Sensoray 7429 - Manual (Page 37)

CUSTOM RESISTIVE SENSORS
The 7429 possesses the capability of interfacing custom resistive sensors to any sensor
channel with the following limitations:
1. The maximum sensor resistance must not exceed 600K ohms.
2. You must provide three 16-bit integers which serve as coefficients of a second-
degree sensor linearization polynomial.
3. A second-degree sensor linearization polynomial is sufficiently accurate for
your application.
Custom resistive sensors are interfaced as follows:
1. If empirical sensor data is available, apply a least-squares fit (or other curve-fit)
algorithm to the data to obtain a polynomial of the form
y = AR
2
+ BR + C
where: R is the sensor resistance (ohms)
y is the channel data value in with appropriate scaling
A, B, and C are coefficients derived from the curve fit
If an equation is already available for the sensor, the equation can usually be
worked into the form of the above expression via various numerical methods (if
not already in the correct form).
2. Install the following command into your host processor's initialization sequence
to define the custom sensor (see DEFINE CHANNEL SENSOR command):
(16 + CHAN),(0CH),
(A MSB),(A LSB),
(B MSB),(B LSB),
(C MSB),(C LSB)
S AMPLE A PPLICATION
A displacement transducer is to be interfaced to the sensor coprocessor on channel 5.
The sensor DC resistance varies as a function of displacement. The following data
points have been measured:
ResistanceDisplacement
(ohms) (mm)
0 -3105
50 245
100 8595
150 21945
A least-squares fit algorithm is applied to the transducer data with the following results: