This invention relates generally to angle measurement devices and more particularly to such devices in which the measurement is made with respect to gravity using a resistive sensor in conjuction with lead circuitry.
Several angle measurement devices are known in the prior art, of which U.S. Pat. Nos. 4,503,622, 4,707,927, 4,547,972, 4,866,850, and 5,083,383 are exemplary. These prior art angle measurement devices typically employ an angle sensor that is physically separated from the electronics employed in the devices. The angle sensors are generally of the resistive type employing a potentiometer having an attached pendulum to reference gravity or of the resistive type employing a liquid-filled glass vial. The resistive type prior art sensors suffer inaccuracies due to friction in the pivot joint, while the resistive type sensors employing glass vials are expensive to produce. The physical separation of either type of sensor from its associated electronics is disadvantageous, particularly in the case of capacitive sensors in which the length of leads connecting the electronics to a remotely located sensor or the movement of those leads affects the capacitance of the sensor.
U.S. Pat. No. 4,644,662 is directed to a capacitive type angle sensor that employs a teflon coating on the sensor plates to insulate them from the fluid contained within the sensor and to serve as part of the sealing mechanism. These sensors are also taught to be remotely mounted. The coating on the sensor plates, in conjunction with rings machined into the sensor housing, serves to seal the fluid inside the sensor. However, this sealing arrangement is prone to leakage if the housing screws are not precisely torqued. In addition, the maximum temperature at which these prior art sensors can operate is limited because at higher temperatures the vapor pressure inside the fluid vessel increases to a point beyond which the sealing rings are not effective, thereby permitting the undesirable loss of fluid. Also, the types of sensors that incorporate a fluid as the sensing medium are generally desigend for static applications because, as the sensor is rotated, acceleration forces acting upon the fluid will cause the fluid to slosh, resulting in inaccurate indications. The rate at which the fluid will slosh is determined by the rate of accleration to which the user subjects the sensor.
It is therefore a principal object of the present invention to provide an improved angle measurement device in which the angle sensor and associated electronics are an integral unit, thereby eliminating the problems associated with prior art angle measurement devices in which those components are physically separated.
It is a further object of the present invention to provide an improved angle measurement device in which the containment of the sensor fluid is accomplished in a way that permits operation at higher temperatures than was previously possible.
It is yet another object of the present invention to provide an improved angle measurement device in which the user may remotely control the operation of an internal lead circuit that serves to compensate for acceleration to which the sensor is subjected.
It is yet another object of the present invention to provide an improved angle measurement device that incorporates a partial barrier inside a fluid vessel to dampen the fluid movement as the sensor is rotated, thereby reducing the sensor's sensitivity to motion transients.
These and other objects are accomplished in accordance with the illustrated preferred embodiment of the present invention by providing a printed circuit board having electronic components mounted on the front surface thereof and having a pair of sensor plates etched into the back surface thereof. Containment of the sensor fluid adjacent the sensor plates and a partial web is accomplished by soldering a sensor cover to an etched sealing ring on the back of the circuit board. The sensor plates etched into the back of the printed circuit board, in combination with the sensor fluid, act as variable resistors whose resistance changes as a function of their angle of rotation from a reference position. An output signal across these variable resistors is in or out of phase with an applied reference signal. The output signal is demodulated with the same reference signal and then filtered to a DC level that corresponds to the angle of rotation. An output stage serves as a driver and electronic lead circuit and also as a means for permitting the user to remotely control the application of the electronic lead circuit in the angle measurement device.