Author Topic: "sprung linear potentiometer" (Read 7768 times)

Simon · « **on:** June 30, 2015, 08:41:19 am »

I've been asked to apply speed control to a DC brushed motor for a sewing machine. the original pedal is literally a bunch of power resistors in series and the foot button pushed on a series of copper strips connected to the "taps" so the more you push the more resistors are bypassed. Naturally it's in poor shape the cooper strips have bent and there is little speed variation left. What I could do with is a sprung pot that I can put under the foot button (that has it's own spring) that is pushed down and then returns to the top under it's own spring when the foot pedal button is lifted. does such a pot exist for sensible money ?

rolycat · « **Reply #1 on:** June 30, 2015, 09:18:18 am »

How about this?

The throttle assembly is £13, and the company also has a spring loaded plunger pot for £24:

LukeW · « **Reply #2 on:** June 30, 2015, 09:35:41 am »

Perhaps one of these may suit.

http://www.digikey.com.au/product-detail/en/3046L-3-502/3046L-3-502-ND/3781595

http://www.digikey.com.au/product-detail/en/404R10KL1.0/987-1025-ND/2408603

http://www.conrad.de/ce/de/product/453518/Positionsgeber-Mono-10-k-TT-Electronics-AB-LM-10-3M29-10-KR-1-St/?ref=detview1&rt=detview1&rb=1

http://au.rs-online.com/web/p/potentiometers/0317780/

They do seem very expensive sometimes... perhaps a spring, a rack and pinion gear and a conventional cheap rotary pot could work.

Simon · « **Reply #3 on:** July 01, 2015, 06:37:05 am »

Expensive indeed, just because it's the MD does not mean he wants to spend any money

. I might try a rotary pot with an arm on it and a spring so that the spring keeps it up and the existing button with it's own return spring pushes down on the arm. At meast i can get mechanical bits made in the machine shop

pickle9000 · « **Reply #4 on:** July 01, 2015, 07:07:44 am »

Quote from: Simon on July 01, 2015, 06:37:05 am

Expensive indeed, just because it's the MD does not mean he wants to spend any money . I might try a rotary pot with an arm on it and a spring so that the spring keeps it up and the existing button with it's own return spring pushes down on the arm. At meast i can get mechanical bits made in the machine shop

A torsion spring on the pot shaft anchored to the body and arm works well. Then you place a small roller on the arm and have that touch the base of the pedal. Issues to watch out for are slop in the wiper, the need to use a large value pot, noise generated by the pot (small travel). This method allows you to use any case. I have also used a linkage to increase pot rotation. A linkage on a sliding pot will work but the sliding pots never seem to have good enough durability.

Simon · « **Reply #5 on:** July 01, 2015, 07:26:37 am »

Well All i need is a bracket to hold the pot, an Arm I can reliably fix to the pot shaft with a partial hole in the bottom to accommodate the spring that pushes on the base to make it return up. If I'm lucky there is a box of springs around or one can easily be obtained.

BradC · « **Reply #6 on:** July 01, 2015, 07:40:05 am »

Just go and buy a second hand rheostat pedal from almost any variable speed sewing machine from the 60's & 70's. They were/are as common as mud around here.

Another option might be a heavy duty slot-car hand control rheostat. Just a big wirewound with a slider.

Simon · « **Reply #7 on:** July 01, 2015, 07:41:12 am »

Quote from: BradC on July 01, 2015, 07:40:05 am

Another option might be a heavy duty slot-car hand control rheostat. Just a big wirewound with a slider.

oh, yes come to think of it i have a few of those around

German_EE · « **Reply #8 on:** July 01, 2015, 10:23:59 am »

I would be tempted to head along to a local music shop and see if they have any scrap guitar pedals. Normally these get thrown because the electronics have died but all you need are the mechanics and the potentiometer.

Psi · « **Reply #9 on:** July 01, 2015, 10:30:15 am »

Maybe you can find some cheap foot pedels from a PC joystick car racing wheel.
They just have a 10k pot inside and a spring.

Ian.M · « **Reply #10 on:** July 01, 2015, 11:43:32 am »

An experienced sewing machine operator can 'creep' the machine forward one stitch at a time using the pedal without even thinking about it.

If you don't reproduce the original 'feel' of the pedal, the boss's wife (or other female relative) is not going to be happy. That results in an unhappy boss and the lack of joy will be passed down the chain to you.

Retrofitting the original pedal with an electronic controller while retaining the feel may be difficult due to lack of space and the need to replace the resistor stack with a spring to keep the force the same.

Unless your company wants to move into the sewing machine accessories market, I'd go with BradC's suggestion and get a compatible replacement of the same general style, identical if possible off EBAY or one of the many sewing machine spares companies.

Simon · « **Reply #11 on:** July 01, 2015, 11:50:49 am »

Well the pedal button is sprung to the pedal case independantly of the input component. At the moment the original feel is on/off so anything will be a guge improvement. He decided to use a 3 speed motor and possibly 3 speed switch so that the button is just a switch but thats not good in my opinion so I'll trying an replicate it as it once was.

Ian.M · « **Reply #12 on:** July 01, 2015, 12:24:04 pm »

Sounds like it might be a Singer Sewing Motor Controller.

They are very common and fully rebuildable though you would need a scrap one of the same voltage rating to scavenge spare carbon disks from to rebuild the variable carbon pile resistor.

Simon · « **Reply #13 on:** July 01, 2015, 01:33:40 pm »

This one is far more crude

dentaku · « **Reply #14 on:** July 01, 2015, 01:41:30 pm »

The most interesting way I've seen a foot pedal work is in an old Foredom drill I have in the basement.
It has a trimpot with a slot through the middle and a twisted rod that goes in it so as the rod moves through the slot it turns the pot.
https://www.eevblog.com/forum/beginners/is-this-a-custom-pot-in-this-drill-speed-controller/

Simon · « **Reply #15 on:** July 01, 2015, 01:44:10 pm »

interesting idea

Ian.M · « **Reply #16 on:** July 01, 2015, 02:03:58 pm »

Yep, that's nasty. If the motor wattage is compatible, swapping in a Singer pedal might be a good option.

If not, I'd simply stick a strain gauge to the top spring contact, and get rid of the existing wiring and resistor banks to make space for some motor control electronics. You could also try some QTC composite material between the top and second spring contacts to get a much larger resistance change than a strain gauge would provide.

Anything else is going to need some serious mechanical design to restore the original 'feel' as the spring rate increases as more of the contact strips are deflected by the button.

If the whole system including the motor is being replaced, a small three phase induction motor with a VFD and a guitar pedal connected to the VFD's control pot input would probably be the best option from a reliability and future maintenance point of view, assuming there is somewhere the VFD can be mounted.

Simon · « **Reply #17 on:** July 01, 2015, 02:07:07 pm »

The original motor us brushed so I'm toying with the idea of driving it with 240VDC with PWM. Although it is assumed to be damaged and the MD has proposed a 24V motor that I can easily do a PWM control for. It's only a sewing machine so no need to go overboard with VFD.

dadler · « **Reply #18 on:** July 01, 2015, 08:40:44 pm »

In the guitar pedal world, we sometimes replace wah/volume pots with LDRs and then use the pedal to progressively occlude light from the LDR. Just an idea.

babysitter · « **Reply #19 on:** July 01, 2015, 08:43:53 pm »

Get a Light source and a LDR in it, place it so the LDR gets more light the further the pedal is pressed, the lower the resistance the faster the motor - wireless speed control !

Simon · « **Reply #20 on:** July 02, 2015, 06:43:51 am »

not a bad idea, I can use an led and LDR and put a "shutter on the bottom of the pedal so that the more it is pressed the lore light is cut off, nice one

max_torque · « **Reply #21 on:** July 02, 2015, 07:58:04 pm »

Just use a car accelerator pedal! All modern cars are "fly by wire" and have usually a set of potentiometers in the pedal assy to encode it's position.

ie:

http://www.ebay.co.uk/itm/FORD-MONDEO-Mk3-TDCI-00-07-THROTTLE-ACCELERATOR-PEDAL-WITH-SENSOR-2S719F836CA-/371360785570?pt=LH_DefaultDomain_3&hash=item5676d3e4a2

Simple, robust, and easy to interface too!


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Author Topic: "sprung linear potentiometer" (Read 7768 times)

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