| | | Drift The drift of a charge amplifier is by global description of the variation in the signal's zero level caused mainly by charge losses in the feedback capacitor and leakage currents at the amplifier input. A leakage current across the insulation resistor causes exponential decay of the feedback capacitor charge with a time constant given by the product of insulation resistance and the capacitance of the capacitor. As sufficiently high insulation resistance leads to a very high time constant, the problem of discharging only affects very long measuring periods. Since extensive research has established very well accepted charge amplifier drift characteristics, their maximum value can be used to evaluate the effect of drift on the calibration result. Charge amplifiers are subject to instability over time. To minimize the effect of this on the results of force sensor calibration, it is advisable to use a precision charge calibrator to calibrate the charge amplifiers of both the reference measuring chain and the measuring chain of the product to be calibrated together with all corresponding cables, display and evaluation devices. This approach also identifies any influences exerted by contacts and electrical connections within the measurement chain. | | Charge calibrator stability Charge calibrators are calibrated at regular (normally yearly) intervals. Over this period the calibration values undergo slight changes, which are characteristic of a specific instrument and as such constant. The stability of the charge calibrator must be considered in evaluating the calibration results. Calibration of strain gage sensors Calibration of strain gage sensors requires the use of a bridge amplifier and a reference standard. Despite being calibrated at the start of the product calibration procedure, the bridge amplifier suffers from a certain inherent measurement uncertainty. The reference signal is characterized by the values shown on the calibration certificate. Crosstalk with multiaxial sensors and sensor systems Complex sensor systems such as measuring wheels are usually fitted with multi-component force sensors that allow simultaneous measurement of up to three force components and three torque values. A multi-component configuration is associated with crosstalk of individual variables | | observed in other measurement components. In other words, with a force loading along one axis minimal signals in the direction of the other orthogonal axes or minimal torque values can be indicated. This crosstalk affects all possible force and torque measurement directions. Its values must be reconciled against measurement uncertainty. Ripple in spinning sensor systems Spinning sensor systems, particularly measuring wheels, exhibit ripple. During coasting the value of this ripple describes the change in sensitivity as a function of the angle of rotation of the wheel in relation to the reference point. In the course of calibration the angular range of a full revolution is measured with a limited number of data points approximated by an optimized interpolation curve. Measurement uncertainties are then considered on the basis of the difference between the interpolation curve and the measuring points. | | |