DC motor speed control for electro mechanical tremolo

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smilan

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Hi,
I'm in the middle of building an electro mechanical tremolo (the same consept of Dearmond 601).
I have here a small DC motor. I connected it to a VARIAC (with a rectifier) ​​and it works well, the variable voltage changes from 0.6V at the lowest speed to 2.6V at the highest speed (in this video you can see the voltage meter connected to the motor supply).
Now I have two Questions:
1. In order to reach a low rotational speed when starting the motor I need to give it a boost of voltage and then lower the voltage to set it on a low speed.
There's a way to make the motor start working at low speed?

2. What a speed controller unit should I get for this motor?
Here you can see and hear it in action:
https://youtu.be/ur4iABtcBzQ
 

ruffrecords

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There are two basic ways to control the speed of a permanent magnet dc motor. Both involve feeding is with a PWN signal to effective vary the average dc voltage the same as you do right now with the variac. Both methods involve determining the speed and using it in a feedback loop to set the speed to a desired value.

in one method, when the motor pulse is off, you measure the back emf which is proportional to the speed. In the second method you attach a disc with castellations around the  and you shine a light through them onto an opto-transistor. The frequency of the resultant pulses is directly proportional to the motor speed.

I don't know of any of the shelf dc motor speed controller.

Cheers

Ian
 

abbey road d enfer

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DC electric motors cannot operate stably below a certain rotational speed, that depends on many characteristics.
Applying servo techniques can improve performance in the instrinsic stability zone, and somewhat extend it, but not much. Adding inertia can be beneficial, at the detriment of acceleration. You may ultimarely try using an epicycloidal gearbox.
 

JohnRoberts

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Back in the 70s I did some work with speed control of small motors. We tooled up a tachometer light interrupter disc mounted to the motor pulley to generate a square wave when the teeth on the interrupter disc broke the light beam in an opto-isolator pair (LED/Photo transistor).

As has been shared these do not start reliably at low speed and it takes some effort to work well even at constant speed.

JR 
 

walter

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Hey this is pretty cool. I am working on a motor controller for a mechanical speedometer, so I might have some ideas. I use an Arduino Uno for the controller. The motor I have is from a printer and includes an optical encoder. This optical disk has 64 holes. For my slow speeds, I use the highest PWM setting and stop for a few milliseconds after every 3 holes. 40ms gives me the 1mph reading, 10ms gets 15mph. Use the MAP command to get the range. After 15mph, the code switches to run the speedometer with PWM.
I looked into the mechanical tremolo awhile ago, but don’t remember how it works. Are you using an optical sensor, and have holes or reflective dots on a spinning disk? If so, you could use that as the encoder depending on how many are on the disk. I just watched a video where someone had a half disk, that probably wouldn’t work as encoder.

I just looked up the DeArmond. That looks familiar, one of those came into the shop I worked at in Texas. It has the motor running at a fixed speed with a cone on the spindle. There is a wheel that rides on the spindle. Depending on where it is on the spindle turns faster or slower. That speed shakes a glass bowl with a transducer emersed in fluid, dish soap or some kind of oil? Can you use a bigger cone?
 

ruffrecords

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JohnRoberts said:
As has been shared these do not start reliably at low speed and it takes some effort to work well even at constant speed.

JR

Constant speed it not really hard. Every Dyno label printer made since the late 80s has one in it.

Cheers

Ian
 

Gus

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PWM works well with brushed motors like the older RC electric motor controllers from torque at low RPM. You do need a high current supply for this to work well with a low duty cycle slow speed pulses

Search something like DIY PWM (pulse width modulated) Brushed DC motor speed control. You should find 555 based to microcontroller stuff

There are small brushless motor and controllers for RC you could adapt. Do a search you will find things

The cone design of the real one is cool IMO one speed no electronic switching to add noise.

Look up Uncle Doug on you tube for a video about repairing a Dearmond there is a diy in it. Most simple DIY electronics will likely cause EMI and therefore possible audio to RF noise issues

 

JohnRoberts

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ruffrecords said:
Constant speed it not really hard. Every Dyno label printer made since the late 80s has one in it.

Cheers

Ian
But the OP doesn't want constant speed.

I have my dad's old notebook pages from measuring wow and flutter in Western Electric film camera motors back in the 30s. Its easier now than it was back then...

Back in the 70s I wasted some bench time trying to extract sync pulses from the cheap cassette deck PM motor brush spikes... I was not successful (apparently the spikes only express when motor is under load, when coasting, no spikes so not a very reliable tachometer).

JR 
 

abbey road d enfer

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Gus said:
PWM works well with brushed motors like the older RC electric motor controllers from torque at low RPM. You do need a high current supply for this to work well with a low duty cycle slow speed pulses
The significant advantage of PWM is that it is seen by the motor as a very low resistance source, and it's more efficient than a regulated PSU, but the actual performane in terms of low speed torque and stability is not better than a very low resistance source.
If it was me, I would use a step motor.
I've used this controller for a lubricating pump:
https://fr.banggood.com/DC-4V-6V-Stepper-Motor-Driver-Controller-Integrated-Board-2-phase-4-wire-Speed-Adjustable-with-Remote-Controller-p-1666601.html?gmcCountry=FR&currency=EUR&createTmp=1&utm_source=googleshopping&utm_medium=cpc_bgcs&utm_content=frank&utm_campaign=frank-ssc-frg-all-1101&ad_id=393874529223&gclid=Cj0KCQjwit_8BRCoARIsAIx3Rj6t0LIV3BkWtpX-8LWo9GsjPsC6WckRoiLlB-x_d9EAamz1IcPjGd0aAhlMEALw_wcB&cur_warehouse=CN

It gives 9 different speeds. Of course, it's not continuously variable.

I believe this one is:
https://fr.banggood.com/DC-4V-6V-Stepper-Motor-Driver-Controller-Integrated-Board-2-phase-4-wire-Speed-Adjustable-with-Remote-Controller-p-1666601.html?gmcCountry=FR&currency=EUR&createTmp=1&utm_source=googleshopping&utm_medium=cpc_bgcs&utm_content=frank&utm_campaign=frank-ssc-frg-all-1101&ad_id=393874529223&gclid=Cj0KCQjwit_8BRCoARIsAIx3Rj6t0LIV3BkWtpX-8LWo9GsjPsC6WckRoiLlB-x_d9EAamz1IcPjGd0aAhlMEALw_wcB&cur_warehouse=CN
 

JohnRoberts

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abbey road d enfer said:
The significant advantage of PWM is that it is seen by the motor as a very low resistance source, and it's more efficient than a regulated PSU, but the actual performane in terms of low speed torque and stability is not better than a very low resistance source.
If it was me, I would use a step motor.
I've used this controller for a lubricating pump:
https://fr.banggood.com/DC-4V-6V-Stepper-Motor-Driver-Controller-Integrated-Board-2-phase-4-wire-Speed-Adjustable-with-Remote-Controller-p-1666601.html?gmcCountry=FR&currency=EUR&createTmp=1&utm_source=googleshopping&utm_medium=cpc_bgcs&utm_content=frank&utm_campaign=frank-ssc-frg-all-1101&ad_id=393874529223&gclid=Cj0KCQjwit_8BRCoARIsAIx3Rj6t0LIV3BkWtpX-8LWo9GsjPsC6WckRoiLlB-x_d9EAamz1IcPjGd0aAhlMEALw_wcB&cur_warehouse=CN

It gives 9 different speeds. Of course, it's not continuously variable.

I believe this one is:
https://fr.banggood.com/DC-4V-6V-Stepper-Motor-Driver-Controller-Integrated-Board-2-phase-4-wire-Speed-Adjustable-with-Remote-Controller-p-1666601.html?gmcCountry=FR&currency=EUR&createTmp=1&utm_source=googleshopping&utm_medium=cpc_bgcs&utm_content=frank&utm_campaign=frank-ssc-frg-all-1101&ad_id=393874529223&gclid=Cj0KCQjwit_8BRCoARIsAIx3Rj6t0LIV3BkWtpX-8LWo9GsjPsC6WckRoiLlB-x_d9EAamz1IcPjGd0aAhlMEALw_wcB&cur_warehouse=CN
+1 indeed stepper motors provide precise low speed control, but require more complex drive electronics.

==

A popular technique used with simple PM motors is use of negative impedance convertors to improve speed vs load behavior.

JR
 

trobbins

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Why not just a simple ww pot and dc plugpack?  I can't see a reason for any speed control other than a dial marked 'speed' and your ear.
 

abbey road d enfer

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trobbins said:
Why not just a simple ww pot and dc plugpack?  I can't see a reason for any speed control other than a dial marked 'speed' and your ear.
This doesn't work at very low speed. The motor hiccups. This is because of solid friction; in order to overcome it, one must apply a higher voltage, then when the motor has started, it goes too fast, so the voltage needs to be reduced. Unfortunately, due to the nature of mechanics, any change in effort may result in stalling.
 

trobbins

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The OP said the motor could operate ok over the voltage range of interest - it was only the starting voltage that couldn't be at the low end.
 

abbey road d enfer

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trobbins said:
The OP said the motor could operate ok over the voltage range of interest - it was only the starting voltage that couldn't be at the low end.
In order to start at low speed settings, there should be either something that gives a starting pulse or a negative impedance driver. A simple rheostat in series is certainly not the solution.
 

Tubetec

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Ive tried a few small dc motor controllers off ebay,
They come in a few different kinds depending on what you need .
5-6 volts output seems like the minimum , maybe changing the motor to one with a more suitable rpm range at 6 volts could be a plan .

My interest was in vari-speeding my Watkins Mk2 copicat,  I wasnt happy with the results using a small cassette deck motor ,lack of inertia or a flywheel on the motor shaft made it jumpy speedwise . It also had that anoying habbit of not starting at a slow speed , however once its going it did become possible to get much slower than the required 'starting' speed . Just getting rid of the big synchronous AC motor did have other benefits as far as induced hum in the heads/recovery circuits etc . Being able to run the motor off the rectified 6.3 volt ac rail keeps things simple .
 

trobbins

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Could have an on/off switch with the tremelo speed pot, so the motor always started at the upper speed limit.  KISS.
 
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