FlexiForce user manual

02/05/09 FlexiForce Sensor User Manual (Rev G)
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SENSOR PERFORMANCE CHARACTERISTICS
There are a number of characteristics of sensors, which can affect your results. This section
contains a description of each of these conditions, and recommendations on how to lessen their
effects.
REPEATABILITY
Repeatability is the ability of the sensor to respond in the same way to a repeatedly applied
force. As with most measurement devices, it is customary to exercise, or "condition" a sensor
before calibrating it or using it for measurement. This is done to reduce the amount of change in
the sensor response due to repeated loading and unloading. A sensor is conditioned by loading it
to 110% of the test weight four or five times. Follow the full procedure in the Conditioning
Sensors section.
LINEARITY
Linearity refers to the sensor’s response (digital output) to the applied load, over the range of
the sensor. This response should ideally be linear; and any non-linearity of the sensor is the
amount that its output deviates from this line. A calibration is performed to "linearize" this
output as much as possible. FlexiForce standard sensors are linear within +/- 3%. FlexiForce
High-Temperature sensors have a linearity that is 1.2% of full scale.
HYSTERESIS
Hysteresis is the difference in the sensor output response during loading and unloading, at the
same force. For static forces, and applications in which force is only increased, and not
decreased, the effects of hysteresis are minimal. If an application includes load decreases, as well
as increases, there may be error introduced by hysteresis that is not accounted for by calibration.
DRIFT
Drift is the change in sensor output when a constant force is applied over a period of time. If the
sensor is kept under a constant load, the resistance of the sensor will continually decrease, and
the output will gradually increase. It is important to take drift into account when calibrating the
sensor, so that its effects can be minimized. The simplest way to accomplish this is to perform
the sensor calibration in a time frame similar to that which will be used in the application.
TEMPERATURE SENSITIVITY
In general, your results will vary if you combine high loads on the sensor with high temperatures.
To ensure accuracy, calibrate the sensor at the temperature at which it will be used in the
application. If the sensor is being used at different temperatures, perform a calibration at each of
these temperatures, save the calibration files, then load the appropriate calibration file when
using the sensor at that temperature.