Servo Control with Sine Cosine Potentiometers 2

[AndreyG]

Servo control with Sine cosine potentiometers.

I have controller and motorized rotating device both rotating over full 360 degrees – no limits. Both Controller and device are equipped with Sine Cosine Potentiometers.

I am looking for simple (and preferably analog) way to compare outputs of the two Sine Cosine Potentiometers and generate error signal.

 

[VEBill]

You're probably not getting any response because your question is unclear.

 

[OperaHouse]

You're probably not getting any response because you want to use positioning technology the rest of us abandoned many years ago. You could use a little PIC if you know how to program.

 

[AndreyG]

Thank you for those who replied. Let me expand the description of the problem:

I have controller and motorized rotating device. The Controller is a knob the User can rotate. The rotating device is Propulsion Nozzle. Both the Controller and the Nozzle can be rotated over full 360 degrees – no limits. Both Controller and device are equipped with Sine Cosine Potentiometers.

I need the Nozzle to follow the Knob. In order to do that I need to compare output signals from the two sine cosine potentiometers and generate error signal. Error signal activate the motor and make the nozzle turn to follow the controller.

I am looking for simple (and preferably analog) way to compare outputs of the two Sine Cosine Potentiometers and generate error signal.

I expect there should be analog way to do it because the sine cosine pots were around before digital techniques came to being.

 

[VEBill]

"...compare outputs of the two [whatever] potentiometers and generate error signal."

Differential amplifiers, opamps.

Or wire one of them up backwards and simply add. Again, opamps.

"Sine Cosine" is probably just a distraction. That's why I replaced them with [whatever] above. I think it clarifies things a bit. Maybe.

 

[AndreyG]

Hi VE1BLL,
Thank you for your comments.

Unfortunately it is more complicated then that. An algorithm is required. If one simply compares say, sin output of one pot to sin output of another pot he does not know singularly where along the circle the device is located neither in which direction to apply correcting action.

I speculate both signals must be combined in some fashion and then only analyzed. There should be standard method to do it.

This is what I am trying to find out.

 

[Skogsgurra]

I do not think that it is an error signal you want to compare. If so, with what are you going to compare what?

Is it possibly so that the sine/cosine signals produce an angle signal (say +10V at 90 degrees and -10V at 270 degrees) that shall be used as a setpoint for the nozzle's angle? Or are there sine and cosine potentiometers delivering actual position of the nozzle? In the latter case, you should probably use normal comparators, perhaps even opamps.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[VEBill]

Remove the two Sine Cosine pots and replace with synchros. Then go for lunch. ;-)

More seriously, I think you could use the two error signals in basically a wired-OR arrangement to drive the motor. If you used only one error signal then it could obviously get hung-up on the wrong solution. By using two in 90-degrees it forces the unique solution.

The wired-OR might be conceptually implemented with diodes, but complicated slightly by the bi-polar nature of the signals.

There might be one quadrant that this simple wired-OR approach may fail. But I'd need a drink first before I could figure it out.

 

[AndreyG]

Sine Cosine Pot example:

http://www.precisionsales.com/potentiometers/singleturn/images/pdf/SSC50S.pdf

These parts have two outputs: each output is sine value of the rotational angle of the Pot. Second output shifted from first by 90 degrees. Hence sine cosine pot.

Directly comparing two signals (say Sin1 and Sin2) does not work because due to periodicity of the function Sin will have the same value in two points around the circle.

Recovering angle from readings of sin and cos results in discontinuity every 180 degrees. Also, this method requires processor with look up table.

There should be analog method to compare sin and cos signals of one pot to sin and cos signal of another pot and derive error signal.

This is what I am trying to find

 

[OperaHouse]

"Directly comparing two signals (say Sin1 and Sin2) does not work because due to periodicity of the function Sin will have the same value in two points around the circle."

If you didn't think we knew that, why did you come here? Of course you would use both signals and that is what the other poster suggested "wired-OR arrangement". I suspect I could do a crude control with just a LM339 quad. It does take me back to the old days of working on CIMEX machines.

 

[AndreyG]

"I suspect I could do a crude control with just a LM339 quad."

I bet you could not. Are you up to the challenge?

But seriously, two days ago I also thought it would be trivial. But it is not so. I made it working on PLC: I detect 4 quadrants and analyze one quadrant at a time. But this is not 'clean and nice'.

There must be strictly analog, simple solution since these parts come from pre-digital days.

 

[VEBill]

I think AndreyG was composing his 15:48 reply at about the same time as my previous post had just appeared 15:50, and thus missed the Wired-OR suggestion.

 

[2dye4]

So you get a sin and cos reading from both knob and actuator.

For the knob you have v1= sin(x) and you have v2= cos(x)

and for the actuator you have z1=sin and z2= cos.

where x is the knob angle and y is the actuator angle.

Essentially you have two complex numbers and you need to know the
angle between them.

angle= angle(v1 +jv2) - angle(z1 + jz2)

angle = angle(v1-z1 + j(v2-z2))

angle = atan((v1-z1)/(v2-z2)) ( with ambiguity on which half of the plane)

I don't see any easy way to do this with just an analog circuit.

 

[Compositepro]

What kind of motor are you controlling? I imagine that sine/cosine pots exist because they allowed for a simple solution to a control problem and are designed to be used with a special motor in a particular way. As was mentioned earlier synchro motors can do exactly what you want and do not require any controls. Somewhere in my collection I have a device similar to what you describe where a pot controls the position of a small motor like an old shaded pole phonograph motor. Except it is not a shaded pole. There are coils that are connected to the pot which control which way the motor rotates. As I recall the the current to these control coils were null when the pot on the motor matched the position of the control pot. There were some gears on the motor as well. It was probably 1940's vintage.

 

[IRstuff]

If this were digitized, the angle reading is simply atan2(sin,cos) or, in analoe, like:

http://findarticles.com/p/articles/mi_qa3957/is_200504/ai_n13617610/

However, as many others have already stated, a digital implementation would be quicker, more reliable, and easier to calibrate.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[VEBill]

That's U.S. Patent No. 6,138,131 in case that wasn't noticed.

 

[VEBill]

Ultra-simple control schemes might be flummoxed by the inversion on the far side of the circle.

Four comparators might just form the basis of a crude and workable controller, each comparator assigned to one (almost linear) quadrant. The final piece of the puzzle would be to switch control between the four outputs.

 

[IRstuff]

The NASA circuit purports to be a 4-quadrant arctangent, just like atan2

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[AndreyG]

Hi VE1BLL, you are catching up with me.
I did on PLC sort of what you described in your last message:
I created two step procedure:
Step one determines quadrant of Knob and quadrant of Nozzle.
If quadrants are not the same I tell Nozzle to go to Knob's quadrant.
When Knob and Nozzle are in the same quadrant I compare analog readings. The comparison rule is different for each quadrant.

But again, it's like cheating with modern equipment. There must be analog way of comparing the two.
I did not look at the patent you suggested yet.
Thank you,

Andrey

 

[VEBill]

IRstuff found the 'patent', I just cut-and-paste the patent number from the page he linked. It's quick and easy to see the diagrams using the patent number as a search term.