Delta Tau GEO BRICK LV User Manual

 

 

Power Supply and Isolation 

In the basic configuration of a Turbo PMAC, the encoder circuitry is not isolated from Turbo PMAC’s 
digital circuitry and the signals are referenced to Turbo PMAC’s digital common level GND.  Typically, 
the encoders in this case are powered from PMAC’s +5V lines with a return on GND.  The total encoder 
current draw must be considered in sizing the Turbo PMAC power supply. 

It is also possible to use a separate supply for the encoders with non-isolated signals connected to Turbo 
PMAC.  In this case, the return of the supply should be connected to the digital common GND on Turbo 
PMAC to give the signals a common reference.  The +5V lines of separate supplies should never be tied 
together, as they will fight each other to control the exact voltage level. 

Isolated Encoder Signals 
In some systems, it is desired to optically isolate the encoder circuitry from PMAC’s digital circuitry.  
This is common in systems with long distances from the encoder to the controller (> 10m or 30 ft) and/or 
systems with very high levels of electrical noise.  Isolation can be achieved using the Acc-8D Opt 6 4-
channel encoder isolator board.  With an isolated encoder, a separate power supply is required for the 
encoders to maintain isolation, and the return on the supply must not be connected to the digital common 
GND, or the isolation will be defeated. 

Simulated Encoder Signals 
Special consideration must be given to systems that have a simulated encoder signal provided from a 
resolver-to-digital converter in a brushless motor amplifier.  In these systems, the encoder signals are 
referenced to the amplifier's signal return, which in turn is connected to Turbo PMAC’s analog common 
AGND.  The best setup in these cases is to isolate the simulated encoder signal from the PMAC digital 
circuitry with the Acc-8D Opt 6 isolator board or similar module.  This will keep full isolation between 
the Turbo PMAC digital circuitry and the amplifier. 

If isolation of the simulated encoder signals is not feasible, Turbo PMAC’s digital circuitry and the 
amplifier signal circuitry (including Turbo PMAC’s analog circuitry) must be well tied together to 
provide a common reference voltage.  Do this by putting jumpers on the Turbo PMAC or interface board 
(E-Points E85, E87, and E88 on many boards), tying the digital and analog circuits on Turbo PMAC 
together, and therefore also the analog signal circuits.  What must be avoided is having the simulated 
encoder cables providing the only connection between the circuits.  This can result in lost signals from 
bad referencing, or even component damage from ground loops. 

Wiring Techniques 

There are several important techniques in the wiring of the encoders that are important for noise 
mitigation.  First, the encoder cable should be kept physically separate from the motor power cable if at 
all possible.  Second, both of these cables should be shielded, the motor cable to prevent noise from 
getting out, and the encoder cable to prevent noise from getting in.  These shields should be grounded at 
the inward end only, that is, to the device that is itself tied to a ground. 

A third important noise mitigation technique is to twist the leads of the complementary pairs around each 
other.  With these twisted pairs, what noise does get in tends to cancel itself out in opposite halves of the 
twist. 

The Turbo PMAC ASICs are set up by default to accept quadrature feedback, but some settings may need 
to be tweaked to optimize operation.