Sensor and circuit architecture for three axis strain gauge pointing device and force transducer

A pointing device for use with computers and other electronic systems incorporates an array of resistive strain gauges on the surface of a substrate. The strain gauges exhibit resistance changes in response to stress or strain applied to the substrate by the movement of a joystick attached to the substrate, the resistance changes being proportional to the extent of movement of the joystick The strain gauges are electrically connected to control circuitry that successively establishes a series of voltage dividers across different ones of the strain gauges to measure the resistance of each of the strain gauges. From those measurements the position of the pointing device may be determined by comparing the measured resistances with known resistance values that correspond to a neutral stick position. The control circuitry establishes the voltage dividers by applying a known voltage across successive pairs of the strain gauges, and then detecting the voltage at the midpoint between the strain gauges that form each strain gauge pair to which the voltage is applied.

CROSS-REFERENCE TO RELATED APPLICATIONS
 Not Applcable
 FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
 Not Applicable
 BACKGROUND OF THE INVENTION
 The present invention relates generally to pointing devices for use in
 connection with computers and other electronic instruments and devices.
 More specifically, the present invention relates to a pointing device used
 to move a pointer or cursor on a display of a computer or similar
 electrical or electronic device or instrument.
 Many operations relating to the use of modern computers, and in particular
 personal computers, require that a pointer or cursor be placed in a
 particular location on a display screen. In addition, many
 computer-operated and video games are operated based on positioning the
 cursor or pointing device in a particular location.
 In some applications, the position of the cursor may be governed by a touch
 pad, which is a small touch-sensitive pad embedded in the casing of the
 device or instrument. Pressure applied to different portions of the pad
 controls the movement of the cursor on the screen.
 In other applications, a joystick is used to control the movement of the
 cursor. On some laptop computers, this joystick may be a very small device
 positioned between several of the keys on the keyboard. The computer user
 manipulates such a small joystick by the end of the user's finger. On many
 computer games, and in other applications, the joystick may be somewhat
 larger, and be manipulated by the user grasping the entire joystick with
 the user's hand. Other applications may use joysticks of different sizes
 to manipulate a position indicator on the screen. The present invention
 will be described in connection with its application to joysticks.
 The position of the joystick, and its movement relative to its central
 "rest" or "neutral" position, should be identified so that such position
 information can be transferred to place the cursor at the appropriate spot
 on the computer screen or display. A variety of devices have been designed
 for detecting the position of a joystick. One particular mechanism for a
 pointing device is described in U.S. Pat. No. 5,640,178 to Endo et al.
 This reference describes a joystick mounted on a resilient substrate.
 Strain gauges are formed on one surface of the substrate. A voltage is
 applied across a pair of the strain gauges in a particular direction, and
 the voltage at a half bridge output terminal between the strain gauges is
 measured. From this voltage, the amount of strain on each of the strain
 gauges of the pair may be determined. From that information, position
 information can be interpreted. However, this measurement technique
 requires that the strain gauges of the pair be exactly matched, with
 exactly equivalent properties. In one embodiment, the strain gauges are
 formed in a particular configuration so they can be laser trimmed to
 ensure that the strain gauges have identical properties.
 SUMMARY ON THE INVENTION
 The present invention is a pointing device that measures strain on a
 substrate, and a method of operating such a pointing device. The strain
 gauges are configured as a series of voltage dividers across the substrate
 surface in a pattern that will allow the changes in the resistances of the
 strain gauges to be measured. From that determination, the position and
 displacement of the pointing device can be defined.
 In accordance with the present invention, a plurality of perimeter contact
 points is formed on the substrate surface, as is a plurality of strain
 gauges. Each strain gauge electrically connects a central contact point
 with one of the perimeter contact points. A controller circuit connected
 to a voltage source successively connects the voltage source across
 selected pairs of the perimeter contact points, and then detects the
 voltage at the central contact point between the strain gauges that are
 connected between the perimeter contact points across which the voltage
 source is connected. Different patterns of high and low voltages are
 connected to the perimeter contact points to establish the voltages across
 the different pairs of strain gauges.
 In one particular embodiment, the strain gauges of the pointing device are
 formed of strain-sensitive ink applied to the substrate surface. In
 another embodiment, a resistor having a known resistance is connected to
 the central contact point and the voltage is applied across that known
 resistance as well as the strain gauge resistances to determine a force
 applied along an additional axis. In still another embodiment, a resistor
 having a known resistance is connected in series with one of the pairs of
 strain gauges to measure the amount of strain when the strain gauges are
 stressed in the same direction. The voltage drop across each of the strain
 gauge resistors is measured to determine the force on each of the three
 axes.

DETAILED DESCRIPTION OF THE INVENTION
 Referring first to FIGS. 1 and 2, a pointing device 21 incorporating a
 preferred embodiment of the present invention is illustrated. The pointing
 device 21 includes a base 31 that may comprise a printed circuit board
 incorporating various circuit elements, as described below. A planar
 substrate 23 is suspended above the base 31. The substrate 23 is
 preferably formed of alumina, but it may also be formed of ceramic. The
 substrate 23 is preferably square, a shape that maximizes strength.
 Furthermore, the square shape optimizes space utilization by the strain
 gauge sensor arrays (as described below), thereby maximizing yields from
 existing alumina and ceramic plates that would be used for the substrate
 23.
 A plurality of conductive solder contact pads 33 permit electrical contact
 between circuitry formed on the underside of the substrate 23 and
 circuitry formed on the base 31. A post or stick 25 projects from the
 upper side of the substrate 23. The stick 25 (which may be in the form of
 a joystick) forms the portion of the device 21 that may be manipulated
 (either directly or indirectly) by the user, and its movement from a
 predefined neutral position applies stress and/or strain to the substrate
 23.
 Referring next to FIGS. 3A and 3B, an array 39 of strain gauges 41, 42, 43,
 44 is arranged on the lower surface of the substrate 23. Preferably, the
 array 39 of strain gauges is applied to the surface of the substrate 23
 that is opposite the surface from which the stick 25 projects. FIGS. 4 and
 5 respectively show a second strain gauge array 39a and a third strain
 gauge array 39b, both of which will be described in more detail below.
 Each array 39, 39a, 39b of strain gauges comprises a pattern of individual
 strain gauges. The pattern of strain gauges in the array is preferably
 symmetrical. For example, in the array 39 of FIGS. 3A and 3B, the strain
 gauges 41-44 are arranged in a cruciform array, in pairs along each of the
 diagonals of the square substrate 23, and they are all substantially
 equidistant from the geometric center of the substrate 23.
 Each strain gauge constitutes a variable resistor, the resistance of which
 changes as a function of strain applied to the portion of the substrate 23
 on which the strain gauge is located. For example, a tensile strain may
 cause the resistance of a strain gauge to increase, while a compressive
 strain may cause the resistance of the strain gauge to decrease. The
 strain gauges are advantageously formed of strain-sensitive ink applied to
 the surface of the substrate. Suitable inks are well-known in the art. The
 other elements of the strain gauge array may be formed by applying
 conductive ink to the surface of the substrate.
 Referring, for example, to the embodiment illustrated in FIGS. 3A and 3B,
 four strain gauges 41, 42, 43, and 44 are provided in a cross-shaped array
 39. FIG. 3A illustrates the strain gauges 41, 42, 43, 44 schematically as
 variable resistors. Each strain gauge 41, 42, 43, 44 connects a central
 contact point 49 with a respective one of several perimeter contact pads
 51, 52, 53, 54. The central contact point 49 may be formed as a contact on
 the surface of the substrate 23, or it may be formed on a circuit board
 (not shown) on the base 31. In the latter case, each of the strain gauges
 41-44 would be electrically connected to the central contact point 49 by a
 discrete conductor, such as a contact pad or a wire (not shown). In the
 physical arrangement shown in FIG. 3B, for example, the central contact
 point 49 (FIG. 3A) is not on the substrate 23, but rather it is on the
 base 31. Each of the strain gauges is connected between its respective
 perimeter contact point and one of a plurality of secondary contact pads
 55, each of which is electrically connected to the central contact point
 49 on the base 31. The perimeter contact pads 51, 52, 53, 54 and the
 secondary contact pads 55 are advantageously formed of a conductive ink,
 as are the connections between each of the contact pads and its respective
 strain gauge, and the connections among the strain gauges. The solder
 contact pads 33 (FIG. 1) may contact the perimeter contact pads 51, 52,
 53, 54 and the secondary contact pads 55 of the substrate to provide
 electrical connection between the strain gauge array and the circuitry
 formed on the base board 31.
 First and second strain gauges 41, 42 along a first axis of the
 cross-shaped array 39 may be used to measure the strain caused in the
 substrate 23 by a movement of the stick 25 in the first axis (which for
 the purposes the following discussion may be described as the Y axis).
 Third and fourth strain gauges 43, 44 on the other (orthogonal) axis of
 the cross-shaped array 39 may be used to measure the strain caused in the
 substrate 23 by movement of the stick 25 in the orthogonal axis 13 (which
 for the purposes of the following discussion may be described as the X
 axis). When the stick 25 is pushed in the Y axis toward the second strain
 gauge 42, a tensile strain is applied to the second strain gauge 42, while
 a compressive strain may be applied to the first gauge 41. The compressive
 strain in the first strain gauge 41 will cause the resistance of the
 strain gauge 41 to change in one direction. For example, the resistance in
 the strain gauge under compression may decrease. In contrast, the
 resistance of these second strain gauge 42 that is under tensile strain
 will change in the other direction. For example, the resistance of the
 second strain gauge under tensile strain may increase. A similar
 phenomenon is experienced by the strain gauges 43, 44 when the stick 25 is
 moved along the X axis. The change in strain gauge resistance will be
 substantially proportional to the distance that the stick 25 is moved from
 its neutral position. Thus, the magnitude of the stick's movement (i.e.,
 its distance from the neutral position) along either axis is determined by
 the magnitude of its resistance change from a nominal value established
 for its neutral position, while the direction of movement is determined by
 which of the strain gauges along a particular axis increases in resistance
 and which decreases in resistance.
 If desired, the device can be configured to measure movement along a third
 (Z) axis orthogonal to the X axis and the Y axis (i.e., perpendicular to
 the plane of the substrate). When a force is applied to the top of the
 stick 25 (that is, a force applied anally to the stick 25), strain is
 applied to the resistors 41, 42, 43, 44, causing the resistance in all of
 the strain gauges to change in the same direction. In this manner,
 movement along the Z axis can be measured. Of course, the stick 25 may be
 moved in a direction that has components along more than one of the X, Y,
 and Z axes, and the affected strain gauges will react in the appropriate
 manner.
 Referring next to FIG. 6, the strain gauge array of FIGS. 3A and 3B is
 connected to control circuitry for selectively applying voltages to each
 of the perimeter contact pads 51, 52, 53, 54 of the strain gauge array 39,
 and for measuring outputs at others of the perimeter contact pads.
 Appropriate control of the application of voltages to the strain gauge
 array allows the control circuitry to determine the resistance value of
 each of the strain gauges 41, 42, 43, 44. By comparing the measured value
 of each of the strain gauge resistances with a previously stored value
 determined when the strain gauge array is not under stress, the direction
 and magnitude of the movement of the stick 25 can be determined. Circuitry
 in the base 31 may include control circuitry such as the control circuitry
 shown in FIG. 6. Alternatively, circuitry in the base circuit board 31 may
 connect the strain gauge array with control circuitry at another location.
 The control circuitry successively applies a voltage across different pairs
 of the strain gauges, and measures the voltage at the central point 49, to
 create a series of voltage dividers. From creating and measuring the
 output from a series of such voltage dividers, the values of, or changes
 in, the resistance value of each of the strain gauges 41, 42, 43, 44 can
 be determined.
 Referring now to FIGS. 7A to 7D, voltage divider circuits are shown from
 which the changes in the resistances of the strain gauges 41, 42, 43, 44
 may be determined. Each of the circuits shown in FIGS. 7A-7D may be used
 with the control circuitry of FIG. 6. Referring first to FIG. 7A, a
 voltage or potential is applied across the pair of Y axis strain gauges
 41, 42. To accomplish this, a high state (V.sub.CC) may be connected to
 the first perimeter contact point 51 (FIGS. 3A, 3B), which is adjacent the
 first strain gauge 41, while a low state (ground) is connected to the
 second contact point 52 (FIGS. 3A, 3B) adjacent the second strain gauge
 42. The voltage at the central contact point 49 may be measured by
 detecting the voltage at one of the X axis perimeter contact points 53, 54
 that are adjacent the orthogonal strain gauges 43, 44. In the schematic
 illustrated in FIG. 7A, the voltage is detected at the perimeter contact
 pad 54 adjacent the strain gauge 44. This configuration produces a voltage
 divider across the Y axis strain gauges 41, 42. Subsequently, the voltage
 across the strain gauges 41, 42 may be reversed, with the high state
 V.sub.CC applied to the second perimeter contact point 52, and the low
 state (ground) applied to the first perimeter contact point 51.
 Measurement or detection of the midpoint voltage may again be taken off
 the fourth contact point 54. This configuration is shown schematically in
 FIG. 7B. Taking the voltages measured at the central contact point 49 in
 the two voltage divider arrangements allows the relative values of the
 variable strain gauges 41, 42 to be determined. By taking two voltage
 measurements at the fourth perimeter contact point 54, one for each
 polarity of the voltage applied across the Y-axis strain gauges 41, 42,
 the effect of the X-axis strain gauge 44 (through which the voltage at the
 central contact point 49 is measured) is essentially negated.
 Similarly, the X axis strain gauges 43, 44 may be analyzed by applying a
 voltage across the X axis strain gauge pair 43, 44, and measuring the
 voltage at the central point 49 between them. Referring to FIG. 7C, a high
 state (V.sub.CC) may be applied to the third perimeter contact point 53
 adjacent the strain gauge 43, and a low state (ground) may be applied to
 the fourth perimeter contact point 54. The voltage is detected at one of
 the other perimeter contact points, such as the first perimeter contact
 point 51. The voltage may then the reversed across the strain gauges 43,
 44, as shown in FIG. 7D. Using the voltage detected at the midpoint or
 central contact point 49 when a voltage is applied across the X axis
 strain gauges 43, 44, the relative values of the strain gauges 43, 44 may
 be determined. Again, the use of two voltage measurements at the first
 perimeter contact 51 substantially negates the effect of variations in the
 resistance of the Y-axis strain gauge 41 through which the voltage
 measurement is taken.
 In accordance with an aspect of the present invention, the control
 circuitry may perform this analysis on the strain gauges 41, 42, 43, 44
 when the pointing device is first powered on, such as when the computer is
 initially turned on. If, at power-on, no pressure is being applied to the
 stick 25 of the pointer 21, the nominal, or "at rest," values of the
 strain gauges 41, 42, 43, 44 may be determined. These nominal values may
 be stored for later comparison with values measured when the strain gauges
 41, 42, 43, 44 are under stress due to manipulation of the stick 25.
 Because the nominal value of each strain gauge is determined, and can be
 used as a comparison point in later measurements, it is not necessary that
 each strain gauge be exactly identical. Differences among the
 characteristics of the strain gauges may be compensated for in the
 calculations made during subsequent measurements. Therefore, manufacturing
 procedures may be simplified, and it may not be necessary to precisely
 trim each strain gauge so that they all have exactly the same resistance
 value and performance characteristics.
 Referring again to FIG. 6, the control circuitry may include a general
 purpose microprocessor 71 having a plurality of digital input/output ports
 P0, P1, P2, P3, Q0, and Q1. Each of the digital input/output ports P0-P3
 of the microprocessor 71 is connected to a corresponding one of the
 perimeter contact points 51, 52, 53, 54. The input/output port Q0 is
 connected to the fourth perimeter contact point 54 through a first fixed
 resistor 77, and the digital input/output port Q1 is connected to the
 second perimeter contact point 52 through a second fixed resistor 79. The
 fixed resistors preferably have resistance values about 1.5 times the
 nominal (unstressed) resistance value of each of the strain gauge
 resistors 41-44.
 A summer 75 combines the states of some of the perimeter contact points and
 some of the digital input/output ports of the microprocessor 71 to produce
 a signal that can be interpreted to determine the strain applied to the
 strain gauge elements 41-44. In the illustrated example, the states of the
 digital input/output ports P0, P3, Q0, and Q1 of the microprocessor 71 are
 combined with the states of the perimeter contact points 52, 54.
 Some of the digital input/output ports of the microprocessor 71 are
 connected directly to the summer 75, while others are connected through
 fixed resistors. Specifically, the digital input/output port Q0 is
 connected directly to a first input of the summer 75. The fourth perimeter
 contact point 54 is also connected to the first input of the summer 75
 through the first fixed resistor 77. The digital port Q1 is directly
 connected to a second input of the summer 75. The second perimeter contact
 point 52 is also connected to the second summer input through the second
 fixed resistor 79. The input/output port P3 and the fourth perimeter
 contact point 54 are connected to a third input of the summer 75 through a
 third fixed resistor 81. Finally, the input/output port P0 and the second
 perimeter contact point 52 are connected to a fourth summer input through
 a fourth fixed resistor 83. In one particular exemplary embodiment, each
 of the third and fourth fixed resistors 81, 83 has a resistance of
 approximately 2.5 times the nominal (unstressed) resistance of the strain
 gauge resistors 41-44.
 A digital to analog (D/A) converter 91, which may be a portion of the
 microprocessor 71, provides a negative the input to the summer 75.
 The output of the summer 75 is applied to an amplifier 93. The amplifier
 output is applied to an analog to digital (A/D) converter 95, which may be
 part of the microprocessor 71. An offset 97 may be a also applied to the
 amplifier 93 to compensate for any DC bias arising from mismatch among the
 individual components of the system.
 The circuit illustrated in FIG. 6 may additionally provide measurement of a
 force applied to the stick 25 of the position control 21 in the Z axis
 (perpendicular to the plane of the substrate 23, as seen in FIG. 2). A Z
 axis force applied to the substrate 23 creates the same strain in the X
 axis strain gauges 43, 44 and in the Y axis strain gauges 41, 42, and the
 strain is in the same direction for all the strain gauges 41, 42, 43, 44.
 The Z axis force can be measured by placing a resistor in series with
 either the X axis strain gauges 43, 44, or the Y axis strain gauges 41,
 42, and applying a voltage to both axes. One of the resistors 77, 79, 81,
 83 may be used for this purpose. By measuring the voltage drop across the
 resistor placed in series with the one axis, the strain applied in the Z
 direction to the strain gauge resistors 41, 42, 43, 44 can be determined.
 For example, the resistor 77 may placed in series with the X aids strain
 gauges 43, 44. The voltage drop across the resistor 77 can be measured
 through the electrical path connecting the perimeter contact point 54 with
 the summer 75 (through the resistor 81).
 To measure the strain in each axis (X, Y, Z), the digital input/output
 ports P0-P3, Q0, and Q1 of the microprocessor 71 are set to successively
 establish the voltage divider configurations shown in FIGS. 7A-7D. To set
 those voltage dividers, the digital input/output ports P0-P3, Q0, and Q1
 of the microprocessor 71 are set to the following states:

Illustrated
 STATE Arrangement P0 P1 P2 P3 Q0 Q1
 Measure Y Figure 7A 0 R 1 R R 0
 (1)
 Measure Y Figure 7B 1 R 0 R R 1
 (2)
 Measure X Figure 7C R 1 R 0 0 R
 (1)
 Measure X Figure 7D R 0 R 1 1 R
 (2)
 Measure Z R 1 1 R 0 0
 In which: 1 is relatively high voltage (V.sub.CC); 0 is relatively low
 voltage (ground); and R is high impedance.
 The microprocessor 71 may be programmed to selectively set the
 microprocessor input/output ports P0-P3, Q0, and Q1 in the appropriate
 states.
 The microprocessor 71 has an output port 99, which may be any of several
 conventional microprocessor output ports. For example, the port 99 may be
 a PS/2 port, an RS232 port, or a Universal Serial Bus. Other types of
 output ports will become apparent to those skilled in the art.
 By successively setting up each of the two voltage divider circuits shown
 in FIGS. 7A and 7B, a succession of signals indicative of the voltage at
 the central contact point 49 is applied through the summer 75 to the
 microprocessor 71. The microprocessor 71compares the voltage value
 received with a voltage value stored in the non-volatile memory of the
 microprocessor 71 that is read on power-up, or at another time when no
 strain is placed on the pointing device. From this comparison, the
 microprocessor 71 calculates the changes in the resistances of the Y axis
 strain gauges 41, 42. From these changes, the microprocessor 71 can, using
 conventionally known procedures, determine the stresses being applied to
 the stick 25 of the cursor control 21 in the Y axis. From these stresses,
 the microprocessor 71 can then further determine the appropriate position
 along the Y axis for the cursor or other pointing device.
 Similarly, by successively setting up each of the two voltage divider
 circuits shown in FIGS. 7C and 7D, a succession of signals indicative of
 the voltage at the central contact point 49 is applied through the summer
 75 to the microprocessor 71. The microprocessor 71 compares the voltage
 value received with a voltage value stored in the non-volatile memory of
 the microprocessor 71 that is read on power-up, or at another time when no
 strain is placed on the pointing device. From this comparison, the
 microprocessor 71 calculates changes in the resistances of the X axis
 strain gauges 43, 44. From these changes, the microprocessor 71 can
 determine, using conventionally known procedures, the stresses being
 applied to the stick 25 of the cursor control 21 in the X axis. From these
 stresses, the microprocessor 71 can then further determine the appropriate
 position along the X axis for the cursor or other pointing device.
 An alternative circuit for measuring a Z axis force applied to the pointing
 device is shown in FIG. 8. In the illustrated embodiment, contact is
 provided directly to the central contact point 49 of the strain gauge
 array. This direct contact allows direct measurement of the voltage
 division across the strain gauges 41-44. The central contact point 49 is
 connected to the input of an amplifier 101. The output of the amplifier
 101 is applied to an analog to digital converter 103 for use by a
 microprocessor 105. The analog to digital converter 103 may form a portion
 of the microprocessor 105. The microprocessor 105 has a plurality of
 digital input/output ports 107, individually labeled P0-P4. These digital
 input/output ports 107 are successively set to high and low voltages, and
 to high impedance to set up the series of voltage dividers.

STATE P0 P1 P2 P3 P4
 Measure Y (1) 0 R 1 R R
 Measure Y (2) 1 R 0 R R
 Measure X (1) R 1 R 0 R
 Measure X (2) R 0 R 1 R
 Measure Z R 1 1 R 0
 An external known (and generally fixed) resistor 108 is connected directly
 between the central contact point 49 and one of the digital input/output
 ports 107 (i.e., P4) of the microprocessor 105. This additional resistor
 108 provides an additional bridge in the strain gauge array that may be
 measured. By setting the states of the digital input/output ports 107 of
 the microprocessor 105 to establish a series of voltage dividers across
 the strain gauges 41-44 in a manner similar to that described above in
 connection with the arrangement illustrated in FIG. 6, the force
 components in the X and Y axes may be determined. A fifth voltage divider
 can be set up across the additional fixed resistor 108 by appropriately
 controlling the digital input/output port P4 of the microprocessor 105.
 Because a Z axis force causes the resistances of the first four strain
 gauges 41-44 all to change in the same direction, the voltage divider
 across the one of the Y-axis strain gauges (e.g., the strain gauge 42),
 one of the X-axis strain gauges (e.g., the strain gauge 44), and the
 eternal fixed resistor 108 allows the Z axis force to be determined. The
 high voltage (V.sub.CC) may be applied to the strain gauges 42 and 44
 simultaneously. The external resistor 108 may be connected between the
 central contact point and a very low potential (such as ground). By
 measuring the voltage at the central contact point, between the strain
 gauges 42 and 44 and the external resistor 108, a voltage divider is
 established. Because the external resistor 108 is fixed, the change in the
 resistance of the strain gauges 42 and 44 may be measured, and the strain
 in the Z aids calculated. As with the embodiment described above in
 connection with FIG. 6, the digital input/output ports 107 are used to
 switch among a high voltage (V.sub.CC), ground, and high impedance states
 around the array of strain gauges 41-44.
 The microprocessor 105 may generate an offset signal to compensate for
 differences among the values of the individual resistors 41-44 of the
 strain gauge array. This offset signal is converted to an analog signal in
 a digital to analog converter 109, which may also be a portion of the
 microprocessor 105. The analog offset signal is applied to the amplifier
 101.
 Referring now to FIGS. 4 and 5, two alternative strain gauge arrays, 39a
 and 39b, respectively, are shown. Each of the triangular strain gauge
 arrays illustrated in FIGS. 4 and 5 includes three strain gauges, and it
 may be applied to a triangular substrate that substantially conforms to
 the strain gauge array.
 FIG. 4, for example, illustrates a triangular arrangement of strain gauge
 variable resistors 141, 142, 143, each connecting a central contact point
 149 with one of three perimeter contact points 151, 152, 153. A fixed
 resistor 144 connects the central contact point 149 with a fourth
 perimeter contact 154.
 A known voltage V.sub.CC may be connected to the fourth perimeter contact
 154, and a series of voltage dividers may be created that divide a known
 voltage across the fixed resistor 144 and, in succession, the variable
 resistances of the strain gauges 141, 142, 143. The midpoint voltage at
 the central contact point 149 is detected on a fifth perimeter contact
 155. From such a series of voltage dividers, the resistances of the strain
 gauges 141, 142, 143 may be measured, and the strain applied to each of
 the strain gauges determined. From this information, the appropriate
 position for the cursor control may be determined.
 In particular, a known voltage V.sub.CC may be applied to the fourth
 contact point 154 that is connected to the fixed resistor 144. The second
 and third perimeter contacts 152, 153 may be connected to high impedances,
 and the first perimeter contact 151 may be connected to a high potential
 through a low impedance. In this fashion, the voltage divider is set up
 across the fixed resistor 144, and the first strain gauge variable
 resistance 141. The output voltage at the central contact point 149
 forming the midpoint of the voltage divider is measured at the fifth
 perimeter contact 155. From the known resistance of the fixed resistor
 144, the value of the variable resistor 141 may be determined. Similar
 voltage dividers may successively be set up for the second and third
 strain gauges 142, 143.
 As shown in FIG. 9, the strain gauge array 39a of FIG. 4 is connected to
 control circuitry for establishing the voltage dividers described above.
 In this control circuit, the known voltage V.sub.CC is supplied to the
 fourth perimeter contact point 154 of the strain gauge array 39a through a
 digital input/output port P3 of a microprocessor 105a. The perimeter
 contact points 151, 152, 153 of the strain gauge array are connected to
 the digital input/output ports P0, P1, P2, respectively, of the
 microprocessor 105a. The digital input/output ports P0, P1, P2 are
 selectively set to establish the voltage dividers across the three strain
 gauges 141, 142, 143.
 The fifth perimeter contact point 155 is connected to the input of a
 summing amplifier 101. An offset may be applied to the amplifier 101 from
 the microprocessor 105a through a digital to analog converter 109. The
 output of the amplifier 101 is applied to the microprocessor 105a through
 an analog to digital converter 103.
 Using trigonometric calculations, the strain measured in each of the
 triangular legs of the array illustrated in FIG. 4 can be converted to X
 and Y axis values, or other appropriate coordinates. Preferably, the
 triangular arrangement of the strain gauges is symmetrical, with a 120
 degree angle between each of the strain gauge legs.
 Yet a third strain gauge array 39b is illustrated in FIG. 5, which
 illustrates a triangular array of three strain gauges 241, 242, 243. Each
 of the strain gauges 214, 242, 243 connects a central contact point 249
 with one of three perimeter contact points 251, 252, 253. Using tri-state
 ports on a microprocessor (not shown), a series of voltage dividers may be
 established across successive pairs of the strain gauges 241, 242, 243. It
 will be recognized that control circuitry similar to that shown in FIG. 9
 may be configured to set up such a series of voltage dividers. Using the
 known angles for the each leg of the strain gauge array 39b, the stresses
 measured on each leg of the array can be converted to an appropriate
 coordinate system.
 The systems and methods described above may consume less power than many
 existing systems and methods for pointing devices. Using the systems and
 methods described above, the pointing device and its controller circuitry
 need not be powered at all times, but need only be powered at times that
 measurements are being taken. In addition, the power is switched around
 the various contact points on the substrate, to successively power the
 different strain gauges, so that not all contact points need to be
 continuously powered.
 An additional advantage of the apparatus and methods described above is
 that automatic gain adjustment can be achieved by simply correcting the
 amplifier offset through the analog output feedback from the
 microprocessor 71 to the amplifier 93 (in the FIG. 6 embodiment).
 A further advantage of the systems and methods described above is that the
 square, triangular, or other regular shape of the strain gauge array
 permits the substrate 23 to be manufactured in a similarly regular shape.
 Such regular shaped substrates are easier to manufacture than the
 intricate shapes often required with other pointing devices.
 Finally, using appropriate electronic techniques (including calculations by
 the microprocessor), differences in the specific characteristics of the
 strain gauges may be accounted for. Therefore, the strain gauges need not
 exactly match one another, and expensive laser trimming of the strain
 gauges to match them is not necessary.
 Those skilled in the art will recognize that modifications may be made to
 the particular embodiments described above without departing from the
 invention. For example, other arrangements of the strain gauges on the
 substrate may be made, and different substrate materials may be used. In
 addition, other types of control circuitry may be used. Moreover, the
 device 21 can be configured for use as a force transducer, with the
 control circuit employing a microprocessor that is programmed to yield a
 force-indicative output signal in response to the sensed displacement of
 the stick 25. Therefore, the particular embodiments described above are
 considered exemplary, and should not be considered limiting. Rather, the
 scope of the invention is defined in the claims that follow.