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There is an interesting article here
http://www.analog.com/en/content/0,2886 … 38,00.html
about oob control using an accelerometer device as used in car airbag systems.
Andy 😀
Interesting indeed. Clever stuff for sure- I’m continually amazed by the amount of logic built into these white boxes…
For years we’ve been using various designs of accelerometers and deflection sensors to detect OOB in lab centrifuges. These are either direct drive motors, or sometimes geared up above motor speed, and are running at up to 30k rpms- little chance to measure rotational speed there…
Basic stuff just trips a microswitch on the chassis if motor mount deflection is excessive, others have used a pendulum and bob-weight to make a contact, modern stuff tends to use a piezo vibration sensor.That is interesting
does the technology work with series wound motors as well as induction motors? since the induction motors are controlled by wave chopping and phase staggering as opposed to tacho pulses that give the velocity ripple that they talk about.I reckon that the accelerator used by airbags could be very fallible when used on a washing machine, that is not just relying on clothes being balanced, but also the speed of water vacating the tub and also the weight of washing, all of these factors will make the washer draw more current when trying to spin, even a dicky suspension leg could stop it spinning using this technology
It sounds too good to be true…can we by pass it?..lol
I was talking to a guy the other day who reckons that nokia is in cahoots with a major domestic appliance manufacturer and they are making units that will be in washing machines that will ring the manufacturer with a fault code when it goes faulty, so if they can get your phone number at point of sale, they can ring you up and tell you that your washing machine needs a service call.
they are supposibly trying to bring it into price range like the iMEMS accelerator for airbags.but the velocity ripple will be at drum speed, not motor speed, so easy enough to discriminate.
Wow rather than flutter, in audio parlance…Re: Detecting out of balance condition
If you noticed this paper was presented in 1999 and i don’t think that these things have appeared on any machines as yet except possibly some more expensive ones?
The accelerometers used on the washers will be of the low g type so not exactly the same as the airbag ones.
As dpm says the device senses tub vibration so really is independent of motor type etc. There are several types of these devices. The analogue peizo types have a crystal sandwiched between a weight (mass) and the accelerometer case which is connected to the vibrating wotnot (the tub in our case). Because of the weights inertia the crystal is subjected to pressure/stretching as the tub moves and consequently produces a charge output proportional to the acceleration (vibration). Another type uses a peizo resistive material producing a resistance that varies with the pressure.
The devices mentioned in the paper also have the signal processing included on chip and can produce a digital output suitable for direct connection to the microcontroller.
You would think that this would provide enough OOB info without any need for motor current sensing etc as it gives a direct reading of exactly what the tub is doing. One of the things that would present a problem i would think is the transverse sensitivity of these peizo devices when used on a wm tub. We know that the movement of the tub is not just in on direction (up and down) but follows a circular motion ie. it has a side to side motion too. This would result in the crystal being subject to shear forces as the weight resists movement from side to side, these shear forces would produce an erronous charge that would be indistinguishable from the true one. I suppose it would be possible for one signal to cancel out the other.
No doubt they will have overcome this somehow possibly by using two crystals??
Andy 😀
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