Cooper Instruments & Systems ELF 4200 Flexiforce Paper Thin Sensor - Manual (Page 41)

CF10 38 6/24/08 Rev. H
calibration will be saved between sessions unless Uncalibrate is selected.
7.0 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.
7.1 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. FlexiForce sensors are generally repeatable within 2.5% once they have been conditioned. 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.
7.2 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 sensors are linear
within +/- 3%.
7.3 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. For a conditioned sensor, with 50% of the full force range
applied, hysteresis is less than 4.5 % of full scale.
7.4 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. In FlexiForce sensors, drift is less than 3%/logarithmic time.
7.5 TEMPERATURE SENSITIVITY
The operating range for FlexiForce sensors is from 15°F (-9°C) to 140°F (60°C). FlexiForce sensor output may
vary up to 0.2% per degree F (approx. 0.36% per degree Celsius). In general, your results will vary if you combine
high loads on the sensor with high temperatures. For loads of less than 10 lbs., the operating temperature can be
increased to 165°F (74°C).
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.
FlexiForce
sensors should be stored at temperatures in the range of 15°F (-9°C) to 165°F (74°C).
7.6 SENSOR LIFE / DURABILITY
Sensor life depends on the application in which it is used. Sensors are reusable, unless used in applications in
which they are subjected to severe conditions, such as against sharp edges, or shear forces. FlexiForce sensors
have been successfully tested at over one million load cycles using a 50 lb. force.
Rough handling of a sensor will also shorten its useful life. For example, a sensor that is repeatedly installed in a