Optical Table CNC Lathe - Glass Linear Scales and Motor Encoders
Based on the experiences with the motor selection, I decided to have dual feedback. First, for decent servo performance, I will have rotary encoders on the motor axes. Then, for dimensional accuracy, I will add glass scales.
I have two Dynamics Research Corporation glass scales for this purpose. They interface as differential encoders, so its is easy to add them to the SnapAmp that I have. One concern is that the SnapAmp does not provide a decent way to connect the differential encoders in a shielded way. As such, the cabling between the SnapAmp and the connector that adds the shielding should be held as short as possible.
The glass scales already have cable connected to them, but the connectors at the ends are missing:
I decided that a good way to connect all the encoders with the control box is with F/FTP or S/FTP network cable.
It is very cheap, but has all the qualities that one might want.
- 4 individually shielded twisted cable pairs (3 for A/B/Z; power does not need twisted pair, but is ok with it)
- pairs twisted at different rates
- additional outer shield
- easily sourced, cheap connectors.
- specified north of 100 Mhz. Far more than my encoders are going to push.
- (While making my connectors, I found out that the cable that is on the encoders is similar in nature - individually shielded twisted pairs)
One problem that presented itself is that the cable on the glass scales, while similar in thickness, is stranded. As far as I knew, network cable is solid wires only. It turns out, however, that stranded network cable is available and so are connectors that can handle it.
After a lot of searching and comparing I came across Platinum Tools EZ-RJ45 Cat5/6 Shielded connectors. These are specified as being able to handle both solid and stranded wires. As an added bonus, their push-through design makes the assembly much easier than the Ethernet connectors of yesteryear:
I am loosely following the pinout on the DRC nameplate for the pin assignment on the plug.
This worked very well! I tested these connectors with both linear scales and one 5000 line encoder. The linear scales I have only moved by hand, but there are no signal quality issues on the encoder even at 1500 RPM
I mounted the DRC glass scale on the long axis. It has a 0.1 mil quadrature resolution.