Angle of elevation detector for "swinging arm".

[langle_j]

I have recently been given the task of displaying the angle of which an swinging arm is elevated by, to be read on an electronic display.

I have a fairly small amount of knowledge surrounding this area of expertise and would really appreciate som guidance for where to get started.

Some other information. The arm in question is aound 1.5m long and when in the vertical (stable) position is about 1m off the ground. Whatever device is used to measure the angle cannot be mounted on the arm itself. There is a small winch contraption used to raise the arm to its desired point of release and there is sufficient space for the devise to be mounted onto the release mechanism.

I was thinking about using an echo sounding devise to measure the distance to the floor but i do not know how to convert that distance into degrees on a electronic display.

Any and all help would be great, as i am at a bit of a loss what to do next!

P.s i am an apprentice so cost will be an issue i.e. i have some money to spend but not a huge amount (around £100 all in)

thanks

 

[duffy]

If you are not allowed to touch it, then ultrasonics might be your best bet.
Sparkfun sells several inexpensive detectors that might help -
https://www.sparkfun.com/products/639

It gives you RS232, Analog, and PWM outputs (you choose which you want to use), works from 0 to 6.5m, costs $25 USD.

 

[crutschow]

It would be easiest to use a PC or microprocessor to calculate the angle from the distance measurement.

 

[cowboybob]

Could the angle detection method (say, a potentiometer) be attached to the axle the arm swings on? That would allow a rotational variable to be applied to a simple metering system (even just a voltmeter) which could be calibrated to read angle.

 

[user_88]

... just curious. What sort of angular resolution do you require?
If you can abide by 5° increments, attach a one inch diameter rope to the end of the arm, and then place contrasting marking tape at appropriate intervals, using a protractor to establish corresponding angles. The marking tape could be detected by a photo diode/LED arrangement, and then connected to a simple counter. The marked rope is routed through a freewheeling pulley, positioned in the vicinity of the winch to allow its movement ... as the arm rotated.
As the arm rotates from zero to its final position, the counter would register each time a marker passed by the photo-pick-up device.
The 1.5 meter arm length would allow something like two meters of length that would require markers ... of some sort.:
S = θ *R = (∏/2) * R = 1.57 * 1.5 m = 2.35 m
It may be possible to place markers on the rope at intervals corresponding to 1° of rotation, if you require a higher resolution.

The one drawback of this scheme is that you will have to have a means of signalling the digital counter as to whether you are raising or lowering the arm. If you are raising the arm, the count should increment ... if lowering it should decrement.

The benefit of this scheme is that the counting output signal is the input to a simple two digit counter with an appropriated readout. A μ-processor is not required. True ... some innovation would be required in marking the measure rope, and designing a reliable, long-lasting mark reader.

... reviewing the original problem description, it sounds like you could attach the length of marked rope to the arm at the same point as the winch cable. When the lift cable is released, the marker rope would just swing free. ... If the winch cable could be marked at the appropriate increments, you might be able to use that instead of a separate rope.

... Can you provide any other useful information about the mechanism, or its purpose? The constraining assumption made in this suggestion is that the winch attachment point is at the extreme end of the arm, not at its rotational axis.

 

[ljcox]

I don't fully understand your limitations, so I'll have it to leave you to consider the practicality of this suggestion.

I would use a Johnson code pattern printed on a transparent film.

It could be either circular, or if that is not practical, linear.

In order to read the code (& therefore the angle) you could have some form of illumination behind the film & photo diodes, photo transistors or LDRs on the other side to detect the light coming through the film.

You would have to decide how many bits are necessary based on the accuracy you need.

The Johnson code is devised such that only one bit changes at any transistion, so there is no ambiguity.

There are various ways that the device could be realised, the above is just the basic idea.