Applications & Questions

  1. Is it important to match the inertia of the motor to the inertia of the load via a gear box or other method?
  2. What is the difference between AC and DC brushless motors?
  3. What is the difference between velocity and torque mode in motor amplifier?
  4. Is an analog tachometer necessary for slow speed stability, and why?
  5. Building a large scale RC boat.
  6. Understanding Mechanical Resonance.
  7. How do I go about Starting My Own Business?
  8. A Simple One Push-Button ON/OFF control circuit ...
  9. Can you give me a definition of the term 'Motion Control'?

Is it important to match the inertia of the motor to the inertia of the load via a gear box or other method?

The answer truly is "It depends". Is the application going to exhibit high load inertia changes during operation? If the answer to the question is "no", such as with pick-and-place units handing chip wafers, it's not be necessary, provided cost or stability for the chosen motor is not a problem. If the unit is a PM DC Brush motor you may have to. Remember that the ideal motor is one that has all the power you need and zero inertia.

After all, what is motor inertia but waste of energy? If answer to the above question is "yes", then by how much? Many motor/amplifier combinations have the ability to help the motion controller offset the changing load inertias.

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What is the difference between AC and DC brushless motors?

Basically . . . Bandwidth!
The Idea for the AC BLDC motor is Power. The idea for the DC BLDC motor is Speed and Response time.
That being the case, AC BLDC motors are frequency dependent which slows down their ability to react. On other hand, the DC BLDC motor, utilizes high response designs for applications such as Indexing, CNC & Robotic actuator controls, etc..

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What is the difference between velocity and torque mode in motor amplifiers?

As their respective names imply, torque mode is one in which the current in the motor windings is adjusted to yield motor torque base of the simple equation:

Torque = (Motor Kt) * (Motor Amps) torque units

In velocity mode, however, the voltage across the motor is adjusted to set the motor speed the simple equation:

Velocity = (Applied Motor Volts) / (Motor Ke) KRPM

The key here is to understand who is controlling what. In torque mode, the motion controller is sensing a change in speed via the feedback unit. It accommodates this change by varying the voltage it sends to the motor amplifier. This, in turn, will directly alter the amount of current flowing through the motor's armature windings.

In velocity mode, feedback changes that are sensed by the controller, will also alter voltage the controller will sent to the motor amplifier, but this is where the similarity ends. In voltage mode, the control voltage is used to change the terminal EMF (Electro-Motive Force) across the motor - not the current through it. As you can see in the above equation, by changing the terminal voltage on the motor, a change in motor current will result. This action by itself will attempt to change the motor's speed to return it to its previous state of balance. Therefore, we have two systems responding to the changes in RPM in the velocity mode - the motor amplifier via the above equation and the motion controller via it's own equations.

If the motion controller bandwidth is not adjusted to respond slower than the motor amplifier package, the system will become unstable. It is due to today's higher bandwidth motion controllers that a torque mode motor/amplifier package is desirable. This mode of operation can be fitted with tachometer feedback to improve both low speed stability and high speed reaction capability.

Since the motion controller has one extra stage to deal with when in the voltage mode, it's bandwidth will never be as high as when operating in the torque mode.

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Is an analog tachometer necessary for slow speed stability, and why?

Yes, it is desirable. As system speed decreases, the time between encoder count edges increases. Now, it is true that many controllers have what is called a 1/T mode, in which the motion update is governed by the receipt of a count edge. When the edge is sensed, the elapsed time period between it and the previous edge is used to calculate motor velocity.

Now this all sounds great, until you are working with a rotary system in which one motor revolution is 113.097 inches on the circumference of a 36-inch diameter disk that needs to run at 0.5 RPM. If we require a resolution of 0.002 inches, then the encoder resolution would have to be 0.0002 inches per count to accommodate computer round-out and other system variables. (Round-out is the natural tendency of the motor to hunt.)

This would yield an encoder line count of 141,371 pulses per revolution. The top speed of the motor would be limited to 1.768 revolutions per second (106 RPM), assuming a top frequency capability of the motion controller of 1 MHz. This is also assuming the encoder can produce that count rate, and that the top speed is also acceptable—in lieu of adding $2000 to the cost of the axis to enable higher frequency count registering. The point is this: If the unit is to operate at 0.5 RPM, there will be a count generated every 212 microseconds with a reduced system top speed. This is using a 565,485 count per revolution encoder! If the encoder resolution is lowered by an order of magnitude, the time increases accordingly, and slow speed stability can suffer in an attempt to accommodate high speed requirements and/or standard off-the-shelf encoders.

Here's the dilemma: If the motion controller is located away from the action, and transient motion occurs due to fiction or some other force, the stability of motion will suffer. By implementing an analog tachometer as an inner velocity loop, the drive can be put to work assisting the motion controller to maintain system motion during the time the motion controller is not actively updating. What you have then is the best of both worlds' slow speed stability and high speed capability.

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I want to build a large scale RC boat using a RoboteQ 2550 controller and a 7.5hp outboard. I require two large dc servo motors w/pots to run throttle and steering. Anything big enough for one of the numerous full scale vehicle RC projects out there that will function with this controller preferably "plug & play" should work fine.

I have attempted to research what's in use on these projects but have found little outside the basic theory offered on the RoboteQ site in regards to closed loop position mode. I would love to read any direct hands on "Do it this way" kind of primers that anyone could point me towards. Any help is greatly appreciated

When about to undertake a motion control project/effort, it's always helpful to know something about motion control.

The ability to output a (remote) signal to a servo drive is actually quite easy. The main issue is really one of signal content. That is, digital, analog, modulated or unmodulated, communication language oriented, etc.

You indicate you want to use pots to control the steering and throttle. Why wouldn't you use a standard "plug and play" off-the-shelf" multichannel radio control transmitter/receiver system. Even if it is multichannel, you only have to use the channels you need. Also, throttle and steering should be modified by a trim control. It is very rare that a servo will be perfectly balanced from start to finish. In addition, I know that my boat motor can reverse the prop, allowing me to backup. Will yours?

If it were me, I would find out what type of output the 'off-the-shelf' radio receiver uses to control the servos, and then either interface them directly to a digital or analog 'off-the-shelf' servo if that is the case, or, use a microprocessor development board to interpret the radio receiver signal outputs and present them to the servos. I suggest you look at PIC and Silabs. They have ready to program/run development boards that cost between $50 and $200 (the Silabs C8051FX20-TB is $150). The next question is, do you have a scope, and can you program?

In this way, you can get the best of all worlds . . . . 'Off the shelf' hardware, minimum time to completion, soft integration requirement, minimum programming, and more.

Check the net for details on radio control receiver output signals, and motion control companies like Kollmorgan, Compumotor, Galil, Delta Tau, Acroloop, Danaher, and others. There is a lot of information available, but you need to understand how to put it all together.

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I am new to motion control. This is a question that confused me. In my system, the closed loop bandwidth is about 4K Hz., but I noticed that the mechanical engineers are working hard to push the mechanical resonant frequency as high as 35K why?

How can I judge if the resonant freq is safe ?

There are many explanations as to why mechanical resonance should be as high as possible in a motion system, and I'll try to present a few here.

Systems in motion are constantly being bombarded by shock waves that are generated by motors, gear boxes, belts with teeth', wheels and bearings against the surface(s) they ride on, electronic control signals, and the like. If I were to take a small rod (metal or otherwise), or even my finger, and tap it against a part on a system, would a vibration occur, and at what frequency? In addition, would that vibration create a sympathetic frequency that would vibrate another part on the system? To make the mechanical resonant frequency as high as possible can never hurt. The question is, however, how high is enough, and also, depending on the application, will the vibrations that occur even be an issue?

I'll put a couple of simple questions to you . . . First, how rigid do machine tools (CNC's) need to be? And Second, in the film industry, when positioning film for exposure or printing, how important is it for the equipment to be motionless' (when exposure or printing takes place), and how fast do you want position film frames (remembering that throughput is money)?

Additionally, when operating a stepper system, if the vibration content of the system were to hit a frequency harmonic of the stepper step oscillation' that occurs while the stepper is stepping', the stepper motor could start turning in the opposite direction, even though it is being commanded to go the right way!

And finally, if the system were using a Laser to perform solid modeling tasks, system vibrations could produce wavy lines instead of straight ones.

There are many reasons for insuring systems are properly damped, but the bottom line is, it enhances product production and extends machine life. A good starting point for system resonance design, is a minimum of five (5) times the calculated or measured motion control vibration frequency to be encountered.

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Hello, my name is Luke,
I graduated from Anoka tech last Spring, and I'm interested in starting my own business. You stopped by class a couple of times and you seemed pretty interesting. I'm looking for some advice in starting some type of electronics business.
Thank you for your time,

Hello Luke,
To answer your question, you need to figure out what your strengths are . . . then what you can do for one or more industries.
For example . . .
Can you program PC's, PLC's, embedded controllers . . . i.e. are you Software capable?
Are you good at system integration, be it Hardware or Software?
Are you good at troubleshooting equipment/Systems . . . i.e. Hardware?
How fast can you pick-up on new ideas and technology?
Are you without fear to sell yourself or your ideas?

Basically, what do you like to do, what do you want to do, what do you enjoy doing?
Industries are many, and you have an opportunity to make good money if you can get a good understanding of your technical capability.

Then, you need to decide if you plan to stay in the state you current are living in.
There are many out-of-state and in-state opportunities as well.
Check out the web for the jobs in the industries you're interested in . . .
Then, knowing what they're looking for, give the main engineer (Plant Engineer, Manager, etc) a call to see if you can assist them with their need as a contractor until they find a permanent employee . . . By the way . . . make up a business card and perhaps a one page 'flyer' indicating your capabilities. Don't put a price on it, but have your costs ready when they ask.

When I first started out, I got with an electrical contracting company and offered them a technical service they didn't have.
They were then able to offer that to their customers to expand their business.
I charged them 25% more than they were paying their staff electricians, and they were very happy to have the capability I was able to provide for them.

Go to local equipment and electrical shows, and get to know people . . . perhaps even write an article or two about something you see need for and can product (but don't give away too much information).
Get a website going - - - good or bad, it's your visibility that counts.
I'm not sure if any of this helped you out, but don't be fearful of just getting a job, then working your way into contracting as you grow in knowledge and ability.

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Generally control is always in the need of a simple, low cost method for turning ON and OFF a control or other device operated by a low current battery. The questions really becomes 'Can a circuit, operating on a low current battery (110mAhr), not draw any power from the battery when not in use?

The following link is to a paper describing that circuit . . .

                             Push Button On-Off Power Control circuit  

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Can you give a definition for the term 'Motion Control'?
Motion control can be defined as: Controlling an object in Three Dimentional Space in which there are other Moving or Non-Moving objects.

Note that motion control has nothing specifically to do with electronics or mechanics, but only to the fact that something in motion needs to be controlled!

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