Why must we use servo motors ?

Not very much PLC related but I wonder If anybody can lead me to the answer with a link or discription. I want to know the reason for using a servo motor in a machine instead of DC motors or Asynchronous ACs.

Search for the definition of 'servo motor' and you'll find the answer.

I gues by DC motors and AC asynchronuous motors you mean simply motors which turn to a certain speed if you feed them a certain voltage / frequency. Think somewhat further in the direction of controlling that speed to some extent.

Regards,

I think tight control of the position of the controlled device that is driven by the motor is the main reason for a servo feedback.
Exact speed is also a possibility, but is not the most common application for a servo motor.

In my view, the single redeeming quality of servo motors is their dynamic performance. By that I mean their ability to make fast speed and direction changes. This is accomplished by very low inertia rotors and other winding characteristics.

As for positioning and speed accuracy, a good high performance AC or DC drive can do that today if a proper positioning controller is placed ahead of the drive. What an AC or DC drive/motor can not do is make the changes quickly.

Just for illustration purposes, I have a laboratory test cell system consisting of a 200hp ABB ACS600 drive using DTC (Direct Torque Control) and a special built water-cooled AC induction motor from Electric Apparatus Company that acheives 4 rpm speed error from no load to full load at 10000rpm. And its done without a tach or encoder on the motor. But, on step changes in speed reference, it takes about 700ms to get the speed to the new level and settle down. A full servo system could likely do the step changes faster.

Reliance Electric also makes a low inertia version of their excellent RPM-AC motor which, for speeds under about 4000rpm, is capable of near servo performance on a DTC-type drive. There may be others that I am not aware of, particularly in Europe and Japan. I have run into some spectacularly high performance AC induction spindle motor designs which have almost unbelievable spec sheets but I've never actually used one.

One thing that lots of people seem to ignore when comparing Servo systems to 'standard' drive systems, is that generally servo systems are designed to be a MUCH better inertial match to the load. That alone results in much better performance than just sticking in any ol' drive/motor with enough horsepower to do the job (and usually lots of extra).

Maybe it's just me, but I see blank looks all the time when I ask a customer what their dynamic loading and inertial constants are on the connected equipment when they are looking for a drive/motor.

Don't take that to mean that servos aren't useful. They can sometimes be the only thing to use in some applications, but like everything else in this industry, the application should determine the solution, not the other way around.

How do you do that?

rdrast said:

One thing that lots of people seem to ignore when comparing Servo systems to 'standard' drive systems, is that generally servo systems are designed to be a MUCH better inertial match to the load. That alone results in much better performance than just sticking in any ol' drive/motor with enough horsepower to do the job (and usually lots of extra).

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What you suggest is that the motor is designed to match the load. This would require a lot of different types of servo motors. One uses gear boxes to match the match the reflected impedence.


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rdrast said:

Maybe it's just me, but I see blank looks all the time when I ask a customer what their dynamic loading and inertial constants are on the connected equipment when they are looking for a drive/motor.

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It isn't just you, I too hear the quiet after I ask what turns out to be an embarrassing question. Fortunately this does not happen all the time or even half the time, but when one gets involved with MANY axes a year it takes only a small percentage to take up a lot of time.
I am sure the application help we provide is worth more than the controller is many cases.

I am pretty sure that a servo is a mechanical/motion system that uses negative feedback. I looked it up in a dictionary. Servo motor is commonly given a more narrow definition for motors used in closed loops systems with high bandwidth.

Inertia matching the motor to the load is part of the design necessary to maximize dynamic performance, as I understand it. It all goes back to my earlier comment about dynamic performance setting servo systems apart from other drive methods.

And, regarding asking customers for load inertia, etc. You servo guys aren't the only ones to get blank stares when that question is asked. I had the pleasure of asking a customer for flywheel inertia on a mid-size stamping press and, predictably, he had no answer. I contacted the press manufacturer and got the response----"we don't know what the wk(squared) is but its enough to do the work at the rated tonnage!". After that, I just stopped asking!

Peter, you are absolutely right with gearboxes being required for proper load matching in most cases. My point though, is that when discussing a DC Motor, or an AC Motor, it seems that all of my customers will choose a standard motor, and any old gearbox to give 'sorta close' to the desired speed output.

On the other hand, in cases where the end user is mechanically sharp, or has real reasons for wanting a servo system, they will make sure that the motor and gear train and load are all well matched for each other.

Heck, I have one customer that uses the same 4 speed gearbox on just about every piece of equipment they own. They must have thousands of them. If they can't get the required output speed, they'll stick two in series, or change drive pulley ratio's, but they never ever ever EVER give any thought to actually matching prime mover load and WK2 to the actual load. <shudder>

Just looking at speed loop tuning parameters, you can get a pretty good idea on how well motor/load are matched.... really low gains generally indicates pretty poor mechanical match. In those cases, you can certainly stick a servo motor/drive on, but will get the same horrible performance.

I don't know, perhaps people are just getting lazier <grin>. Too often lately it seems that the folk doing the electrical controls are required to go to great lengths to 'tune out' mechanical problems, because, obviously, it's easier to change a number then bolts and gears. /sigh

(Edit)
Oh, and the simplest way to think about the benefits of inertia matching is to think of simple electrical circuits. Consider if you will, a battery and a resistor. You get the most stable, and maximal power transfer from source to load when the resistor value equals the internal source resistance of the battery.

In other words, when load impedance = source impedance. In most cases, mechanical inertia can be thought of directly as electrical impedance.

One more characteristic of servo motors that hasn't been mentioned yet:

They are designed to deliver 100% rated torque at zero speed - continuously, without forced cooling.

Originally topic started with question:
Why must we use servo motors ?

Well we don't have to. If you can do the job any other way
(I'm not talking about human labour), it's probably more
economical than using servo. Thanks to lower prices and
better performance of embeded controls many VFDs are
sporting more and more features. So if they can do the job - great!!!
But don't confuse them with true servo drives. When it's hard to
choose anything else, it's probably because you do need servo.
With servo you get all goodies that other drives handle
only partially.
- full torque at zero speed,
- very fast response
- very tight controls (position, torque or speed)
- coordinated motion of multiple axes (not just electronic gearing)
- full programmability with multiple modes of operation that
can be conveniently changed on fly - or run simultaniously.

Some servos setups aren't that much more expensive any more.

Use the right tool for each application. I have never been a big fan of rolling you own to save a few pennies.

As Freeman Dyson said

"A good scientist is a person with original ideas. A good engineer is a person who makes a design that works with as few original ideas as possible. There are no prima donnas in engineering. "

This is a case where technology has passed up a naming convention. As Peter stated earlier, any one of a number of motor technologies can perform a 'servo' function. In fact, the first servo motors were brush style DC servos, if I remember correctly. They were the only motor type that could easily support closed loop control back in the day.
Today, most people equate the term 'servo motor' to one of the magnet-on-rotor, electronically commutated motors. Again, as stated above, you only need this motor type if you have a 'high bandwidth' application. There's another term that technology is passing by. High bandwidth means something different every year as processing power and motor technology continue to advance.
Rick hit the nail on the head. Use what works for you. I have used brushless DC motors in what could be handled as an AC vector designs simply because of size and price. But as a general rule, your best torque-to-inertia ratio in a motor will come from something with permanent magnets on the rotor.
That brings me to inertia mismatches. 'Conventional wisdon' says you need to match the motor to the load inertia. If that was all there was to it every motor manufacturer in the world would make double shafted motors with the express purpose of hanging a flywheel off the back end of the motor. What better way to match up motor and load inertia? Why isn't this done? Because it's not just about motor to load inertia ratio. It's about torque to inertia ratio. What seems to get lost in the talking is that the two most popular inertia matching techniques (stiffer gearing and bigger motors) increase the torque to inertia ratio, providing better system response. The downside to stiffer gearing is you inadvertently increase the motor acceleration for a given load acceleration profile. So you don't get an n^2 performance increase, you get an n performance increase. The downside to a bigger motor is you increase the system inertia, biting into some of your gain.
Now, before someone jams the driver-to-load matching math down my throat, I do agree that optimal power transfer occurs when the driver and load inertias match. However, if I can get a low inertia motor to produce HUGE amount of torque (read high temperature superconductors), what do I really gain much by trying to get that motor to spin 100,000 RPM and get gearing that can stand up to it just to match up the inertias?

But I ramble.
Keith

a servo motor is just a motor with a feedback on its position.

yes i do know they have servo motors with special like short shaft etc, but it stays a simple electric motor.

Very old thread, Shooter. You can ignore Mr. copy/paste spammer...



-Eric

Eric Nelson said:

Very old thread, Shooter. You can ignore Mr. copy/paste spammer...



-Eric

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It may be old but it is still good info. In the electrical vehicle community I am actually convincing some people they must KNOW THY LOAD before choosing a motor.

Dan Bentler