Best way of getting an analog readout on linear/rotational movement - Page 1
Forum section: electronics / projects

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 16, 2020, 04:42:04 pm
content: Hi all!I have a little side project in converting a diffraction grating UV-vis spectrometer.The grating, as seen in picture 1 was rotatable with a motor, and I want to control it via a knob (or possible beter motor in the future) and have a proportional voltage out (to make quick, simple vis spectra with a X-Y recorder).The mechanism for turning the grating is seen in pic 2. the axle rotates a screw that translates forwards and pushes on a brass cam, that in turn rotates the grating.If I wanted a voltage out proportional to the rotation, what would be the best approach?mounting a potentiometer directly on the grating axle? having the screw push on a slide potentiometer?This is a quick project, so simpler is better.thanks for the look!

author: KT88
author_rank: Frequent Contributor
timestamp: September 16, 2020, 04:45:47 pm
content: That totally depends on your requirements...

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 16, 2020, 04:49:37 pm
content: Of course it does, as said it should just be 'pretty good' - I'd prefer it to be potentiometer based, I'm just unsure how to mechanically implement it with the least backlash/ slop.

author: KT88
author_rank: Frequent Contributor
timestamp: September 16, 2020, 05:06:27 pm
content: A coupler, similar to the one you used to couple the motor to the rod would be fine.If you don't have the highest accuracy demand heat shrink tubing (maybe multiple) could also work, if the axes are precisely aligned.There are potentiometers on the market for that purpose.https://www.megatron.de/

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 16, 2020, 05:23:53 pm
content: I see, so there is. If I can find them used, might be a good solution!This may be very 1950's, and a little on the Rube Goldberg end of the scale, but what if some dial string (very rigid) was tied to the cam, wrapped a few times around the axle of a 10 turn linear potentiometer, and then secured with a tension spring? There would need to be some friction on the potentiometer axle of course, maybe heat shrink tubing.A potentiometer shaftB threadC tension spring

author: Gribo
author_rank: Frequent Contributor
timestamp: September 16, 2020, 05:28:45 pm
content: Depending on the accuracy, I would use a shaft encoder.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 16, 2020, 05:41:39 pm
content: that's an option, but it would require some amount of circuitry to obtain a voltage out, like this:

author: TimFox
author_rank: Super Contributor
timestamp: September 16, 2020, 06:05:12 pm
content: How many turns of the shaft do you need to encode?

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 16, 2020, 08:28:49 pm
content: About 8 turns.The string thing actually works remarkably well, until a better solution can be implemented!

author: TimFox
author_rank: Super Contributor
timestamp: September 16, 2020, 09:30:17 pm
content: If less than 10 turns, why not a 10-turn pot?

author: viperidae
author_rank: Frequent Contributor
timestamp: September 17, 2020, 05:59:09 am
content: You could glue a magnet on the pivot of the brass bit and use a hall effect rotation sensorhttps://au.element14.com/bi-technologies-tt-electronics/6127v1a60l-5/sensor-hall-0-2v-20v-to-10v-60deg/dp/2319671is an example

author: ace1903
author_rank: Regular Contributor
timestamp: September 17, 2020, 07:16:31 am
content: Can't see well but I think that you have DC motor with embedded optical encoder. I have motor that looks similar and it has optical encoder already mounted.If you have same model, you can modify encoder cover to add another coupling behind encoder and drive motor shaft by hand by rotating shaft attached to that coupling.I used STM32F103 to count pulses and to display position and small L298 h bridge to drive motor to desired position.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: September 19, 2020, 03:49:13 pm
content: Once upon a time, I had the task of controlling the rotation of the outdoor antenna from a computer. I put a mouse in the antenna drive, pressing the wheel to the motor axis, and connected the buttons to the limit switches.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: September 20, 2020, 02:37:44 am
content: I would try a ten-turn Hall effect potentiometer.

author: David Hess
author_rank: Super Contributor
timestamp: September 20, 2020, 09:52:01 pm
content: Potentiometers intended for use in angular and linear position sensing are available and work very well.

author: KT88
author_rank: Frequent Contributor
timestamp: September 20, 2020, 10:40:29 pm
content: @ ChristofferB: I like your implementation because of it's over all simplicity.I would use most likelyuse a larger pulley on the pot and stay with a single-turn. This causes less drag and would give better lon-term repeatability (less or no slip - the string could be attached to the pulley). Together with a larger diameter pot you won't see little if any disadvantge in accuracy over a multi-turn at probably lower lower cost...CheersAndreas