User's Manual

PMAC2 User Manual
Setting Up PMAC2 for MLDT Feedback 73
SETTING UP PMAC2 FOR MLDT FEEDBACK
PMAC2 can provide direct interface to magnetostrictive linear displacement transducers (MLDTs), such
as MTS's Temposonics brand. MLDTs can provide absolute position information in rugged
environments; they are particularly well suited to hydraulic applications. In this interface PMAC2
provides a periodic excitation pulse output to the MLDT, receives the echo pulse that returns at the speed
of sound in the transducer, and very accurately measures the time between these pulses, which is directly
proportional to the distance of the moving member from the stationary base of the transducer. The timer
therefore contains a position measurement.
Interface Type
MLDTs are available with several different interface formats; for this interface, a format with external
excitation is required, because PMAC2 provides the excitation pulse. Usually this format has an RS-422
interface, because the excitation and echo pulses are at RS-422 levels. The PMAC2 MLDT interface
inputs and outputs are at RS-422 levels.
Note:
If the Handwheel port of PMAC2 will be used either to provide the excitation
pulse or to accept the return signal, the DSPGate2 (MACRO gate) must be at least
a DSPGate2B. The modifications between the DSPGate2A and DSPGate2B allow
the functions used to perform pulse creating and acceptance on the Handwheel port
to be used.
Signal Formats
There are two common signal formats of the external excitation type; MTS calls them RPM and DPM. In
the RPM format there are two short pulses returned from the MLDT; an immediate start pulse, and a
delayed stop pulse. In the DPM format there is only one long pulse returned from the MLDT.
Since PMAC2 uses the first rising signal edge returned after the falling edge of the output pulse to latch
the timer, the key setup issue in this format is to make sure that the output pulse width is large enough so
that the falling edge of the output pulse occurs after the rising edge of the return line’s start.
Pulse Out
Return
Start Stop
Figure 14 RPM Signal Format
The rising edge of the return pulse in the DPM format is the equivalent of the rising edge of the start pulse
in the RPM format. The falling edge of the return pulse in the DPM format is the equivalent to the rising
edge of the stop pulse in the RPM format. Because PMAC2 is expecting a rising signal edge to latch the
timer, in this signal format the return signals should be inverted so that the ‘+’ output of the MLDT is
wired into PMAC2’s ‘-‘ input, and vice versa.
Pulse Out
Return
Start Stop
Figure 15 DPM Signal Format