort

of

>misery as the people who are losing step pulses at random. I suspect the
>encoders that came with the motors have open-collector outputs, and need a
>fairly stiff pull-up to avoid any noise pulses getting accepted by the

Gecko drive.

>Anyway, I hate to see you having to REDUCE resolution by 2:1.
>
>Jon
>FAQ: http://www.ktmarketing.com/faq.html
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