[noeltam75]

Hi all, this is my first thread, if i am not in the correct forum, pls redirect me.

I need some help on choosing the right rotary encoder for my slow speed, large diameter turntable, for angle control.

The turntable is 1 meter in diameter, and I can only add the encoder to the outer rim (gear to gear). This means one complete round of the turntable is equals to many rounds on the encoder. My turnable turns slowly between 0.2rpm to 10rpm. I intended to hook up the encoder to my PC's parallel, separated by a optoisolated CNC4PC breakout board.

My ultimate goal is to enter a value between 1 and 360 degrees. I would like Mach3 to energize turntable, compare the user input value with the rotary encoder reading, the de-energize tuntable when the user input value and the rotary value are equal.

First thing first, that is choosing the right rotary encoder. I am almost sure to use an incremental, as my idea was example 100 pulse count represents 1 degree of turntable. I was browsing usdigital.com vs encoderoutlet.com. Price varies alot, not sure why. And I am not sure how much CPR to buy. I have tried the online calculator, but confuses me even more.

I hoped someone in this forum can give your advise.
Thank you.

[wizard]

If your only access to the table is the rim and you intend to drive the table with a servo motor at the rim then use what ever encoder the motor comes with. If going DIY for the servo package just about any encoder would work.

Now if you need extreme precision you may have to consider a high resolution encoder.

Ultimately you will need to do some engineering here that includes determining the specs for the rim gear. From there you can engineer a solution for reasonable resolution.

[Al_The_Man]

Is the motor you intend servo or stepper?
If servo, the encoder can be scaled in the drive or controller to suit your desired resolution and will automatically position, the same as any other commanded axis?
Also is your motor driving the table on the rim, or through separate geared mechanism?
Al.

[noeltam75]

Wizard and Al
My turntable is similar to this:
http://www.rapol.com/TurnTables.html]
I am using the motor that comes with it to drive the table.
I was intending to connect the control panel (CW/CCW switches and Speed pot) to PC and control it via Mach3. So to speak, i am driving blind. Therefore, I need an encoder to feedback to Mach3 it's current angle postition.
The motor drives the table via some mechnical gear system below. it actually had a larger gear that spread 1 meter in diameter. The rotary encoder which I am getting will be gear to this large gear. I have no problem driving the table, and spins the encoder with it. I am only not sure which CPR inclemental encoder to choose.

I read somewhere PC's parallel port have frequency limitation. If CPR to high, PC will tends to miss pulses. Is this true? Then what's the max freq? If it is true, I need to calculate CPR range of the encoder, given the speed range 0.2-10rpm. Then I found this online calculator:
QDI Optical Encoder Calculator
It confuses me further by adding in "line count". What is a line count?

Maybe I worry too much, complicate myself. Wizard said any encoder will work. So I will just get any high cpr for better resolution?

[Al_The_Man]

You may be OK if you are not controlling the table motor as a servo due to normal backlash on those kind of tables, if you are just looking to input an encoder and detect a match to a entered position then if you go with a system that multiplies the encoder line count x4 as many drive do, you would could go with a 90 pulse/turn encoder for a x4 to get 360, if your system software just reads the pulses as-is then you can get encoder with 360 pulses/turn.
Which in a welding rotator is usually enough resolution.
I just realized that is no good if you are reading off the edge of the table.
You may be able to run an encoder of the edge of the table and use the marker pulse on an encoder only as a 1 pulse per degree, you would have to look into the mechanics to obtain the suitable gearing for that.
Al.

[night owl]

I agree with Al, find a way to get a 1:1 ratio between the table and the encoder so that one rotation of the table results in one rotation of the encoder. In this way you can use the z pulse of the encoder for reference and eliminate the need for a seperate reference switch. Even some cheap encoders have thousands of pulses per revolution, so you still have many pulses per degree. This also keeps your frequency low, and the programming simple.

[Joe S.]

The largest turntable in the link you posted is about 1 meter.
I have to ask questions:
1. What is the application?
2. What level of precision must you have?
3. Do you have any experience with micro controllers?
4. Do you need position only or do you need speed and direction too?
5. Indoor or outdoor operation?
6. Resolution? 1 degree or tighter, if so how much?
7. Budget?

[noeltam75]

nope, can't do 1:1 bcos I can't reach the center of table, only the rim. If i can, i won't be in this forum. I estimated 6:1.
Ok, let me work backwards. See if I made sense, cos I am an idiot on encoder terminlogy.

Assuming 1deg=1pulse. 360pulse/turn, ratio 6:1, I will need an encoder 60pulse/turn (ppr)= 60ppr/4 = 15CPR. Oops, no such encoder!!

Smallest CPR I can find is 180CPR, this below:
Model 15S On-line Order Page

Assuming I buy this one 180CPR encoder. That's 720ppr(ppr=line count). Using the online calculator to find freq (given speed of turntable 0.2RPM-10RPM).
QDI Optical Encoder Calculator
Freq=0.002 KHz (720 Line count, 0.2RPM).
Freq=0.120 KHz (720 Line count, 10RPM).

Wow!! Freq seems low, but I guess is ok, it is high freq which I should be worried, right?

JoeS, I will answer your question in the next post.

[Al_The_Man]

For very low resolution it is often possible to make a simple encoder, opto slotted disk, or two Prox switches or the kind shown in the post on making home limits from sensors etc.
For that application and product you are using, very fine resolution is probably not needed?
Also if not using a standard quadrature x4 input system, you can pick a resolution of x1,x2 or x4.
Al.

[CnC4Now]

look up renishaw, no wheels , scales, tape on. I am working on a similar table, 2 rpm max. Need .5 arc sec accuracy, not using pot for speed.
Regards
CnC4now

[noeltam75]

look up renishaw, no wheels , scales, tape on. I am working on a similar table, 2 rpm max. Need .5 arc sec accuracy, not using pot for speed.
Regards
CnC4now

Cool thks CNC4now! Renishaw's tape on encoder is also another great alternative. I will definitely study this option in details b4 i comment.
Just curious what did u used for speed control? I used PC to control pot bcos it seem like the easiest, non-destructive method. I hoped to explore other methods.