Mechanical decoupling between motor and load
Decoupling happens when a section of the mechanical linkage changes in a way that causes the motor to sense variations in the inertia of the load. Examples of decoupling include twisting of a shaft, flexing of a mechanical coupling, elasticity of a timing belt, and gear backlash.
As the gain of the speed controller is increased, the drive's commands and the motor's responses become more rigid or synchronized. The stiffness of this command-response between the drive and motor is very similar to mechanical stiffness. In fact, once this drive-motor stiffness surpasses the stiffness of any of the mechanical linkages, those linkages decouple. The image below shows three separate decoupling events. The 29-53Hz valley-peak is one, the 180-210Hz valley-peak is another, and the 320-350Hz valley-peak is the third.
This Bode plot shows three separate decoupling events.
As the frequency increases logarithmically from left to right, a valley is observed at 29Hz. This is known as the natural or locked rotor frequency. At 53Hz, a peak -- known as a pole frequency -- is shown. If these were the only peak and valley in the entire Bode plot, this system would be known as a two-mass system, where the two masses would be the inertia of the motor's rotor and the inertia of the load. The plot line below 29Hz would represent the characteristics of the motor, and the plot line above 53Hz would represent the characteristics of the load. The plot section between 29 and 53Hz represents the decoupled region. The drive is unable to control these frequencies, so ideally it is best if these decoupled frequency regions are kept to a minimum.
If you are having trouble visualizing the concept of decoupling, perhaps this analogy will help. Imagine you have your hand outstretched holding one end of a rubber band. At the other end of the rubber band is a one-pound ball hanging from gravity. If you gently move your hand up and down, you will sense variation in the load as if the weight of the ball were changing. When the band is being stretched, the mass seems higher, and when the band is contracting, the mass seems less. This is similar to what the motor experiences when a shaft twists or a coupling flexes or a belt stretches. These changes are linear and don't seem so abrupt to your senses. However, gear backlash is nonlinear, and the above analogy is not adequate.
To imagine gear backlash, we start with the same scenario of a one-pound ball at the end of a rubber band. However, this time, the rubber band is being cut so that we instantly sense a change from one pound to zero. Our hand might actually jerk up for an instant until we adjust our arm muscles to the fact that there is no longer a weight to hold up. Just as we adjust to this no-load condition, the band is magically restored to its original condition, and we instantly sense the one-pound ball again. This time, our hand might drop down until we adjust our arm muscles to compensate for the new weight.
No matter if the decoupling is linear or nonlinear, the result is the load seen by the motor shaft changes. Stability of a drive controller exists when the gain of the speed controller properly matches the inertia of the connected load. When sections of the mechanical load decouple, the motor shaft senses less inertia. The controller gain is no longer properly matched, because the perceived inertia is less. If enough of the load decouples, the gain-to-inertia ratio can reach a level that creates instability. Nonlinear decoupling (gear backlash) is the worst type of decoupling, because the perceived inertia value changes so drastically.
Marcus Schick is the industry business developer for the motion control business of Siemens Industry Inc.
Good points about adaptive gains, tuning and resonances. I've often seen that the effect of mechanical resonances and changes over time are not taken into consideration when tuning a servo. The problem is that nine times out of ten you can get away with it, so the tenth time seems like a "mystery" when it occurs.
I have been thinking more about this rubber string analogy and there was an error in my earlier post too.
The system consists of two masses, the ball and your hand, and a spring between them, the rubber band. Your muscles provide the force to your hand that is moving it. In a servo drive the force is the air-pap torque, the hand corresponds to the inertia of the servomotor's rotor and the ball corresponds to the driven load. The rubber band corresponds to the shafts and the coupling between the motor and the load.
Now when you slowly use your muscles to move your hand up and down the ball will follow. When you increase the frequency of the movement you will find that it becomes more and more "stiff", that is, more difficult to move your hand although the ball is moving up and down. This is the anti-frequency resonance where it is difficult to get the hand tomove. By increasing the frequency further it becomes again easier to move your hand and the amplitudes of the hand and the ball oscillations increases a lot when the frequency is approaching the resonance frequency. Above the resonance frequency the ball movement decreases and your muscles are moving mainly your hand.
The anti-resonance frequency makes it difficult to control the motor movement near and above it and thus limits the dynamics that is possible to be achieved. Note that the anti-resonance frequency is defined only by the torsional stiffnesses of the coupling and the shaft and the inertia of the load. Thus the servo control system cannot improve the control response beyond that. As a rule of thumb the shortest possible response time of the speed control is the inverse of the anti-resonance frequency.
Reduction gear is often used with servo drives. The reduction gear decreases the ioad inertia seen on the motor shaft. Thus in practice the anti-resonance frequency is important only for servo drives that do not have reduction gear.
Don't forget friction! Friction is a nasty thing that makes accurate position control difficult. Slip stick itself is one of the reasons for growling at low speeds. However, sometimes a dither (vibration) signal is added to the torque reference in order to keep the mechanical system in a small movement all the time. This avoids the slip stick phenomena but unfortunately has audible noise as byproduct.
By the way, there is a slight error in the explanation of the Bode plot (apparently this is a transfer function from motor air-gap torque to motor speed). The plot line below about 20 Hz describes the motor and load moving together as one piece. Around 29 Hz only the motor moves but not the load (this frequency is also known as anti-resonance frequency). Between 29 Hz and 53 Hz the load is more and more accelerating when the motor is decelerating and vice versa thus finally reaching the resonance at 53 Hz. Above 53 Hz the plot shows more and more only the inertia of the motor.
You can check this with the rubber band and ball. If you move your hand very slowly up and down, the ball will follow. When you increse the frequency you will find there is a frequency where your hand moves but the ball does not move. This is the anti-resonance frequency. Increasing the frequency still you will finally get into resonance frequency where the ball is moving very much even when your hand movement is small. Increasing the frequency yet higher (if you can) you will find that the ball is again more or less standing still although your hand moves a lot.
Wish I could afford a driver like this one. I accidentally hit the end of travel on one of my servos on my CNC mill. The momentary stall ended up letting all the "magic smoke" out of one of my servo drivers. A driver that would properly compensate for drive error like the above one would be most handy.
As long as a servo drive permits access to and real time changes in its gains, then it's not strictly necessary to have a modern servo drive in order to take advantage of the concept. The system controller could change the gains and feed them to the drive. It's not as elegant as a self-contained drive, but it does permit use of this neat concept.
Marcus, thank you for an informative article. I was just at a seminar given by a semiconductor vendor on a new microcontroller targeted at the motor control market. These incorporate motor control timers as well as fast A/D converters. All of these are built in to the SoC, so that the measurement and correction strategies you discuss can be implemented.
An analysis of what’s needed to implement Design for Disassembly and Design for Recycling results in eight strategies engineers can use to design an intentional end-of-life stage into their products.
Government regulations, coupled with growing consumer sensitivity about data and identity theft, require that data storage organizations demonstrate proper protection and due diligence in protecting sensitive information stored inside datacenter enclosures.
When a crane doesn't have a monitoring system, crane owners schedule service every six months and simply scrap the parts they replace, even if a part has had little use and doesn't need replacing. This can cost thousands.
From Dell / Intel® New Paradigms in Design Work Scott Hamilton, vertical market strategist for Dell Precision workstations, 5/2/2013 3
Early in my career, I worked as a draftsman and remember the days of drawing on vellum with numbered pencils and Mylar with plastic lead. This was a fun experience in the sense that I ...
I've been using workstations for more than 10 years and love finding ways to get more performance from my system. With demanding professional applications that require more power each ...
A lasting memory from my first job as an engineer in an auto assembly plant is standing on hard concrete at six in the morning, vending-machine coffee clutched in hand, listening to ...
A quick look into the merger of two powerhouse 3D printing OEMs and the new leader in rapid prototyping solutions, Stratasys. The industrial revolution is now led by 3D printing and engineers are given the opportunity to fully maximize their design capabilities, reduce their time-to-market and functionally test prototypes cheaper, faster and easier. Bruce Bradshaw, Director of Marketing in North America, will explore the large product offering and variety of materials that will help CAD designers articulate their product design with actual, physical prototypes. This broadcast will dive deep into technical information including application specific stories from real world customers and their experiences with 3D printing. 3D Printing is
To save this item to your list of favorite Design News content so you can find it later in your Profile page, click the "Save It" button next to the item.
If you found this interesting or useful, please use the links to the services below to share it with other readers. You will need a free account with each service to share an item via that service.
Tweet This
9 
