OVERVIEW
This section outlines Sensor Construction and Application.
SENSORS
The FlexiForce sensor is an ultra-thin and flexible printed circuit. Sensors are available in three
full-scale force ranges: Low (25 lbf ), Medium (150 lbf ), and High (1000 lbf ). The length of the
sensors can be trimmed by Tekscan to predefined lengths of 2”, 4” and 6” or can be trimmed by
the customer. If the customer trims the sensor they will have to attach a connector to the sensor.
(This can be accomplished by purchasing staked pin connectors and a crimping tool. They can
also use a conductive epoxy to adhere small wires to each conductor). The "active sensing area"
is a 0.375” diameter circle at the end of the sensor. The sensors are constructed of two layers of
substrate, such as a polyester film. On each layer, a conductive material (silver) is applied,
followed by a layer of pressure-sensitive ink. Adhesive is then used to laminate the two layers of
substrate together to form the sensor. The silver circle on top of the pressure-sensitive ink defines
the “active sensing area”. Silver extends from the sensing area to the connectors at the other end
of the sensor, forming the conductive leads. FlexiForce sensors are terminated with a solderable
male square pin connector, which allows them to be incorporated into a circuit. The two outer
pins of the connector are active and the center pin is inactive.
The sensor acts as a variable resistor in an electrical circuit. When the sensor is unloaded, its
resistance is very high (greater than 5 Meg-ohm); when a force is applied to the sensor, the
resistance decreases. Connecting an ohmmeter to the outer two pins of the sensor connector and
applying a force to the sensing area can read the change in resistance.
APPLICATION
There are many ways to integrate the FlexiForce
sensor into an application. One way is to
incorporate it into a force-to-voltage circuit. A
means of calibration must then be established to
convert the output into the appropriate
engineering units. Depending on the setup, an
adjustment could then be done to increase or
decrease the sensitivity of the sensor.
An example circuit is shown above. In this case, a
5 V DC excitation voltage drives it. This circuit uses an inverting operational amplifier
arrangement to produce an analog output based on the sensor resistance and a fixed reference
resistance. An analog-to-digital converter can be used to change this voltage to a digital output.
In this circuit, the sensitivity of the sensor could be adjusted by changing the reference resistance
(RF); a lower reference resistance will make the sensor less sensitive, and increase its active
force range.
02/26/04 FlexiForce Sensor User Manual (RevC) 4
