Electronics > Projects, Designs, and Technical Stuff
Splitting differential servo signals
Corryl:
So, this may be really basic. Actually, I hope it is. But my google-fu appears weak at the moment.
What I’m trying to accomplish is reading a differential servo signal (well, 2 signals since they’re in quadrature) without disrupting where they’re currently going. The signals in question are being generated and read by a haas st-20y turning center). I need the haas control to not lose the axis.
This is for a fun little project (double crowned helical gear cutting). I need the “transverse” (z axis) position of the hob. The double crowning is achieved by varying the synchronization between the “work gear” and the hob based on the transverse position. The haas control doesn’t have this ability, so my plan is to add an encoder to the spindle if the c axis encoder isn’t active when in “cutting mode”, or read the c axis encoder, bolt a new spindle/axis to the turret with the rotational position controlled by a pid loop on a micro controller I’ll add, and then, read the z axis position from the machine. The haas control will be in charge of the spindle motor and the z axis like normal.
My first thought was just to y the connections...but many encoders seem to be able to source very little current, so I’m not sure if this is safe. My initial thought was to use a very very very high input impedance opamp...could split the signal and decode it in one go. This just seems way too simple. A corralary to Murphy’s law states that nothing is ever simple. Therefore, I find this to be likely incorrect.
OM222O:
I'm not sure I understand the question (I don't think many others will either without some pictures/ schematics). If I understand correctly however you want to split an encoder output signal into 2? (2 inputs converted to 4 basically)?
It would also be helpful if you can share some details as to voltage levels and the speed of the encoder pulses.
rstofer:
If I read that correctly, you have 2 sets of differential signals that just so happen to be in quadrature and you are concerned about loading the signals.
How about a pair of differential op amps to convert the signals to single-ended yet still in quadrature:
https://www.allaboutcircuits.com/textbook/semiconductors/chpt-8/building-a-differential-amplifier/
With all 4 resistors having the same value, the gain is 1. You can use fairly high value resistors like 10k which shouldn't load the signals very much at all. You can change the gain if it helps your project but I might not do that with the first stage and simple use a non-inverting circuit to amplify the now single-ended signals.
https://www.electronics-notes.com/articles/analogue_circuits/operational-amplifier-op-amp/non-inverting-amplifier.php
OM222O:
--- Quote from: rstofer on April 22, 2019, 07:39:48 pm ---If I read that correctly, you have 2 sets of differential signals that just so happen to be in quadrature and you are concerned about loading the signals.
How about a pair of differential op amps to convert the signals to single-ended yet still in quadrature:
https://www.allaboutcircuits.com/textbook/semiconductors/chpt-8/building-a-differential-amplifier/
With all 4 resistors having the same value, the gain is 1. You can use fairly high value resistors like 10k which shouldn't load the signals very much at all. You can change the gain if it helps your project but I might not do that with the first stage and simple use a non-inverting circuit to amplify the now single-ended signals.
https://www.electronics-notes.com/articles/analogue_circuits/operational-amplifier-op-amp/non-inverting-amplifier.php
--- End quote ---
how does converting them to single ended helps? if anything it makes the signal integrity worse!
besides I would question the terminology used by the OP ... it doesn't sound very technical and I doubt any encoders produce differential signals. maybe differential as in phase shifted by 90 degrees between the pair of signals?
He really needs to post more details and a clear objective as to what he wants to achieve
duak:
I've done a bit of repair work on CNC machine tools using Mitsubishi and Fanuc controls and have a fair understanding of how they go together. Just to make sure we're on the same length of wave here, how old is this machine and what control and servos are in it? I haven't worked on a Haas but I believe they use Fanuc, although they may have realized their own.
So you're thinking of tapping off the encoder signals back into the control? Since the mid 90's most encoders for CNC machine tools are serial with proprietary protocols. Unless you know they are quadrature they may not actually be. I'd have to look at the spec but they are probably something like RS-485 that requires termination to match the impedance of the cable. Any tapping off will require a short stub to prevent altering the electrical characteristics.
Cheers,
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