Delta Computer Systems VMC 186/40 Manual page 17

VMC 186/40 Motion Control Module
The value used for the Scale parameter is affected by the number or recirculations selected (hardware jumper).
Recirculation selection is based on the transducer length and desired position resolution. When using four
recirculations, the most common position resolution is 0.0005". With two recirculations, the position resolution
is usually 0.001" and with one recirculation it is 0.002" or 0.05 mm. For more information on recirculation see
page 33.
Linear transducers have a calibration number (typically 9.1 µs per inch) and the VMC 186/40 has a 55.5 MHz
oscillator for transducer position measurement counters. To determine the Scale for a magnetostrictive
transducer use the following formula:
Scale = 
Position Unit * Cal Number * 55.5 * Number of Recirculations
The calibration number must be specified in the same base units as the position units (inches, mm, etc.). If the
calibration number is expressed in inches then position units would be expressed in inches (i.e. 9.1392
microseconds per inch for a calibration number and .001 inch resolution is desired)
The Scale parameter can also be adjusted or fine tuned by using position readings from the VMC 186/40 and
actual physical measurements in the following formula:
Scale = Old Scale * 
Where Old Scale is the default Scale or a previously calculated Scale, and measured position is in position units
(thousandth of an inch, etc.) from a physical measurement of the system. Position readings are taken using the
Actual Position parameter.
Note: If a negative value is obtained for Scale, the absolute value must be used and the Direction parameter
changed from 0 to -1 or vice versa. Also, when the Scale parameter is changed, the Extend Limit, Retract Limit
and Offset must also be changed.

32768
Position Reading 1 - Position Reading 2
Measured Position 1 - Measured Position 2

17
Control Parameters