View Full Version : Encoder IC?
01-30-2012, 11:08 AM
I recently purchased an Electrocraft servo motor, and have since modified it to incorporate an encoder.
The device that it was installed on was a shop built mechanism to interrupt a beam with a slotted copper disk, I think it came out of the physics department at UW Madison, WI.
Since the servo only had a tach drive, the builder had constructed an encoder that uses 4 led slot photodiode detectors to 'read' the slots in the disk to determine angular location and generate an index signal. Great idea, but none of the electronics was included in the deal...
The size of this disk would be appropriate to incorporate on the spindle of the lathe conversion I am doing.
I am looking for information on construction of an encoder and possible IC's that could be used for the encoder.
01-30-2012, 12:23 PM
I am guessing you are looking for a Opto slot detector such as the H22A1 this is often use in this application, depending on what you intend feeding it into depends on whether you add a buffer on the output such as a 2n7000.
The LED side just requires a series resistor to limit LED current.
The spec sheet should show a typical application.
If it is just an index marker, you only need one slot in the disk.
I would suggest only using this for index, if you need a quadrature encoder type output, you would be better off using a US digital optical encoder.
A quadrature output will require exact spacing of the LEDS at 90° signal shift.
01-30-2012, 03:14 PM
I should have posted the pics... I don't know exactly what I am looking for as far as IC's or schematics are concerned, but given the parts that I have it becomes easier to describe what I need.
The fist pic shows 2 quad pickups and 2 index pickups on the housing, followed by the wheel and the modified servo. the servo had it's tach removed and it's shaft reduced from .500 to 8mm to accommodate a CUI VT 103. The encoder parts are large enough in diameter that they could be fitted to the 1.250" main shaft in the headstock. I just thought that it would be nice to reuse the parts that I had, and while it really isn't necessary to provide angular data for the spindle of the lathe, I have it if i want it. if all I need is an index that would be available as well.
So what I need is an IC or schematic that will accept data from the 4 slot detectors that I have and provide an output that is similar to that of the data stream from the CUI for connection to a smooth stepper board.
01-30-2012, 04:30 PM
What slot Opto devices are you using and do you want single ended or differential drivers for the quad signals?
Differential drivers/receivers for RS422/RS485 are quite common.
Otherwise a ULN2008 driver or descrete 2n7000 FET buffers should work.
Or just the open collector output of the Opto, again it depends on what you are inputting to?
Personally I would have though the $50.00 for a US digital encoder would have been more practical, if you are relying on this encoder for PID loop, it will be fairly coarse resolution.
It looks like smoothstepper takes differential or single ended, if you want noise immunity, use differential driver IC75ALS191 or 26LS31.
01-30-2012, 10:11 PM
Looks like you got an encoder already installed. Figure out what those devices are by looking for some numbers.
Are you wanting this feedback to a CNC or for RPM Sensing?
OR - are you looking for the chip that takes the quadrature and usesw it for something useful?
01-31-2012, 09:49 PM
I was looking for the chip that took the signals from the quadrature sensors and output a signal that was the same as that output by the CUI encoders so that I could input the angular position and speed to the controller. Is that something the servo drives and the controller boards do on board? Perhaps I'm reading too much into this.
The motor in the photo used to have a tach, and what you see is the shaft turned down and shortened ready for an encoder. I retained the end plate from the tach housing because it was already tapped for mounting an encoder. The brush caps just plug the holes now.
I will more than likely just install a disk with a hole to index the spindle for threading and measure rpm to allow MACH to adjust the spindle speed via the VFD that will connect to the new drive motor.
The ability to read angular position is not necessary... I just thought it would be cool. The link to the chip is more for my own education... I know enough about the details of how all this works to be dangerous... but always willing to learn.
I appreciated all the work that others have done over the years to allow me to tackle this more or less as a plug and play proposition compared to what would have been necessary when I first started to follow the EMC project years ago.
01-31-2012, 10:28 PM
The cui encoders are quad out just like you will get out of that setup with the two opto interrupters. One will be A phase and other other B. No chip necessary.