Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
2002-03-20
2004-03-02
Le, N. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
Reexamination Certificate
active
06700391
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a capacitive distance measuring system and in particular a capacitive length measuring system for electronically detecting and displaying a position or a displacement distance.
BACKGROUND OF THE INVENTION
A distance measuring system is known from EP 0 442 898 B1, which system uses a capacitive sensor designed as differential capacitor, as well as a sigma/delta converter, consisting of an integrator and a comparator. The differential capacitor comprises two side-by-side arranged transmitting electrodes and a joint receiving electrode, which is arranged a short distance from the transmitting electrodes on a scale, so as to be displaceable relative to these transmitting electrodes. Depending on the displacement position, the receiving electrode overlays the individual transmitting electrodes differently, which leads to changes in the capacitances for the partial capacitors of the differential capacitor. These changes represent a measure for the displacement distance to be measured.
In order to determine the partial capacitance ratio, the individual transmitting electrodes of the differential capacitor are fed during each cycle with charge packets of a predetermined size but different polarity. A positive charge packet is transmitted to the one transmitting electrode, if the total charge amount transmitted via the receiving electrode to the integrator and integrated therein is negative. A negative charge packet is transmitted to the other transmitting electrode if the total charge is positive. The number of respectively transmitted packets is detected with associated interpolation counters. The ratio of these counters relative to each other reflects the sought after ratio of the partial capacitances.
Several transmitting electrodes are arranged in a row for the measuring of longer lengths. If the measuring range that is fixed by one pair of transmitting electrodes is abandoned during the displacement, then a change to the next pair occurs. The number and direction of the change operations are also counted and are used jointly with the interpolation counters for determining the distance traveled.
The known distance measuring system has proven itself in practical operations. However, interference signals resulting from the triggering of transmitting electrodes, the display and other sources of interference are also integrated and lead to non-linearity, reflected in the measuring accuracy. In addition, a supply source must be provided that transmits charge packets with a specific polarity at the selected moments to the corresponding partial capacitor.
SUMMARY OF THE INVENTION
Starting with this premise, it is the object of the invention to create a suitable distance measuring system, in particular for battery-operated hand-held devices, which ensures the highest possible measuring accuracy with the highest possible resolution. In addition, the distance measuring system should operate with a simple energy supply.
The distance measuring system is provided with one or several capacitive sensors, which are designed as differential capacitors. The capacitive distance measuring can be realized with power savings compared to prior methods. The capacitances used are in the pF (pico-farad) range, as a result of which the transmitted charge amounts and the resulting currents are very low. Each sensor comprises at least 2, for example 16, transmitting electrodes that are preferably arranged in a row with a constant distance there between. The sensor further comprises one or preferably several, for example 64, counter electrodes that are also arranged in a row. The electrodes are arranged on a measurer, such as a ruler or measuring tape, opposite the respective transmitting electrodes and at a short, equal distance to these. The measurer is positioned such that it can be adjusted and in particular can be displaced relative to the transmitting electrodes. Thus, at least two partial capacitors are formed in this way, wherein the capacitance of at least one of the partial capacitors changes proportional to the distance distance.
A triggering device is used to feed a binary signal to selected one or groups of transmitting electrodes for generating individual measuring signals at predetermined times. The signals for triggering the two partial capacitors are fixedly offset in phase relative to each other, so that the resulting measuring signals do not overlap in time if possible. If voltage pulses are used for the triggering, the measuring signals essentially are the transmitted current surges that form a charge amount (a charge packet). If current signals or charge packets function as trigger signals, however, voltages are measured.
A switch unit is advantageously provided, containing switches that are closed for a predetermined interval to define time windows. The time windows include selected edges of the trigger signals positioned therein. Inside the time windows, the associated measuring signals are allowed to pass through to a processing device, which evaluates these signals and uses the results to determine the partial capacitances of the differential capacitor. The processing device determines the distance to be measured from the partial capacitances.
The energy supply can have a very simple design. The trigger signals transmitted to the one partial capacitor can have the same curve shape as the trigger signals for the other partial capacitor. They can even have the same polarity and assume the same values, so that positive or negative rectangular signals, for example, are particularly suitable for use as trigger signals. Simple battery cells are sufficient for the energy supply.
It is advantageous if the phase offset and the time window are fixed such that only one selected signal edge can be observed inside one time window. With a 90° phase offset, an equidistant spacing of one fourth of the clock cycle is obtained between the edges of the trigger signals for the two partial capacitors. The time window is smaller than the pulse duration of a trigger signal. It is advantageous if the time window essentially includes only the respective edge change, meaning it represents only a fraction of the total signal period. Interference signals consequently do not influence the evaluation during the complete signal duration, but only briefly during the edge change where the interference distance is high. As a result, the linearity of the measuring system is improved and a high measuring accuracy is possible.
The processing device can contain a capacitive sigma/delta converter that essentially consists of an integrator with a downstream-connected comparator. The integrator integrates the charge received from the differential capacitor. The comparator provides an output signal that corresponds to the mathematical sign for this charge and determines the signal edge to be evaluated next. If both partial capacitors are triggered with positive rectangular signals, for example, either the rising edge of the trigger signal for the one partial capacitor or the decreasing edge, offset by half a cycle, of the trigger signal to the other partial capacitor is selected in dependence on the comparator output. With trigger signals having different polarity, either the front or the rear edges of the trigger signals are always evaluated.
The time window should be sufficiently large, so that the received charge packets are almost completely integrated in an integration capacitor of the integrator, taking into consideration the internal resistance of the source supplying the integration capacitor. The time window preferably is larger than the sum of the values for the trigger signal rise time and ten times the time constant &tgr;=R
i
C, where R
i
characterizes the internal resistance of the voltage source used and C the maximum possible measuring capacitance. On the other hand, the time window should be as small as possible, meaning it should essentially only detect the signal edges and should be synchronized as accurately as possible with these edges, to limit the i
Steiner Christian
Strack Peter
Carl Mahr Holding GmbH
Kerveros James
Le N.
Smith Stuart I.
Venable LLP
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