The present invention relates in general to sensors and in particular to a piezoresistive membrane having a centered weight for detecting angular or relative displacement such as for detecting the tilt of a body.
Many tilt sensors are of the electrolytic type. A typical electrolytic sensor includes a glass or ceramic envelope that is partially filled with a conductive fluid. The fluid moves in response to tilting of the sensor where the fluid is under the influence of gravity, such as with a carpenter""s spirit level. In other embodiments the fluid may be under the influence of the acceleration of a body. Platinum contacts may be sealed flush with the inside walls of the envelope. When such a sensor is at its zero position the electrical impedance of the fluid from the center electrode to each of the left and right electrodes is equal. Tilting the sensor disturbs this balanced condition and the impedance changes in proportion to the tilt angle. Cost and size of a typical electrolytic sensor limit their use in certain environments. Many electrolytic sensors are sensitive to temperature change and temperature compensation needs to be provided in most of the signal conditioning electronic units. Also, with respect to glass electrolytic sensors, great care must be afforded to the thermal and mechanical stress related characteristics of glass during installation and alignment. This may limit the range of applications of such sensors.
Another known category of tilt sensor types is switch sensors, which may be a variation of an electrolytic sensor. A switch type tilt sensor doesn""t use a linear output with respect to inclination angle. Instead, a signal is generated once the inclination reaches a predetermined threshold.
While not used specifically for detecting the tilt of a body, a common structure for measuring pressure are pressure transducers. Pressure transducers may be diaphragm-based transducers that convert an applied pressure into stresses in the plane of the diaphragm. The stresses may be measured and converted into electrical signals by use of piezoresistive sensors that are an integral part of the diaphragm. Depending on the application, the diaphragm may be fabricated of metal or a semiconductor material such as silicon. Such configurations are known to be used in microphones, the automotive industry such as for checking tire, gas and air pressure, the biomedical industry such as for determining blood and fluid pressure, various instrumentation and vacuum sensing. The piezoresistive effect varies as a function of the structure of the silicon""s crystal lattice. Resistance in particular is dependent on changes in length and resistivity of the piezoresistor caused by stress. The following equation applies:       R    =          rL              A        ⁢                  
                ⁢                  R          =          resistance                          L    =    length        r    =    reistivity        A    =    area  
The relationship between stress and resistance change for silicon may be expressed by:                                                         Δ              ⁢                              xe2x80x83                            ⁢              R                        R                    =                                                    π                L                            ⁢                              σ                L                                      +                                          π                T                            ⁢                              σ                T                                                    ⁢                  
                ⁢                  R          =          Resistance                ⁢                  
                ⁢                              Δ            ⁢                          xe2x80x83                        ⁢            R                    =                      Resistance            ⁢                          xe2x80x83                        ⁢            change                          ⁢                  
                ⁢                              π            L                    =                      Longitudinal            ⁢                          xe2x80x83                        ⁢            piezoresistive            ⁢                          xe2x80x83                        ⁢            coefficient                          ⁢                  
                ⁢                              π            T                    =                      Transverse            ⁢                          xe2x80x83                        ⁢            piezoresistive            ⁢                          xe2x80x83                        ⁢            coefficient                          ⁢                  
                ⁢                              σ            L                    =                      Longitudinal            ⁢                          xe2x80x83                        ⁢            stress                          ⁢                  
                ⁢                              σ            T                    =                      Transverse            ⁢                          xe2x80x83                        ⁢            stress                                              (        1        )            
Equation (1) includes two piezoresistive coefficients xcfx80L and xcfx80T. These two piezoresistive coefficients are longitudinal and transverse as indicated by their respective subscripts. The longitudinal piezoresistive coefficient relates the relative resistance change due to an applied stress in a piezoresistive element when the stress is in the same direction as the current flow through the element. The transverse piezoresistive coefficient relates the relative resistance change due to an applied stress in a piezoresistive element when the stress is at right angles to the current flow through the elements. It should be noted that the transverse piezoresistive coefficient could be defined for a stress in the plane of the sensor as well as for a stress normal to that plane.
The change in resistance of the piezoresistive elements, or piezoresistors, can be used to create a voltage output by means of a conventional Wheatstone bridge circuit. In this respect, the resistance in the bridge legs changes in response to pressure applied to a diaphragm, for example, of which the piezoresistors are an integral part. A known configuration for such an arrangement may be semiconductor devices with the resistive bridge legs formed such as by appropriate doping of selected portions of material in the semiconductor material. For example, U.S. Pat. No. 5,614,678 discloses a device using semiconductor material that is lightly doped N- or P-type silicon in a portion of the crystallographic plane. The piezoresistive elements of that device may be fabricated from P+ or P++ silicon in the crystallographic plane using known techniques. The piezoresistive sensing elements may be arranged in a Wheatstone bridge circuit so that two piezoresistors are positive changing and the other two are negative changing. The disclosed arrangement allows for an output voltage to be generated, which is indicative of an applied pressure on the device.
Considering the cost and size advantages of using a semiconductor sensor relative to known tilt sensor types such as an electrolytic sensor, for example, it would be advantageous to provide a low cost semiconductor sensor for detecting tilt that could be adapted for a range of environments such as those requiring small scales in size.
An exemplary embodiment of a sensor for detecting tilt of the present invention takes advantage of the crystallographic structure and piezoresistive effect in semiconductor materials. Such a tilt sensor may include a membrane where a plurality of piezoresistors may be diffused onto the membrane. A weight may be integral to the membrane and placed near the membrane""s center. In one exemplary embodiment four piezoresistors are diffused onto or formed integral with the membrane to form a conventional Wheatstone bridge circuit. A constant voltage input may be applied to the membrane. A change in resistance of one or more of the piezoresistors may be used to create a voltage output by means of the Wheatstone bridge. The voltage output from the Wheatstone bridge may be responsive to the angular displacement or tilt of the membrane from a reference position such as horizontal. The voltage output at any point in time from the Wheatstone bridge may be indicative of the stresses incurred by the membrane due to the membrane""s tilting. This may allow for a qualitative or quantitative measurement of the membrane""s angular displacement or tilt.
In one exemplary embodiment of the present invention two piezoresistors may be placed within the membrane perpendicularly or transverse to an applied stress and the other two may be placed parallel or longitudinally to the applied stress. The piezoresistors may be placed in locations that allow for maximum sensitivity of the sensors. This may be at or near the edges of the membrane where maximum stress occurs under an applied pressure. Sensor sensitivity may be defined as the ratio of change of voltage output to applied pressure. With no stress applied to the membrane in this configuration the voltage output is zero. As the applied pressure or stress is increased the voltage output will increase allowing for a determination of the membrane""s angular displacement or tilt. One advantage of this configuration is that resistance changes resulting from temperature variations tend to cancel each other. In this respect, the sensor is at least partially immune to the effects of temperature.
The membrane according to one aspect of the present invention may be selected to have an appropriate thickness so that at least a portion of it will deflect in response to an applied pressure caused by a weight proximate the center of the membrane when the membrane is tilted. The deflection may stress the piezoresistors thereby changing their respective resistance value. A Wheatstone bridge circuit may be used to transform the changes in resistance value into a voltage output, which may be indicative of the amount the membrane has tilted. The composition of at least a portion of the membrane may be substantially of silicon or a silicon-based alloy provided that electrical isolation is achieved among the piezoresistors. The weight may be composed of a variety of suitable materials such as copper or nickel, for example, and be affixed or secured to the membrane by known techniques such as bonding or electrodeposition, for example.
One advantage of tilt sensors in accordance with the present invention is that exemplary embodiments may be fabricated in varying sizes. Empirical simulation using commercially available finite element analysis software such as software from Ansys, Inc. demonstrates that the dimensions of the membrane and weight vary in proportion to one another and may consequently be adapted for a wide range of applications requiring specific size sensors.