Servo question

[Space Varmint]

What are the active components in differential amplifiers used to drive servo motors. I'm not getting specifics in my Google searches. What I am seeking is due to the fact that it is a feedback system, what device or devices are used to produce the needed feedback information?

 

[Papabravo]

There are at least two answers to the question. I think the one you are looking for is rotary encoder. It has two digital outputs called A and B and they are 90 degrees out of phase. As the motor moves the digital signals A and B beat up and down. Counting the transitions allows you to figure out how many radians the shaft has moved. You can also tell direction by looking at which transition happens first as you start to move.

 

[HarveyH42]

Try searching for something about converting a servo for continuous rotation (robotics). Been a long time, but remember a site that went into great detail on how servos work, and most importantly how to bypass the feedback. If I remember it, there is a pot attached to the motor shaft for feedback. There is a physical stop for the limits, don't remember any switches.

 

[Space Varmint]

There are at least two answers to the question. I think the one you are looking for is rotary encoder. It has two digital outputs called A and B and they are 90 degrees out of phase. As the motor moves the digital signals A and B beat up and down. Counting the transitions allows you to figure out how many radians the shaft has moved. You can also tell direction by looking at which transition happens first as you start to move.

Appreciate your input. I think your talking digital I'm sure. I am more interested in analog. It is so much faster. I'm looking for what is the device that compares the results. Now obviously people may respond with the answer of "comparator". It may be but I would like to hear from someone with experience in this circuitry. That way I know for sure. I saw some vague references to common mode outputs but they sort of cut it short without getting to the meat & potatoes.

 

[Mikebits]

The Harvmyster is right.
https://www.kronosrobotics.com/an116/GAN116.shtml

Here is another link.

 

[Space Varmint]

You should find this most interesting...

https://www.freepatentsonline.com/4050002.html

I'm talking real servo systems. There is no digital and no use of PWM. It is a purely analog feedback loop system.

 

[jpanhalt]

Are you referring to a selsyn motor/drive?

John

 

[Space Varmint]

Are you referring to a selsyn motor/drive?

John

Wow! Good find! That looks like the active transducer in the transmitting device.

 

[Papabravo]

You should find this most interesting...

https://www.freepatentsonline.com/4050002.html

I'm talking real servo systems. There is no digital and no use of PWM. It is a purely analog feedback loop system.

That's crap. Real servo systems use any and all available techniques. My experience with servo systems is flight controls for military aircraft. Is that real enough? As an inherently unstable airframe, the B2 would auger in without its fly-by-wire systems. You might consider resolvers, rate gyros, and the lowly pot coupled to the shaft. Rotary optical encoders are just one of many techniques.

A comparator is generally inappropriate in this context since its digital output can represent only two states. At the point in the system where you need to compare the "Command" signal with the "Error" signal a simple summing amplifier would work just fine. A summing amplifier is made from an operational amplifier with a large open loop gain, by placing a simple resistor from the output to the input. Is that what you are after?

I guess the original question was too vague to allow a precise answer.

Relative to the patent abstract:
Rectification is done with diodes, just as in a power supply. The diodes can be configured as half-wave, full-wave or bridge rectifiers. Subtraction of two signals is performed by inverting one of them and adding them together. Nowadays you get functions in a single chip that used to take a significant piece of an ATR rack.

 

[Space Varmint]

Apparently the receiver too. I would like to see the electronics around the unit. Still, it's pretty slick! I believe you got it right there.

Electronic terminology tends to adapt over time. I have always heard this technology referred to a servo mechanisms whereas now days I have noticed with the growth of robotics they tends to use this terminology but it is actually incorrect. Most of that is digital stepper type technology. It really first appeared with things like printers to spin the daisy wheel and feed paper through the platen.

 

[Space Varmint]

A comparator is generally inappropriate in this context since its digital output can represent only two states. At the point in the system where you need to compare the "Command" signal with the "Error" signal a simple summing amplifier would work just fine. A summing amplifier is made from an operational amplifier with a large open loop gain, by placing a simple resistor from the output to the input. Is that what you are after?

Right there! That's the electrical circuits I was referring to. Sorry for being so vague as I have not delved into these system, hands on per say.

Where could I get more information on these circuits?

Analog beats the hell out of digital for speed. Let's not generalize too much. Take an analog computer for instance...Fast!!!

 

[Space Varmint]

Rotary optical encoders are just one of many techniques.

Optical encoders are purely digital.

It is my understanding that servo devices refer to analog. A stepper motor is not a servo device.

Look at the date on the the mechanism Jpanhalt came up with. It's like 1925. Servo has been around a very long time and was never digital. It's just a blending of the terminology in recent years.

 

[Papabravo]

Right there! That's the electrical circuits I was referring to. Sorry for being so vague as I have not delved into these system, hands on per say.

Where could I get more information on these circuits?

Analog beats the hell out of digital for speed. Let's not generalize too much. Take an analog computer for instance...Fast!!!

Well Google and Wikipedia are my favorite starting points. Here are some links on operational amplifier circuits.
https://en.wikipedia.org/wiki/OpAmp
https://www.electro-tech-online.com/custompdfs/2008/09/tl082m.pdf
https://www.electro-tech-online.com/custompdfs/2008/09/slob090.pdf
https://www.electro-tech-online.com/custompdfs/2008/09/AN-A.pdf

Search the forums for other keywordws that you encounter in the articles and follw the links at the end of the Wikipedia article. Have a ball, and knock yourself out

 

[Papabravo]

Optical encoders are purely digital.

It is my understanding that servo devices refer to analog. A stepper motor is not a servo device.

Look at the date on the the mechanism Jpanhalt came up with. It's like 1925. Servo has been around a very long time and was never digital. It's just a blending of the terminology in recent years.

A stepper motor with an optical encoder would be a legitimate closed loop servo mechanism system. Not that anybody would do it that way, but it is possible.

Yes it has been around for a long time. So have I and my father before me. Like everything else it has grown, evolved, and adapted. It now encompases digital techniques including the use of wicked fast VLIW Digital Signal Processors, A/D and D/A converters with unheard of precision.

I can one up jpanhalt on the date. Check out the flyball governor on the steam engines that drove the industrial revolution circa 1850. There is an analog servo mechanism that is truly a thing of beauty.
https://en.wikipedia.org/wiki/Centrifugal_governor
https://en.wikipedia.org/wiki/Servomechanism
Note the picture includes the motor, the reduction gears, and an optical encoder.

 

[Space Varmint]

A stepper motor with an optical encoder would be a legitimate closed loop servo mechanism system. Not that anybody would do it that way, but it is possible.

Yes it has been around for a long time. So have I and my father before me. Like everything else it has grown, evolved, and adapted. It now encompases digital techniques including the use of wicked fast VLIW Digital Signal Processors, A/D and D/A converters with unheard of precision.

Oh, granted the precision is far superior with DSP but there is still that speed limitation. The Nyquist Theorem stipulates the the processor clock speed must be at minimum twice the frequency your dealing with. Hence you have that limitation in the processor. Actually processors are tapped out now days which is why you see dual core and multi-core processors come along. It has to do the the size of the interconnecting wires internal to the chip. The diameter of the wire has reached it's limit for the amount of current it must carry. That's why tantalum is so hot. It enables them to shrink the wire that extra degree or two.

 

[Papabravo]

Gee I think 3 GHz. is plenty fast enough for most control applications that come readily to mind. How exactly is that too slow for your tastes?

 

[Space Varmint]

I like it! **broken link removed**

Hey now. Never discredit the old timers. Man some of them were shear geniuses!

I always flip out when I think of how Single Sideband was actually invented. Some guy sat down and figured out this phasing array using at least a hundred diodes and capacitors to actually phase out the carrier and one sideband...lol.

These guys were nuts!

 

[Space Varmint]

Gee I think 3 GHz. is plenty fast enough for most control applications that come readily to mind. How exactly is that too slow for your tastes?

Oh, my picture didn't come out on your centrifugal doo-hicky.

Trust me. We always want faster faster faster. Real time! Robotics will demand it. I am certain.

 

[Space Varmint]

Anyway that coil arrangement that Jpanhalt came up with is fascinating! A while back I had got into a discussion with a really smart guy on another electronics board. He had built a balancing mechanism into a two wheeled robot. Well he was successful but the way he went about it just seemed like too much of an over kill. It disturbed me. He used a neat transducer which I had not thought of, an "inverted pendulum". But then the movement of the pendulum device was converted into data and run through a software "Kalman" filter. This data was used to control the motors needed to correct the slight variations for balance.

I felt this was very slow and demanded too much processor time and effort. I always look at the man for robotic features. The man uses what, the brain stem for balance I believe. Not the brain itself. Kind of like reflexes where your hand will yank back from a hot pan or something and later you feel the pain (reflex). There is less processing required and a shorter path in the circuitry so to speak. Brain stem to spinal cord to fingers and back.

I think this coil mechanism is ideal for such an application. The inverted pendulum could be mounted to the coil arrangement and the motors could respond directly to the movement of the coils. The accuracy is not even an issue because balance is always in motion and constantly correcting itself. You do not want a processor (brain) to constantly be preoccupied with the balancing process. If it were that way for the man, we would not be able to think. Most people cannot walk and chew gum at the same time (a figure of speech). What do you think? Anybody? Does anyone think we should keep the Kalman filter? Interested in hearing your argument.