Simulated TrackPoint Strain Gauge Sensor

In order to eliminate inconsistencies in calibrating for the TrackPoint circuit, a 'fake stick' with switches is built to simulate a particular type of a (real) sensor. The resistance switches electronically simulate the application of force applied to the sensor along each axis. This allows the circuit to be tested with signals which represent a force applied with precise magnitude and exactly along the specified direction.

The following measurements and calculations are the preliminary steps for building the simulated sensor with an onboard series resistor.

1. Collect a number of TrackPoint sensors and measure R_ZG and R_VZ, where R_ZG is resistance from Z to ground. (It is recommended to use a low-current meter and a 4-wire resistance measurement technique.) Record in the following format.

Sensor #	RZGround	RVZ
1	xxx.xxx	xxx.xxx
2	xxx.xxx	xxx.xxx
3	xxx.xxx	xxx.xxx
n	xxx.xxx	xxx.xxx
Average	xxx.xxx	xxx.xxx

Calculate and record the averages for the two separate resistance measurements.

NOTE: In the real sensor, R_VZ ~ R_ZG ~ R_XG ~ R_ZX~R_BASE

For Steps 2, 3 and 4, measure and record the voltage supply to the 10^-5 accuracy.

2. Apply voltage supply at Z.

Record voltages at X, Y (as Vx, Vy table below).

Apply 170g of force on an axis and record the voltage with weight, V_W.

Sensor #	Vx	VxW	Vy	VyW
1	x.xxxx	x.xxxx	x.xxxx	x.xxxx
2	x.xxxx	x.xxxx	x.xxxx	x.xxxx
3	x.xxxx	x.xxxx	x.xxxx	x.xxxx
n	x.xxxx	x.xxxx	x.xxxx	x.xxxx
Average	x.xxxx	x.xxxx	x.xxxx	x.xxxx

3. Calculate the delta Vx and Vy (averages):

delta Vx = |Vx_AVG - Vx_W,AVG |

delta Vy = |Vy_AVG - Vy_W,AVG|

4. Calculate %delta of Vx in order to attain %delta Rx:

(delta Vx) / (Vx) = %delta Vx = %delta Rx

Where %delta Rx is the percent resistance change in the x-axis with weight applied.

5. Apply voltage supply +V.

Record voltage at Z and apply 500g of force.

Record voltage measurements.

Sensor #	Vz	VzW
1	x.xxxx	x.xxxx
2	x.xxxx	x.xxxx
3	x.xxxx	x.xxxx
n	x.xxxx	x.xxxx

6. Calculate the delta Vz (average):

delta Vz = |Vz_AVG - Vz_W,AVG|

7. Calculate delta R_Z, using delta Vz.

Refer to Figures A and B.

We know,

R_TOTAL = 2 R_BASE + delta R

V_Rbase = Vcc [R_BASE /( R_TOTAL )]

Thus, use delta R_Z = (Vcc * R_BASE)/V_Rbase - 2 R_BASE

%delta R_Z = delta R_Z

Note that %delta Vx is NOT EQUAL to %delta Rx

When the vertical z-force is applied on the real sensor, R_ZG (bridge resistance) increases by

delta R_Z and R_VZ remains constant. In the simulated sensor, either R_VZ or R_ZG may be designated to be constant and the other to have either an increase or a decrease of resistance by delta Rz. The delta Rz may be implemented in series with R_ZG or in parallel with R_VZ, as long as the TrackPoint circuit detects the desired %change of voltage when z-force is applied.

8. Build the simulated sensor. Use calculated delta Rx and delta Rz from steps # 4, 7 respectively.

IN BUILDING SIMULATED SENSOR, REFER TO SIMULATED SENSOR SCHEMATIC and PARTS LIST.

Parts List

***VERY IMPORTANT***

Try combinations of series resistors and measure them in targetting for .001 - .01% accuracy.

Record measurements as you build the fake stick.

FOR THE X-axis:

For the static state, a DPDT switch will be used to activate the force applied on the x-axis. Designate either R_ZX or R_XG as the open leg and the other to be the closed leg. In this design, R_ZX is the open leg, and R_XG will be the closed leg at static state.

For R_ZX:

Static State Choose two series resistors, R5 and R6 and set R6>>R5. In order to maximize accuracy, use R8 to trim R6 until R_ZX ~ (R_{BASE VALUE} +5-20 ohms) ~ value close to the measured average value of the real sensor. (See Preliminary Set -up for Fake Stick.)

Dynamic State Use R7 to be switched on in parallel with R5 to decrease the resistance of R_ZX by a value of delta R_X, change of resistance when weight is applied in the x-axis.

For R_XG:

Static State Just as done for R_ZX, choose two series resistors, R9 and R10, setting R9>>R10. Position the parallel, trimming resistors, R11 and R12 with R9 and R10 respectively, where the resulting R_XZ ~ R_{BASE VALUE}.

Dynamic State Note that when R12 is switched off by SWX, there should be an increase of delta R_ZG equal to the decrease of delta R_ZX, which is also equal to delta R_X.

FOR THE Y-axis:

For R_ZY and R_YG:

Set two series resistors R13, R15 to equal R_{BASE VALUE}, using R14 and R16 to trim the series resistors.

For simplicity, no switches need to be implemented in the y-axis since it behaves similar to the x-axis.

FOR THE Z-axis:

For R_VZ:

Choose two series resistors R1 and R2, setting R2>>R1. Use R4 to trim R2. Use SWZ to switch R3 in parallel with R1, resulting a decrease of delta R_Z .

Prepared by

Fritzie Mateo

11/97