Delta Tau GEO BRICK LV User Manual

 

 

The timer registers are processed in the conversion table as parallel feedback representing the position, 
just as an absolute encoder would be.  The filtered parallel read (method digit $3) should be used to reject 
errors due to extra or missing echo pulses.  Consult the Setting up the Encoder Conversion Table section 
in this manual and the specification for variables I8000 – I8191 in the Software Reference Manual. 

If the MLDT is connected to the MACRO Station, the data must be processed through conversion tables 
on both the Station and the Turbo PMAC (although the table on the Turbo PMAC really just copies the 
data off the ring).  The conversion table on the MACRO Station is contained in setup variables MI120 – 
MI151.  This table should do a filtered parallel read of the timer register, the filtering able to reject errors 
due to extra or missing echo pulses.  The result is passed back over the ring to Turbo PMAC, where it is 
treated just as any other type of feedback from the ring would be.  Consult the section Setting Up the 
Encoder Conversion Table in this manual, the specification for variables I8000 – I8191 in the Software 
Reference Manual, and the manuals for the MACRO Station. 

Absolute Power-On Position Address and Format: Ixx10, Ixx95, MI11x 

Because MLDTs are absolute position sensors, no homing search move is required if they are used for 
position feedback.  Turbo PMAC variables Ixx10 (Motor xx Power-On Servo Position Address) and 
Ixx95 (Motor xx Power-On Position Format) can be used to establish this absolute position.  The Ixx10 
description in the Turbo PMAC Software Reference Manual has a complete table of the possible address 
settings for the MLDT timer registers, and also for getting absolute position from a MACRO Station.   

Ixx95 should be set to $170000 for an on-board MLDT timer register.  It should be set to $740000 to 
specify power-on position from a MACRO Station; station variable MI11x should be set to $17nnnn, 
where “nnnn” represents the Y-address of the timer register on the MACRO Station. 

Position Reference Offset: Ixx26 

When used with an absolute position read, Ixx26 specifies the difference between the sensor’s zero 
position and the motor zero position.  With an MLDT, the sensor’s zero position is at the 
transmitter/receiver module, outside the possible range of motion of the sensor.  If it is desired that the 
motor’s zero position be within the range of travel, Ixx26 should be used to define the offset between 
sensor and motor zero.  Ixx26 has units of 1/16 count, where a count here is an LSB of the timer. 

Power-On Mode: Ixx80 

Ixx80 controls whether the absolute position read for Motor xx is performed immediately at power-
on/reset, or whether the absolute position read is delayed until a $* or $$* command is issued.  Because 
the output frequency for the MLDT is not established yet when a read would be done immediately at 
power-on, it should be delayed with an MLDT.  This requires that Bit 2 of Ixx80 be set to 1, making the 
value of Ixx80 equal to 4 if the motor is not to be enabled immediately on power-on/reset, or equal to 5 if 
the motor is to be enabled immediately. 

Motor Units 

For a motor set up with MLDT feedback, a count is one increment (LSB) of the timer.  The physical 
length of this increment, which is the resolution of the measurement, is dependent on the speed of the 
return pulse in the MLDT, and the frequency of the timer in the Turbo PMAC.  The speed of the return 
pulse (the speed of sound in the metal) varies from device to device, but is always approximately the 
inverse of 0.35 

µsec/mm, or 9.0 µsec/inch.  The frequency of the timer is 117.96 MHz.  The resolution 

can be calculated as: