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 Post subject: Re: Speed circuit
PostPosted: Jul Sat 15, 2017 3:44 pm 
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Oh ok so I could use that chip and feed it to the inverting input.

Hmmmmmmmm..........that would solve the issue of needing to rectify the tach signal.

But I do like your idea using the transistors.

Which would be the best choice?


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 Post subject: Re: Speed circuit
PostPosted: Jul Sat 15, 2017 5:24 pm 
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Sometimes the best choice is simply the one that works......

At this point, I'd like to see a summary of requirements and parameters....eg:
Motor voltage and current
Control range...i.e. How far off the speed can be and the circuit will still correct it
Speed accuracy required...I guessing it should be 1% or so.

I recommend specifying modules independently. I see three functions:
Frequency to voltage converter
Differential amplifier, including reference voltage
Motor driver ( possibly just the emitter-follower in my scribble)

For each stage, we need input and output voltages and currents ( or voltage and source/load impedances)

One note: with the differential amp, I doubt if you need the 9-volt regulator...maybe just a bit of decoupling between the motor drive and the diff-amp.

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 Post subject: Re: Speed circuit
PostPosted: Jul Sat 15, 2017 5:27 pm 
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Tube Radio wrote:
Think I saw that one earlier, but it increases its voltage with frequency and I need the exact opposite effect.


That's easily fixed use a LM337 instead of a 317 .

http://www.rivertronics.com/swingmaster ... ster2.html

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Last edited by battradio@ on Jul Sun 16, 2017 11:09 pm, edited 1 time in total.

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 Post subject: Re: Speed circuit
PostPosted: Jul Sat 15, 2017 10:27 pm 
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The regulator was to provide a fixed voltage to the speed circuit.

I currently use a 12 volt switch mode supply to power the motor, but that may not always be the case so I may wind up using a 7812 to supply everything.

Speed regulation needs to be 1% or better as the slightest change in speed is noticed.


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 Post subject: Re: Speed circuit
PostPosted: Jul Sat 15, 2017 10:47 pm 
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That chip will do it , but use all metal film and silver mica or polystyrene capacitors in critical places to get the best thermal stability .

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 Post subject: Re: Speed circuit
PostPosted: Jul Sat 15, 2017 11:57 pm 
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I might try the differential amplifier idea first as I have ready access to various transistors.

I'll list the ones I have at home tonight when I get back home.

EDIT:

Here's the transistors I have at home

MPS2369
http://www.mouser.com/ds/2/308/mps2369-d-303613.pdf

2N4410
http://www.onsemi.com/pub/Collateral/2N4410-D.PDF

MPSH04
http://www.datasheets360.com/pdf/3197485207720696829

MPS3646
http://www.mouser.com/ds/2/308/mps3646-d-304089.pdf

2N5818
http://www.mouser.com/ds/2/68/2n5814-2n5819-42960.pdf

2N3414
http://pdf1.alldatasheet.com/datasheet- ... N3414.html

2N5832
http://pdf1.alldatasheet.com/datasheet- ... N5832.html

2N4124
https://www.onsemi.com/pub/Collateral/2N4123-D.PDF

I may have other NPN transistors at work if none of those will do the job.


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 Post subject: Re: Speed circuit
PostPosted: Jul Sun 16, 2017 11:14 pm 
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do you have any mpsw-05 they are very thermally stable , , bond the pair with epoxy so they will track together .

http://www.rivertronics.com/swingmaster ... ster2.html

no drifting was very crucial in this project .

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 Post subject: Re: Speed circuit
PostPosted: Jul Sun 16, 2017 11:18 pm 
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Don't forget my request for requirements and parameters.....

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 Post subject: Re: Speed circuit
PostPosted: Jul Sun 16, 2017 11:49 pm 
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At 627Hz the control voltage to the motor is 2.4408Vdc.

Speed regulation needs to be 1% or better as due to the size of the brass piece on the motor even a change of 1Hz causes a noticeable to me speed change, although the actual platter RPMs is only changing by a good bit less than 1RPM.

I can run the motor tomorrow at work and try to manually hold its speed at 627 Hz then measure the unloaded output of the frequency to voltage converter.

I'd like to remove the brass piece as it would make the motor run faster and a change in motor speed won't affect the platter speed as much, but I'd be concerned about motor vibration as I got the rotor balanced by drilling some small holes, but it still vibrates a little at full speed.

Also as it is now the motor is running about the same speed as it would have in the VCR.


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 Post subject: Re: Speed circuit
PostPosted: Jul Mon 17, 2017 12:14 am 
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Tube Radio wrote:
I'd like to remove the brass piece as it would make the motor run faster

I am curious how removing the brass piece will make the motor run faster.


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 Post subject: Re: Speed circuit
PostPosted: Jul Mon 17, 2017 12:20 am 
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Meant to say it would allow me to run the motor faster as the brass piece fits on the shaft and is in contact with the drive wheel.


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 Post subject: Re: Speed circuit
PostPosted: Jul Mon 17, 2017 3:55 am 
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Tube Radio wrote:
At 627Hz the control voltage to the motor is 2.4408Vdc.

Speed regulation needs to be 1% or better as due to the size of the brass piece on the motor even a change of 1Hz causes a noticeable to me speed change, although the actual platter RPMs is only changing by a good bit less than 1RPM.

I can run the motor tomorrow at work and try to manually hold its speed at 627 Hz then measure the unloaded output of the frequency to voltage converter.

I'd like to remove the brass piece as it would make the motor run faster and a change in motor speed won't affect the platter speed as much, but I'd be concerned about motor vibration as I got the rotor balanced by drilling some small holes, but it still vibrates a little at full speed.

Also as it is now the motor is running about the same speed as it would have in the VCR.

It appears you are talking about changing the effective "gear ratio" from motor to turntable. That will not affect stability....A 1% change in motor speed will still give a 1% change in turntable speed.

Back to parameters:
What is the current required by the motor? This tells us what the driver transistor must supply.

I don't understand "control voltage to the motor". I asked for the voltage output from you frequency to voltage convertor....i.e. the voltage we're going to put into one side if the differential pair and compare to a zener-stabilized reference.

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 Post subject: Re: Speed circuit
PostPosted: Jul Mon 17, 2017 5:04 am 
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The motor control voltage is what it takes to make the motor spin at the right RPMs.

The motor's current doesn't matter as its driver chip is controlled by a voltage so required current is minimal.

True about motor speed.

But if the motor is running faster and its speed changes by 1RPM the platter won't change its speed as much as it would if the motor is running slower.

The other measurement I mentioned that I would measure it tomorrow at work.


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 Post subject: Re: Speed circuit
PostPosted: Jul Mon 17, 2017 2:15 pm 
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Tube Radio wrote:
The motor control voltage is what it takes to make the motor spin at the right RPMs.
...and there's a driver, so we don't need that extra drive transistor I showed......can I assume that the motor controller has an input impedance of at least 100 Kohm?
Quote:

But if the motor is running faster and its speed changes by 1RPM the platter won't change its speed as much as it would if the motor is running slower.

The trick here is that tolerances are normally expressed as a percentage.....in this case, we need the overall design to regulate better than 1%. Specifying things this way, the gear ratio does not matter.....it should be set based on the torque required to turn the platter.

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 Post subject: Re: Speed circuit
PostPosted: Jul Mon 17, 2017 3:30 pm 
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To measure the output of the frequency to voltage converter was quite convoluted as I had to figure a way to get the speed stable enough at 627Hz to take an accurate reading which was not at all easy to do.

At 627Hz the output of the F to V converter is 3.082Vdc.

I just did a test by having the drive wheel contacting the motor's shaft directly and the motor does turn fast enough, but at the higher speed the motor has vibration and I would have to balance it better.

That said the I'll leave the brass piece in place, but the only problem is the brass piece needs to be a little longer to make full contact with the full thickness of the rubber on the drive wheel.

I do have another VCR video head motor which does have a brass piece the video head drum attached to. I'd have to cut it then figure a way to shave it down some.

Here's the data on the driver IC for the motor.

http://www.datasheets360.com/pdf/4719735905870063851

I am using the Ecv pin which is pin 22 on the chip.

Not sure what the input impedance of that pin is, but if I put a 10K resistor between the Ecv connection and the control voltage the speed jumps to 700 RPMs. I'm measuring 2.528mV across the 10.19K resistor which gives me a current of 248uA.

The nice thing about the VCR video head motors is most use the same size shaft and bearings which is good because I don't have to worry about mucking up the current shaft while trying to get that other brass piece to work as I can just install it in the motor as is.


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 Post subject: Re: Speed circuit
PostPosted: Jul Mon 17, 2017 8:57 pm 
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If I have interpreted this thread correctly, the OP is trying to drive a VHS head motor open loop, when it is designed to be driven by feedback from a Hall effect sensor.

Isn't the task just a matter of using a Hall effect sensor or opto coupler as the input and connecting a voltage divider to the voltage control input Ecv from the output and vary the divider ratio until the required speed is achieved?

Although also open loop, this is why many turntables had alternating black and white stripes on their rim so that the speed could be fine tuned manually by means of a stroboscope built into the turntable.


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 Post subject: Re: Speed circuit
PostPosted: Jul Mon 17, 2017 9:16 pm 
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I had the same reaction when I saw Hall effect.

When using the VCR motor with the turntable, we would surely have to use a speed that the controller was not designed to deliver.

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 Post subject: Re: Speed circuit
PostPosted: Jul Tue 18, 2017 12:19 am 
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It's not that simple.

The two hall sensors are part of the driver circuitry.

The tach signal is fed to speed control circuitry on the VCR's main board which then sends a control voltage to the motor's control wire and that's how the speed regulation is done.

From having worked on VCRs in the past, the motor's speed is somewhat close to the speed the motor would have ran in the VCR, but is capable of any speed from its minimum up to its maximum.

Controlling the motor's speed open loop is near impossible given how little of a voltage change causes the motor to change RPMs.

The best meter I have at work has a resolution of 100uV when displaying from 1.0000 to 9.9999 volts and I do know that a change of at least 100uV or less will affect the tach frequency which means the motor's RPMs have changed.


Interestingly enough when I looked at the motor I was going to take the brass piece from I noticed it looked real similar to the motor I currently have and it is extremely close. Only major difference is that it uses a BA6414FS and according to the only datasheet I could find (one listing several ROHM chips for VCR motors) it is similar to the BA6827FS only it lacks Ecv control only having Eci control.

I may try the motor tomorrow using the existing speed control as both motors use the same connector.

I will then see whether Ecv or Eci gives the better speed control keeping in mind that the Eci input current is only limited by a 500 ohm internal resistor.



EDIT:

I tried the motor and it does spin but there's two things:

1. Speed regulation is poor, but that is to be expected when using a circuit set up for Ecv control to drive an Eci input.
2. Motor coil is noticeably noisy, whereas the other one is quiet.

Using that motor board would require possibly adding a V to I converter though.

One thing I wonder is with what the motor is used for how the manufacturers could get away with a rotor that is so off balance that it causes some vibration when it runs.

As I discovered with the rotor to the other motor it was relatively easy to balance.

1. Test motor.
2. Drill a small hole on side that is too heavy.
3. Check for vibration.
4. Repeat until vibration is minimal.


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 Post subject: Re: Speed circuit
PostPosted: Jul Tue 18, 2017 3:53 pm 
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I'm lost....

Do we know for sure if that controller is intended for control of speed using a voltage or current input?

Is this project turning out to be more difficult than simply finding the right motor?

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 Post subject: Re: Speed circuit
PostPosted: Jul Tue 18, 2017 5:06 pm 
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The motor I am using was set up originally for Eci (current) control, but I swapped a connection to the chip and made it Ecv (voltage) controlled.

The other motor is indeed Eci controlled and I brought that up as I didn't know if current or voltage control would be easier.

The only difference is the Ecv input on the original motor goes to a built in V to I converter on the driver chip, whereas the chip on the other motor only has an Eci input.


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