Ultrasonic Z-height sensing? February 13, 2014 06:19AM |
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Re: Ultrasonic Z-height sensing? February 13, 2014 06:51AM |
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Re: Ultrasonic Z-height sensing? February 13, 2014 07:12AM |
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Re: Ultrasonic Z-height sensing? February 13, 2014 07:16AM |
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Re: Ultrasonic Z-height sensing? February 13, 2014 07:32AM |
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Re: Ultrasonic Z-height sensing? February 13, 2014 07:51AM |
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Re: Ultrasonic Z-height sensing? February 13, 2014 08:42AM |
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Re: Ultrasonic Z-height sensing? February 13, 2014 09:02AM |
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Quote
dc42
Instead of measuring the phase, which would require 2 transducers, I think it may be possible to rely on the fact that at 2mm or so range, the standing wave between the bed and the transducer will affect the impedance of the transducer, and hence the amplitude and phase of the signal across it when it is driven from a high impedance source. So we could look for a maximum or minimum in this signal, and calibrate the height at which this maximum or minimum occurs.
Re: Ultrasonic Z-height sensing? February 13, 2014 09:20AM |
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Quote
Radian
Quote
dc42
Instead of measuring the phase, which would require 2 transducers, I think it may be possible to rely on the fact that at 2mm or so range, the standing wave between the bed and the transducer will affect the impedance of the transducer, and hence the amplitude and phase of the signal across it when it is driven from a high impedance source. So we could look for a maximum or minimum in this signal, and calibrate the height at which this maximum or minimum occurs.
Now that's a darn good idea but it would have to have be a well-defined response such that you knew you were somewhere in the first and only cycle.
Re: Ultrasonic Z-height sensing? February 13, 2014 10:08AM |
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Re: Ultrasonic Z-height sensing? February 13, 2014 03:28PM |
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Re: Ultrasonic Z-height sensing? February 13, 2014 05:13PM |
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Quote
dc42
I just did a quick test on an ancient ultrasonic transducer I had lying around. The greatest difference in amplitude I was able to see between the first maximum and first minimum was about 10%. So I think this method may be feasible. However, the maximum and minimum are not sharp peaks, so it may be better to calibrate at the point halfway between.
Re: Ultrasonic Z-height sensing? February 13, 2014 05:45PM |
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Quote
Radian
Quote
dc42
I just did a quick test on an ancient ultrasonic transducer I had lying around. The greatest difference in amplitude I was able to see between the first maximum and first minimum was about 10%. So I think this method may be feasible. However, the maximum and minimum are not sharp peaks, so it may be better to calibrate at the point halfway between.
Right, but what sort of source impedance was driving the transducer? At close range I was expecting the reflected wave to have more than a 10% effect. I just tried driving a 5V square wave through 10K and get something like 30%. The first peak (constructive return) looks dead easy to threshold. If it was synchronously sampled it could easily be "ridden" to locate the maximum - within around 4mm of the bed. And just calibrating for the maximum looks as though it would be reliable enough to determine that it was in the first cycle. BTW this is with a £1.50 transducer.
Re: Ultrasonic Z-height sensing? February 13, 2014 06:25PM |
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Re: Ultrasonic Z-height sensing? February 13, 2014 06:56PM |
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Re: Ultrasonic Z-height sensing? February 14, 2014 08:13AM |
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Re: Ultrasonic Z-height sensing? February 14, 2014 11:59AM |
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Re: Ultrasonic Z-height sensing? February 14, 2014 12:15PM |
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Re: Ultrasonic Z-height sensing? February 14, 2014 01:00PM |
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Re: Ultrasonic Z-height sensing? February 14, 2014 04:19PM |
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Re: Ultrasonic Z-height sensing? February 15, 2014 03:29PM |
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Re: Ultrasonic Z-height sensing? February 15, 2014 05:14PM |
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Re: Ultrasonic Z-height sensing? February 15, 2014 06:20PM |
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Re: Ultrasonic Z-height sensing? February 15, 2014 07:01PM |
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Re: Ultrasonic Z-height sensing? February 17, 2014 08:14AM |
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Re: Ultrasonic Z-height sensing? February 17, 2014 03:26PM |
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Re: Ultrasonic Z-height sensing? February 17, 2014 06:25PM |
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Re: Ultrasonic Z-height sensing? February 17, 2014 09:14PM |
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Re: Ultrasonic Z-height sensing? February 18, 2014 10:11AM |
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This sounds very promising indeed. Maybe modulate the drive frequency at around 100Hz and 5% and take the maximum reading over each frequency variation cycle?Quote
dc42
The above tests were done with the bed cold (temp about 21C). I then tried heating the bed to 70C. This had the unfortunate effect of reducing the voltage across the transducer, throwing the calibration too far out even to use with a speed-of-sound correction applied. Reducing the drive frequency by about 3% restored the sensitivity. I was then able to re-establish a suitable calibration point (at 1.1mm this time, higher because of the lower frequency and higher speed of sound in the warmer air), and z-homing worked reliably again So I think the issue is variation of the resonant frequency of the transducer with temperature.
I'll think about how to compensate for this, and do more tests tomorrow.
Re: Ultrasonic Z-height sensing? February 18, 2014 04:02PM |
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