Resonances in a scanning mechanism
Resonances in a scanning mechanism
(OP)
I am trying to understand the resonances in a scanning mechanism being developed where I work. The mechanism is driven by a stepper motor via an offset bearing and pushrod to give a reciprocating motion of a few degrees. The stepper motors are only of the order of 2 watts, so we are not talking about a large mechanism here.
The existing mechanism gives clear resonances as heard, and seen on a sound meter. With the sound meter a few inches from the mechanism the resonant level is up at 90dB compared to the non-resonant level at around 74dB. Hence the resonances are very pronounced.
Talking in terms of pulses per second to the stepper motor (pps), I can get clear resonances at around 250pps,500pps,750pps,1500pps. There are others at lower speeds as well.
Should I deduce that the resonance is actually at the 1500pps rate and the others drive the 1500pps resonance due to the harmonics in their rectangular pulse shapes?
This is a real problem for us. We have just spend loads of money with a contract mechanical design house to get a smaller mechanism made. This is half the size and weight but resonates in a similar way to the previous mechanism. Evidently the same mistake has been made again. What should I be looking for? (other than competent mechanical designers!)
My brand new mechanical design book on dynamics and statics has very little to say about resonances.
For me I think that this full metal design has loose couplings and no damping. The stepper motor pulses kick the mechanism hard and the whole mechanism is available to ring like a bell at certain frequencies. I therefore feel that some damping is needed but I can’t put my finger on where it should go. For example, if I put rubber mounts on the pushrod to the mirror the worry is that the position of the mirror will be uncertain (wobble). Is there such a thing as an adjustable damper so the response can be tuned up rather than calculated?
The existing mechanism gives clear resonances as heard, and seen on a sound meter. With the sound meter a few inches from the mechanism the resonant level is up at 90dB compared to the non-resonant level at around 74dB. Hence the resonances are very pronounced.
Talking in terms of pulses per second to the stepper motor (pps), I can get clear resonances at around 250pps,500pps,750pps,1500pps. There are others at lower speeds as well.
Should I deduce that the resonance is actually at the 1500pps rate and the others drive the 1500pps resonance due to the harmonics in their rectangular pulse shapes?
This is a real problem for us. We have just spend loads of money with a contract mechanical design house to get a smaller mechanism made. This is half the size and weight but resonates in a similar way to the previous mechanism. Evidently the same mistake has been made again. What should I be looking for? (other than competent mechanical designers!)
My brand new mechanical design book on dynamics and statics has very little to say about resonances.
For me I think that this full metal design has loose couplings and no damping. The stepper motor pulses kick the mechanism hard and the whole mechanism is available to ring like a bell at certain frequencies. I therefore feel that some damping is needed but I can’t put my finger on where it should go. For example, if I put rubber mounts on the pushrod to the mirror the worry is that the position of the mirror will be uncertain (wobble). Is there such a thing as an adjustable damper so the response can be tuned up rather than calculated?





RE: Resonances in a scanning mechanism
Finding resonances in mechanical systems is quite hard, particularly if the system is very small compared with the wavelength of the problem. If you are lucky it will be a structural resonance of one part, in which case you can redesign that part with a different resonant frequency.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Resonances in a scanning mechanism
Should I deduce that the resonance is actually at the 1500pps rate and the others drive the 1500pps resonance due to the harmonics in their rectangular pulse shapes?"
If you have done a sweep and have confidence those are the only resonance within that range, my personal opinion is that you have come to the right conclusion.
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RE: Resonances in a scanning mechanism
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Resonances in a scanning mechanism
When the current pattern in the coils changes, if there is enough current, the rotor changes position almost instantly. For a size 23 stepper, typically about a millisecond. For any given stepper and load combination, there is really only one step rate where it just cleanly goes to the next position and _doesn't_ ring.
A book and a thesis about "Phase Plane Analysis of Stepping Motors" or some such came out of the University of New Hampshire some years back. I think the authors are Taft and Gauthier. The material is opaque at first, but concentrated study provides a new understanding of stuff that is covered only empirically elsewhere.
You may be able to make some improvements with mechanical damping and/or phase current shaping, but I think if you want the mechanism to be able to scan at anything other than one rate defined by the mechanism itself, e.g., rate sweeping or arbitrary rates, you are using the wrong mechanism.
Start looking at voice coils.
Mike Halloran
Pembroke Pines, FL, USA
RE: Resonances in a scanning mechanism
I agree with Greg that the harmonics are fishy and I will check the other frequencies more carefully if I get the chance. I am sure there is at least one more set of resonances below 250pps but I didn’t measure those with such interest.
Hopefully Hartog will arrive in a few days and will be enough to answer a few basic questions.
Is it reasonable to suppose that a gap in a bearing/pushrod will resonate at a lower frequency if the gap is bigger?
RE: Resonances in a scanning mechanism
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Resonances in a scanning mechanism
What is your objective? Is it to remove the noise or is it to remove the ‘resonance’?. If this is a variable speed/excitation system, then you need to ensure that all mechanical natural frequencies are above or below the minimum or maximum frequencies of excitation – not easy (otherwise any changes you make are likely to just chase the ‘resonance’ to another frequency in the operating range).
If it is safe to do so, maybe you could run the mechanism at a ‘resonance’ for an extended period of time – this should result in a fatigue failure pointing towards the ‘problem’ component. If you don’t get a fatigue failure after a certain amount of time, then you could ask the question ‘do you have a problem that needs to be solved’?
I don't know if it is 'doable' but you might want to consider mounting the entire mechanism on a shaker table (especially if you can make a jig to allow shaking in x, y and z axes): fire up the table at the frequencies of interest and carry out a detailed visual examination (with no power applied to your motor) using a strobe that is slightly detuned from the excitation frequency. If you really do have a resonance, you might be able to ‘see’ the resonant component moving relative to other parts of the jig.
RE: Resonances in a scanning mechanism
http://www.machinerywatch.com
RE: Resonances in a scanning mechanism
TPL, the idea is to reduce the noise primarily but an undamped resonace will additionally give a scanning error which would be difficult to isolate.
Turns out the scope didn't have FFT capability. The other one does but it is not real time (takes 10 seconds to calculate). I have hired an analyser which should arrive the day after tommorow.
I'll have to look and see if I can get a small accelerometer as well. As I recall I went for a microphone rather than an accelerometer because a three-axis accelerometer is more complicated(?)
RE: Resonances in a scanning mechanism
http://www.machinerywatch.com
RE: Resonances in a scanning mechanism
The secret seems to be in how you drive the stepper. If you pulse it brutally it will behave rather like an impact air wrench, and behave accordingly. Trying to drive something with an impact wrench is never going to be a silent or a gentle process, no matter how the resonances are carefully rearranged, hehehe.
You need to research "micro-stepping". Instead of just switching the motor windings hard between supply and ground, they need to be driven with sine waves originating from a pair of digital to analog converters. The motor windings can be driven with sine and cosine waves (displaced ninety degrees). Motor torque will be really smooth, and the stepper will function all but silently.
It can also be driven to a position and held, in between "steps", hence the term micro-stepping.
I eventually bought a commercial XY plotter, but the learning experience and the challenge made the whole protracted drama well worthwhile.
RE: Resonances in a scanning mechanism
RE: Resonances in a scanning mechanism
Microstepping digitally is a cheap option that doubles the number of available motor steps by simultaneously energuising pairs of motor windings, but it is not really true microstepping.
With a three phase mains connected induction motor, you have three sine waves energising the triple windings phased 120 degrees apart. Because of the characteristics of sine waves this produces a true smoothly rotating magnetic field. No jumps or steps, smooth torque, and no noises.
Here is an application note how to digitally drive stepper motors with sinewaves:
http://w
RE: Resonances in a scanning mechanism
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Resonances in a scanning mechanism
RE: Resonances in a scanning mechanism
A resonance at 167Hz gets stimulated at 167pps but when pumped by 334pps the 167Hz still gets excited. I have even seen the 167Hz come up when driven at around 1400pps. Since I am half-stepping the motor I suppose it is reasonable that the divided-by-two sub-harmonic is present.
I did try tapping the mechanism as suggested to see where it rings, but there seem to be so many excitable elements that the situation is far from simple. At the lower pulse speeds the response has so many harmonics the response looks like equal amplitude harmonics in a comb.
Since hitting the stepper hard makes it ring hard I turned the voltage drive down so the current ramps up more slowly (due to the stator inductance). Reducing the drive voltage from 40V to 15V knocked the resonant sound level down by 5dB, a good improvement.
RE: Resonances in a scanning mechanism
You can make some improvement by shaping the coil current rise and fall, especially damping the induction spike on turnoff. Microstepping takes that a step farther by sinusoidally modulating the coil currents to achieve smoother motion. Modules to do it are getting cheaper all the time.
Mike Halloran
Pembroke Pines, FL, USA
RE: Resonances in a scanning mechanism
http://w
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.