How to track motion of a moving ovendoor

Question

At work, we are measuring various parameters to test out a newly designed oven prototype. To make things more efficient, I am trying to create an electronic setup to automate the measurement, in particular of the door's motion (from fully open to fully shut).

The goal is in measuring the instantaneous velocity of the door at the various points of its motion, which is a fixed radial path. Note the emphasis on instantaneous velocity logging, as opposed to an average velocity as determined by timing alone.

What is a way I can measure the ovendoor's velocity / track its motion with reasonable accuracy? (Reasonable implying error < 1-2 cm/s)

Since it is only fixed radial motion (and no component along any other axis), and since there is a moving component and a stationary component, I presume this should be an easier problem than, say, measuring the velocity of an independent body like a person or hand.

My first thought was an accelerometer-gyroscope-IMU-based method, i.e. integrating the acceleration over time to get the velocity, but reading up on this subject suggests that this will lead to sizeable errors.

Answer 1

One suggestion is to have a set of inexpensive low power lasers arranged in an arc above the upper plane of the door's rotation, pointing directly downwards. A corresponding arc of PIN photodiodes or phototransistors just below the lower plane of the door's rotation can be hooked up to GPIO pins of a microcontroller, to detect the light interruption of each beam in sequence.

I've made a rough illustration of the arrangement, using 5 degree sensing angles. This precision could well be brought down to 1 degree per sensor, if needed.

This 5 Volt 5 mW dot-type red laser would suffice (around $1 on eBay including international shipping):

For the sensor, a PIN photodiode such as the OP906 ($0.59 in single units from Digikey) can be used:

In your microcontroller code, the timings for the successive laser beam interruptions will provide a precise instantaneous rotation speed between any two points separated by 5 degrees, as the door is opened or closed.

The ADC sample rate, timer precision and processing speed of the microcontroller will determine the data precision achievable.

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One useful simplification of this design (thanks, @jippie, for getting me thinking in this direction), if precision is not super-critical, is to use a single narrow tube light, such as a 8 watt fluorescent or CFL tube, above the door, to replace the lasers. The light detectors would remain as they are, light to them would be interrupted in sequence as the door closes.

Answer 2

I am not fully aware of the context of this oven door; but I would like to contribute a suggestion.

How about a quality potentiometer at the hinge of the door? Consider this, you get both the position and the velocity of the door this way; this method is simple and robust. Obtaining the position of the door is trivial, through some testing and measuring, you could find what values the pot outputs at given inputs, or door positions.

Also, if you differentiate the position with respect the time (in practice, you can simply divide the rate of change in position you obtain over the rate of change in time), you would get the angular velocity. Then from there you can calculate the velocity the door is traveling at.

From the picture above, the angular velocity would be little omega. V is the velocity the end of the door travels. V, obviously obtained with angular velocity multiplied by radius.

Answer 3

I'm surprised that no one has mentioned using a string encoder or a string potentiometer It's a fairly simple and accurate solution. Basically a string is connected to a spring-loaded encoder and the encoder measures the string movement. There is very little moving mass and it doesn't require that the encoder shaft be colinear with the hinge or require any modification to the hinge such as gearing or belts.

Answer 4

If the primary movement is along a circular path, a single gyroscope should give sufficient accuracy. Would require some math to go from angular velocity to velocity of center of mass, but that's a one time calculation.

Basically, if the velocity is limited to angular velocity, a gyroscope+math can generally give you the instantaneous velocity.

The issue with IMU's and dead wreckoning type approaches is that over a long time you get compounding velocity error from integration of linear acceleration. This happens because there's generally no good way of generically determining linear velocity. This gets much worse when trying to determine position, accounting for rotational offsets of linear velocity, etc. etc.

Depending on the kind of testing you are doing, you can augment the IMU with a sensor for determining when the door is closed/at a stop position, then use that to re-zero the IMU. With that approach the IMU only needs to be reasonably accurate over the time it takes to open/close the door (presumably only a few seconds), which I believe is reasonable.