Cowboy Analog Servo Motor Modifications?

[BDAZ]

I don't get what the 2K pot in series is doing for you. unless you are using that as a fine adjustment once you have the 100K set to an approximate speed. I did a very crude test and figured on my system (remember it's also about pulley sizes) 60 SPM equates to around 60K Ohms. I like the idea of moving the pots off the motor and accessible without sticking one's hands past the belts and pulleys.

For those who are analog challenged here's a chart:

When I have the log pot installed I should get 50% of the knob rotation controlling the first 10 ohms, so I doubt the second pot is necessary. It would be with a linear pot.

Such a simple fix for a significant issue.

Cya!

Bob

[JoMama]

The problem is not with the multi position switch and there is no potentiometer on this motor.

The Speed is controlled by a linear hall sensor that varies the control voltage based on the strength of the magnetic field from the magnet in the black plastic arm. The problem is, the way the magnet is position in the arm, the field strength goes from zero to 100% with only a small movement of the arm.

Here's how you fix it.

1. Remove the left end cover and the screws that hold the electronics box under to motor. (let it hang on the wires)

2. Remove the screw (5mm hex key) and nut from the arm that holds the brake pad. (Brake pad already removed in picture)

3. Release the spring from the tab just below the brake pad.

4. Pry the arm off the shaft with an open end wrench or small cressent wrench that can stradle the shaft and spring. You may want to wedge a flat head screw driver into the slot and pry it open a bit, but too much force can break it, since it is only cast aluminum.

5. Remove the black plastic arm with the shaft and square magnet pressed in.

6. Use a Sharpie to mark the side of the magnet that faces out.

7. Either pry the magnet out with a small sharp screwdriver or drill a small hole through the plastic behind the magnet and push it out.

8. Using a Dremel or small sharp wood chisel, angle the bottom of the magent pocket down as shown in the picture.

9. Reinstall the magnet and glue in if necessary.

10. Reassemble, in reverse order paying attention to the clocking between the plastic arm and the brake arm.

I removed the brake pad completely, since I never run it that fast and like to be able to turn it by hand without stepping on the pedal. There is a stop tab that prevents the brake arm from contacting the wheel when no pad is installed. The control will now have more range of movement and be more linear. To get even more control, move the pedal linkage from that little tab that bolts to the top edge of the pedal to the side of the actual pedal. (you'll need to drill a hole) The closer it is to the pivot, the more control you will have. With the brake pad removed, you will have to preload the linkage when you tighten the clamp, or there will be a lot of deadband before it starts to spin.

Edited May 24, 2014 by JoMama

[BDAZ]

The speed control IS a 220K linear pot and changing the value to a 100K log pot WILL achieve the desired effect but your system may enhance the effect assuming there is a magnetic sensor on the motor.

Cya!

Bob

[JoMama]

Maybe it is has different electronics.

The knob on the front (circled in blue) is the speed LIMIT and only sets the maximum speed. On mine it is a multi-position rotary switch with detented positions, but it could just as well be a potentiometer which would be smooth, while turning.

The speed CONTROL comes form the linear hall sensor circeld in red and it varies the speed between zero and the maximum set by the knob, in relation to how far the foot pedal is pushed down.

Modifying the magnet angle increases the range of motion that the arm travels between zero and maximum (set by the knob) and makes it more controlable.

[BDAZ]

On mine it is a 220K pot connected to a connector on the board. The initial problem was setting the maximum speed more accurately since only a few degrees of rotation of the pot results in the full range of required top speed. So both mods should result in much better control of the motor.

Thanks!

Bob

[JoMama]

The magnet modification made the biggest difference on my machine. With the knob set to the maximum speed, I can still control the foot pedal to get anything between about 50spm to 1500spm. The only problen with that long, skinny, 2 pole motor design is that is does not as have much torque when run in the bottom 5-10% of its range. It is a motor designed to run at 3400 rpm and anything less than 340 RPM, they don't like and it can oherheat the windings, which is why I have a speed reducer on the way. That should take it down to 17-500spm of controlable speed.

[msdeluca]

Bdaz,

I'd be interested in a tutorial when you have this figured out to your liking.

Mike

[Constabulary]

Very interesting thread!

So no matter which servo motor you have it seems a speed reducer in addition is always your friend.

[BDAZ]

I am still waiting for my pots and connectors to arrive. The pot mod is mostly about controlling speed between 0 and 60 SPM. I have ordered connectors, leads and pots from China so I can test various components and produce a "plug and play" configuration.

I'm hoping to get my components this week.

Bob

[Trox]

Please let me know when the Chinese manage to make an modern sewing machine motor that actually works as it should.

For those who cannot afford a Efka or a Ho Sing; Try a 3 phase clutch with a Frequency converter (and a speed reducer if its a heavy stitcher). Thats the system with the worlds best low speed control; for sure. Its the cheapest options also, normally you get those old 3 phase clutch motors thrown after you. Its easy to find a used frequency control on EBay too. The wiring its easy, all you need then is a sewing machine and a single phase outlet.

Good luck

Tor

By the way nice topic!

Edited May 26, 2014 by Trox