Turntable and rotary encoder

Hi all,

I have a motorised turntable, which is driven by a synchronised AC Motor, and I need to get angular position of the rotation. Because it was hard to hack electronic structure of the table I have decided to use rotary encoder.
But now I am facing another problem as you can see on the photo I cannot put my rotary encoder in the middle of turning plate because I have a screw there. I was thinking to put the rotary encoder in the side of the plate (fig 2) and to wear both the encoder and the plate with kid of a rubber or something else that would make the friction higher. Do you guys think that this would work if not dose anyone have another idea how to do it.

And if this is not the right place to ask this question where can I?

Best,
Ilir

Ooh, that encoder will be spinning for it's life! Can you instead put some optical marking on the outer rim of the wheel and monitor that?

Hi,

Thank you for you're fast replay, I could maybe but i do not know how exactly can I do it. But I have another idea to make like a root or canal in the turning plate and to put the encoder there, do you think that this can be done.

Best

If the plate is run by a synchronous AC motor, its rotational speed will be highly stable and accurate. You could probably get by with a little bump like a screw head or something on the bottom of the plate to operate a microswitch once per rotation. Then you can use millis() and compute the table position as a function of time since last switch transition. I think you could get within 1/2 degree, as long as the table is still plugged in and moving all the time.

You can't put the shaft directly against the turntable. You have to design a mount that holds the motor with a rubber wheel from and RC car (hobby store or online) on the shaft of the encoder so the plate with the hole in it that the encoder mounts on has the rubber wheel installed on the shaft after the encoder nut is tighted on the threaded part. The metal plate is a 1 inch wide strip of aluminum or thin steel mounted flush with the bottom coming out perpendicular to the table and then up and over with a right angle bend in toward the table so the encoder goes into the plate from the bottom into the hole with the shaft point straight up (perpendicular to the floor) and the nut screwed on from the top and then the rubber wheel mounted on the shaft so the wheel is in the same horizontal plane as the surface of the table and the encoder is below the surface of the table and to the side , shaft up.

I need to get angular position of the rotation

Hi, you say you need angular position, this means that the encoder will have to be connected to the turntable so that no slip is involved and one turn of the turntable = one turn of the encoder. OR do you want to measure angular speed or velocity? There is a big difference in the setup you would need. Also the type of encoder.

Hope to help.

Tom...... :)

You need to measure the diameter of the wheel attached to the encoder and calculate the ratio with the diameter of the table. The OP already stated that he cannot mount the encoder centered to the table so your suggestion won't work. If the wheel attached to the encoder is 3" in diam and the table is 12" in diam (for example , round numbers), then you don't have to calculate circumference because you automatically have a 4*:*1 ratio.

CrossRoads: Ooh, that encoder will be spinning for it's life!

Well it's a 2 r.p.m. turntable.

Sounds like a 3D laser scanner project.

Maybe print an optical encoder wheel to attach underneath the table and use a reflective sensor to detect.

Just google optical wheel encoder and there are a few programs that will print out one with about any degree of complexity.

Since this is just going in one direction no need for direction detection but need to sense a "start" timing mark.

I think gardners suggestion is pretty easy to test without a lot of hardware setup.

Thank you for you're replies, Gardners ide is nice but the turntable is not going to move constantly the people need to interact with it. Justone idea Is really cool and if I can not arrange somehow with ready encoder I will need to try you're idea to create an optical rotary encoder in the bottom of the table. Raschemmel ide seem to be the most possible even though I did not understand completely. Do you mean to make kind of canal in the side of the turning plate so the encoder wheel will fit on it ?

Thank you all

Hi, see reply #5, do you want to measure turntable angle/position or speed?

Tom....... :)

Hi Tom, I need to measure angel position. I need to get information of the table movement of around every degree.

Raschemmel ide seem to be the most possible even though I did not understand completely. Do you mean to make kind of canal in the side of the turning plate so the encoder wheel will fit on it ?

No, that's not what I meant. To get an idea of what I meant. Get a wire coat hanger and dis-assemble it.

Straigten it out .

Now lift the turntable up about 1/8" and slide 3" of the wire under it and then let the turntable down on top of the wire so it holds it there.

Now measure about 4 inches out (horizontally) from the turntable and make a 90 degree bend in the coat hanger to it points at the ceiling.

Now squat down and site across the turntable and make another 90 degree bend in the coat hanger TOWARD the turntable at the level about 1" below the surface of the turntable so you have a total of two 90 degree bends so far (one up, and the to the left toward the turntable)

Now cut the coat hanger about 2 to 3 inches from the edge of the turntable. If all the dimensions are correct, if you look at the coat hanger from the side and imagine that it represents the cross section of a 1" wide piece of aluminum or sheet steel , the same length as the coat hanger and with all the same bends in it. , then if you drilled a hole in the center of that 1" wide piece of metal at the place where it is closest to the turntable (but about 1" below the surface) and mount the encoder shaft pointing up, when you tighten the nut onto the threaded part of the encoder, you can mount a rubber wheel of the type used on RC model airplanes or cars onto the shaft of the encoder and the wheel will make contact with the turntable causing the encoder to turn smoothly. The proper way to do this is have a machinist make an adjustable base with a threaded lead-screw so you can adjust the distance of the encoder/wheel assembly from the edge of the turntable the wheel is rolling on.

justone: Maybe print an optical encoder wheel to attach underneath the table and use a reflective sensor to detect.

+1

A variation on this would be to print a strip that attaches to the edge of the turntable. It may be easier to install the photo-detector beside the turntable rather than under it. Som of the record turntables (remember them) used this technique to maintain the correct speed.

Remember that detecting the exact position will be a lot easier than making the turntable stop at a predetermined location.

...R

Robin2:

justone: Maybe print an optical encoder wheel to attach underneath the table and use a reflective sensor to detect.

+1

A variation on this would be to print a strip that attaches to the edge of the turntable. It may be easier to install the photo-detector beside the turntable rather than under it. Som of the record turntables (remember them) used this technique to maintain the correct speed.

Remember that detecting the exact position will be a lot easier than making the turntable stop at a predetermined location.

...R

I have a higher end Marantz turntable that works like that. The edges of the turntable have polished reflective surfaces between black non-reflective strips. Then there is a plastic module mounted looking at the strips spinning past, that has a neon light source and a photosensor detector that is used for sensing the speed and adjusting the servo controlled platter motor. It has the added advantage that just the 60 Hz lighting from the neon light source would give you a visual display of the strips such that they would 'freeze' still when locked to proper speed, but there is an pot knob you could manipulate to cause the record to spin a little faster or slower if you wished.

I would think the main thing is to KNOW WHERE the turntable is at and not just count pulses so you would also need a reference signal (pulse) to let the program have a known position.

As far as having an encoder being driven by the turntable over time there would rolling errors due to the mechanism not being a true gear train ( 300 teeth/30 teeth) but rather a not very accurate system ( a 10.0xxxx inch rolling another 1.00xxx wheel).

If you don't care at what position the turntable is at then why even have an encoder?

Thank you all of you it was really helpful. I will try all of the ideas . And just one the main idea is to find the turntable position.