User's Manual

PMAC User Manual
Setting Up a Motor 57
1/T Sub-count Interpolation
There are two optional methods on PMAC for achieving sub-count resolution with incremental feedback.
The first is called 1/T decoding. Each encoder channel has two timer registers associated with it. The
first register holds the time between the last two encoder transitions. Velocity is estimated as being
inversely proportional to this time — a very accurate estimation, particularly at low speeds.
The second timer holds the time since the last transition. Fractional distance traveled since the last
transition is estimated as the value of the second timer divided by the value of the first timer (see figure 7-
3). This interpolation provides added smoothness to low speed moves, but it does not provide accurate
interpolation at rest. 1/T decoding requires the $00 conversion format (see below).
Parallel Sub-count Interpolation
The second method of interpolation allows PMAC to read up to five bits of parallel fractional information
to supplement the integer quadrature count. Usually this information is derived from analog sine/cosine
quadrature signals of encoders or interferometers through analog-to-digital converter circuitry. This
circuitry, which creates digital quadrature and the parallel fractional bits, must be external to PMAC,
either on Delta Tau’s Acc-8D Option 8, Analog Encoder Interpolator board, or on user-provided circuitry.
PMAC provides simultaneous latching of the quadrature counter and the parallel inputs to ensure
synchronicity of the data.
This interpolation method, unlike the 1/T method, provides accurate interpolation at rest as well as during
movement. If the A/D conversion circuitry does not provide accurate interpolation at high speeds, it is
possible to change on the fly between 1/T and parallel interpolation.
This type of sub-count extension may be done only on odd-numbered encoders. The five bits are the five
inputs associated with the next higher-numbered encoder: FAULT (MSBit), +LIM, -LIM, HMFL, and
CHC (LSBit). Parallel sub-count extension requires the $80 conversion format (see below).
T
1
T
2
T
1
T
2
A
B
Servo
Interrupts
Velocity Estimation : V =
n
T
2
T
1
Position Estimation : P = Counter +/-
n
K
T
1
Figure 7 PMAC 1/T Extension