| Capacitance
Capacitance is the ability of a device to store an electronic charge. In a parallel plate capacitor, the amount of charge that can be stored is directly proportional to the area between the two plates and inversely proportional to the distance between the plates. If the plates are separated by a spring, upon application of a force, the distance between the plates is reduced by the amount of spring deflection. This deflection leads to a change in capacitance, which can be calibrated and used to deduce unknown loads.
Why Capacitance?
Using a variable capacitor can avoid a number of key problems that arise with load cells made from strain gauges or piezo resistive sensors.
Strain gauges generate signal in the micro to milli volt range in response to strains that are in the 10E-6 to 10E-3 range with an applied input voltage of typically 10 V. This means the span of the signal is at most 20-40 mV for the entire range of measurements.
The Loadstar capacitive approach generates an output signal that is several hundred times higher in value, and has a large 2-3V pre-amplification dynamic range that can be used to measure loads. Thus dramatically lower amplification is needed, resulting in greater sensititivity and range of measurement. Loadstar also conveniently provides an amplified output in the 0 to 5 V range for input to data acquisition and control systems.
Furthermore, since our variable capacitors are made of robust mechanical components (not delicate strain gauges) there is negligible effect of temperature and other environemental factors. This leads to much more reliable performance and requires far fewer number of calibrations under operating conditions.
Why Capacitance to Voltage?
Capacitance cannot be easily carried over long distances; it is prone to errors from adjacent electromagnetic fields and losses in the wiring system. This is a key reason why capacitance may not have been used widely until now. We have developed an elegant solution for this problem with a product that converts the raw capacitance to a digital signal. The digital signal can be processed to eliminate noise and extraneous factors and can then be converted into any other analog signal such as a DC signal in the 0 to 5 V range that are popularly used in DAQs and PLCs.
|
