ultra low latency rotational position sensor - Page 1
Forum section: electronics / projects

author: max_torque
author_rank: Super Contributor
timestamp: August 31, 2016, 01:00:13 pm
content: I've been asked to identify the phase/positional offset vs rotational speed for a sensor that reads the position of a spinning wheel that spins between 0 and 20krpm.In order to do this i need to know the reference position of that wheel with high accuracy and low latency. The sensor we are bench marking is optical and hence already probably has a very low phase shift with increasing speed, but it still will have some as the trigger sweeps over the optical receiver, resulting in a slight difference in the time domain as to the point at which the sensor is "triggered". At low rpm, time delay causes a minimal position shift, but at 20krpm, where 1 uS is 0.12deg, it starts to become important (for our application)So, what other sensor could i use/make as a reference trigger for the wheel position? I need a position to better than 0.01 deg really at 20krpm, which is 83nS!!I'm thinking of perhaps an optical sensor with a very narrow beam width, which i might be able to make out of some optical fibres and some IR leds? (output will be into a scope, so can have a pretty low output power)

author: Kleinstein
author_rank: Super Contributor
timestamp: August 31, 2016, 04:49:08 pm
content: The idea of the optical sensor is already good - fast versions can be really fast. Making the beam narrow does not really help except having a sharp edge and thus good position resolution - but it does not help wit speed. For a small delay it is about making the receiver fast, just like fiber optics receivers. Here high intensity (e.g. a laser) helps as less amplification is needed. With a good model of the receiver part one might get an good estimate of the actual speed of the circuit, so that some of the delay could be compensated.

author: qwaarjet
author_rank: Regular Contributor
timestamp: September 01, 2016, 04:16:55 am
content: a capacitive sensor on the wheel might do the job. The Bently Nevada linehttps://www.gemeasurement.com/sensors-probes-transducers/proximity-probes/bently-nevada-3300-xl-series-proximitor-systemwith proper calibration might meet the specs you need.

author: Marco
author_rank: Super Contributor
timestamp: September 01, 2016, 05:04:58 am
content: Quote from: max_torque on August 31, 2016, 01:00:13 pmOptical gate with a fast photodiode, take the derivative with a RC filter, do zero crossing detection with a fast comparator? (Gated so that only the zero crossing after the signal has increased above X is detected.)

author: Ian.M
author_rank: Super Contributor
timestamp: September 01, 2016, 07:21:59 am
content: Maybe a setup like a mirror galvo could help. A tiny first surface mirror on the wheel, with a carefully collimated laser beam hitting it and an ultrafast photosensor two meters away guarded by a narrow slit. The mirror deflects the beam at double the angular rate of the wheel, so the spot will move 0.698mm for 0.01 degrees of rotation.

author: LaserSteve
author_rank: Super Contributor
timestamp: September 01, 2016, 04:43:19 pm
content: On most CT Machines, this is done by embedding a "Synchro" or Resolver transformer in the edge of the wheel, around its whole diameter. A reference phase is transmitted off the wheel optically and the synchro stator is fixed around the patient in the frame. It is then possible to interpolate the position of the wheel very accurately. The machine I worked on had a fine and course tracking loop and could resolve 0.01 Degree absolute.The way to emulate that is to place two rows of dots around the wheel (sine and cosine channels) plus a reference dot and read them optically or using capacitance. You then have a PLL that interpolates the position.For an optical lever, the issue is two things. Laser beams have a profile or fine structure, and most have a Gaussian energy distribution across the beam. So you need to pick your comparator/slicer levels very carefully. Thus your detector sees a sinusoid like waveform.Issue two is resolution. For a very fine, well defined location, typically multiple mirrors are used with a pinhole to analyze the reflected beam, the idea being that there is one and only one set of bounces thorough the array of mirrors (usually a "Figure Four" shape) that satisfies the geometric condition for a valid detection.You need one heck of a good absolute encoder, or a Binary or Grey code inscribed around the diameter.Steve

author: max_torque
author_rank: Super Contributor
timestamp: September 01, 2016, 07:38:23 pm
content: Interesting ideas chaps! The mirror/laser idea has got to be worth a short as it's pretty easy to set up, and with a relatively powerful laser diode i won't need a lot of amplification in the receiving diode. Using a visible wavelength diode will make it a lot easier to set up, just need to find a suitable small mirror or highly polished surface to use as the reflector!Increasing the radius of measurement will bring greater positional accuracy as well

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: September 01, 2016, 10:08:21 pm
content: My favorite reference, "Instrument Engineers' Handbook" suggests that for this kind of measurement good options are Stroboscopic Tachometer and/or Fiber-optic Stroboscopes. These work at speeds up to 1M rpm.

author: max_torque
author_rank: Super Contributor
timestamp: September 02, 2016, 12:26:14 pm
content: Quote from: Alexei.Polkhanov on September 01, 2016, 10:08:21 pmI want to get a rotational positional fix, not an average rotational speed from the wheel, hence the need for very high timing accuracy / repeatabilityAssuming that the max circumferential error possible is half the diameter of the sensing diode (assuming the beam width is narrower than the sensing diode), then using the galvo idea with the laser / mirror should be a good one. It is of course limited by the minimum response time of the photodiode and amplification system, where the further the receiver is away from the target wheel the better the angular resolution, but the shorter the pulse of photons is that needs sensing.With say a 3mm photodiode on a 2m radius, the resolution should be in the order of 0.043deg

author: Ian.M
author_rank: Super Contributor
timestamp: September 02, 2016, 01:04:10 pm
content: Don't forget the speed of light delay from the rotating mirror to the sensor + in the cabling (x whatever the velocity factor is, typically 0.66).As 83ns @ C is just under 25m, and the proposed beam length is 2m, with probably several meters of cabling, it could be a significant contribution to your angular error.

author: LaserSteve
author_rank: Super Contributor
timestamp: September 02, 2016, 02:22:13 pm
content: Large spinning objects are rarely stable in rotational speed, and smaller objects are usually worse. You need a minimum of two position datum per revolution. With two datum you can gate a time interval counter and gain accuracy, rather then measure one whole rotation and then correct later. With two pairs of datum points your angular accuracy climbs by a factor of four.. With continuous data in quadrature you can use a positioning chip and know very precisely where you are. If your processor is fast enough, you can interpolate.For example, LSI Computer Systems Inc. makes register chips with 10 Mhz quadrature inputs.Steve

author: David Hess
author_rank: Super Contributor
timestamp: September 05, 2016, 05:09:29 am
content: I do not understand why this would be a problem except for the sensor itself.An optical quadrature encoder and its decoding logic would need to operate at 12 MHz (I calculated the same number you did which is reassuring). For the quadrature decoding logic and counter to produce a parallel output of the position with a latency less than 10s of nanoseconds or even nanoseconds is trivial but I am not sure how you are going to use it in a timely manner. The photodiode needs to be relatively fast but 12 MHz is no problem.I think the most difficult part will be finding a quadrature encoder designed to operate at 20K RPM with the proper resolution simply because it is an unusual requirement.

author: CM800
author_rank: Frequent Contributor
timestamp: September 05, 2016, 08:33:11 am
content: Quote from: max_torque on August 31, 2016, 01:00:13 pmI would strongly suggest contacting Renishaw Encoders directly and discussing this with them, they will very likely have an answer as they are right up there as one of the leading manufacturers.http://www.rls.si/

author: viperidae
author_rank: Frequent Contributor
timestamp: September 07, 2016, 10:05:28 am
content: Quote from: max_torque link=topic=73437.msg1017320#msg10173202m radius and 20,000rpm is around mach 7 isn't it? How big is this thing that's spinning around?

author: amyk
author_rank: Super Contributor
timestamp: September 07, 2016, 10:46:22 am
content: Quote from: viperidae on September 07, 2016, 10:05:28 amThe only objects I can think of that can spin at 20k RPM are nowhere near 2m. If my calculations are correct, that's nearly 4.2km/s(!!!) at the edge and a 1g object there experiences a force of approximately 8760kg

author: max_torque
author_rank: Super Contributor
timestamp: September 07, 2016, 06:17:56 pm
content: Quote from: viperidae on September 07, 2016, 10:05:28 amThe only think spinning around is a beam of laser light! So luckily, as it doesn't have any mass (theoretical physicists can discuss this one !!) we are all good!The actual target wheel is only 50mm diameter, but i'm reflecting a laser off it, and by moving the sensor to a much greater radius, gaining angular resolution that way!

author: JS
author_rank: Frequent Contributor
timestamp: September 08, 2016, 03:01:30 am
content: Quote from: max_torque on September 07, 2016, 06:17:56 pmMatch 10 wouldn't be a problem for the image of a beam of light, as far as I care it could be traveling twice the speed of light and not brake any known laws of physics, and is not a problem to archive so if the distance is long enough. Point a laser to the moon, then swipe it across of it very fast. From side to side it's 3500km, let's say it's 350.000km from home, one tangent away it's 1/100th of a radian, I promised twice the speed of light, so it needs to take 1/100th of a second in going across, nice numbers man! 1rad/sec rotation of the laser pointer would do the trick. Only takes 10rpm to do it, and the IMAGE of the beam will be traveling twice the speed of light. They don't need to be photons, I could be throwing potatoes to the moon with a rotating machine gun and the potatoes will arrive to each end of the moon with 1/100th of a second to each other. I do need one heck of a potato guns to throw at least 100 potatoes a second and then arrive to the moon.Coming back to the topic, you need to calibrate your device, so you need a more precise device to characterize it, and if you find it's reliable enough using some correction you could use the one you already have. If you find the delay of it for example, you could correct for that and know the "exact" position of your thing a few ns ago. Using an interferometer may be the easier way to obtain a few points of very precise data to calibrate your sensor. I guess you can live with a small delay as long as is a known one, you probably wouldn't get much done in a closed loop situation in that order of time.For the mirror and long beam approach you don't need to account for the size of the diode, you could use a narrow (long) window and put the diode behind, you could get your sensor closer (than the moon?) and may be easier to calibrate. There is no sense in making the window a round hole, and you are interested in as much energy as you can get. Bigger diode, narrow window (slim rectangular sensor, not round) will give you the maximum energy (area) with the greater definition (length).JS