Abstract:
An apparatus and a method for controlling a feedback force applied by a joystick to a user gripping the control handle of the joystick. In the preferred embodiment, a light path between an infrared light source and a light sensor is interrupted by a user gripping the handle of a joystick. The output signal from the light sensor is employed by a processor to control the feedback force. When based upon the output signal, the processor determines that the user has just gripped the control handle, the processor enables the feedback force to increase slowly, over a sufficiently long time to enable the user to firmly grip the control handle before the feedback force becomes very strong. The processor immediately disables generation of the feedback force as soon as the user releases the control handle. The processor periodically energizes and then de-energizes the light source, determines an average value of the output signal when the light source is both energized and de-energized, and based upon a comparison of these average values, determines if the user&#39;s hand gripping the control handle has interrupted the light path between the source and light sensor. Additionally, the processor checks for a failure of the light source, and if detected, halts the generation of the feedback force.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to an apparatus and a method for controlling a haptic feedback force transmitted to a user by a device, and more specifically, to an apparatus and a method for preventing a force feedback device from applying a feedback force to a control handle unless the user is gripping the control handle. 
     BACKGROUND OF THE INVENTION 
     A device having energy that can be potentially harmful to a user or which might damage the device should employ an interlock that enables the safe utilization of the device by the user. In those instances where life safety is an issue, a keyed switch is often used as an interlock to guard against exposing the user to potentially hazardous forces or actions. The provision of an interlock serves to put the user on notice that a particular device is associated with a potentially harmful energy, since the interlock must be successfully engaged before the user can interact with the device. For example, an electrical enclosure having a keyed switch (interlock) is often employed to prevent a user from contacting a dangerous enclosed energy, such as a high voltage. The keyed switch causes devices contained within the electrical enclosure to be de-energized when the user opens the enclosure to access its interior. Although interlocks are often employed to de-energize devices having potentially life threatening energies, they are also used to enable a hazardous device to be energized. For example, a user must be typically seated on a lawn tractor to engage an interlock (keyed switch) under the seat that is actuated by the weight of the user, in order to operate the power take-off drive for the mower blades. 
     Interlock switches that must be continually actuated by a user to enable a device to operate are commonly referred to as “deadmnan” switches. This term refers to the concept that when the user&#39;s actuation of the switch goes slack or “dead,” then the interlocked device should be de-energized. Typically, a deadman switch is provided on devices that are firmly grasped by the operator and if released, can independently produce a rapid or sudden movement that may be unsafe to the operator. For example, a hand-held chain saw may include a deadman switch that must be engaged (compressed by the user&#39;s grip) before the chain can be driven. If the grip of the operator loosens on the handle, the deadman switch disengages, stopping the chain. 
     Although a haptic force feedback joystick is not as dangerous as a chain saw, if the control handle is not firmly gripped when the force is applied to the control handle, it can potentially injure a user, possibly causing bruises, or worse. In addition, since the control handle of a force feedback joystick is intended to be held by the user while the force is applied, the drive mechanism and control handle may be damaged if a substantial force is applied while the handle is not grasped. In the prior art, mechanical and/or retro-reflective switches have been employed to stop the movement of a force feedback joystick handle when the user is not gripping the handle. A retro-reflective switch employs a light source that produces a light beam, which is reflected from a user&#39;s hand applied to grip the control handle so that the reflected light actuates a light sensor. These prior art solutions have several problems. First, mechanical switches typically require periodic maintenance to prevent failure due to oxidation or breakage of their electrical contacts. Second, mechanical switches having a high duty cycle rating are relatively expensive. Third, retro-reflective switches do not work reliably when the light beam of the switch is broken by a non-reflective material, such as a darkly colored glove or a user&#39;s hand that has a dark skin tone. Fourth, ambient light sources can easily cause false triggering of retro-reflective switches. Moreover, the prior art teaches a simple interlock that allows the full magnitude of the force to be applied to the control handle as soon as the user grips the handle. If the user&#39;s grip has not yet tightened, the application of maximum force can literally tear the control handle loose from the user, so that it causes injury. Significantly, the prior art does not teach slowly increasing the magnitude of the force when the user initially grips the joystick&#39;s handle. 
     Ideally, a user would prefer that the force be applied slowly as the grip on the control handle is set, but any force being applied should immediately subside when the user loosens a grip on the handle. Further, any sensor applied to detect the user&#39;s grip on the control handle should be insensitive to the reflectivity of the user&#39;s hand or glove worn on the hand and should also be insensitive to ambient light variation. Currently, none of the prior art interlock or deadman switches for a haptic force feedback joystick addresses these problems. It would also be desirable to employ the technique used to disable force applied to the control handle for safety reasons to interrupt force applied to the control handle when the user elects to pause the execution of an application controlling the computer. If a user presses a keyboard button or other control designated to pause a game or other application running on a host computer, the application of force to the haptic joystick control handle would then be disabled just as if the user had released the control handle. Alternatively, by sensing the user&#39;s grip on the joystick control handle, it would be possible to automatically pause the game if the user releases the control handle. Thus, in addition to immediately disabling the application of force applied to the control handle in response to this condition, the application or operating system could be programmed to respond by pausing the application or by causing the host computer to enter a “sleep” mode. None of these features related to pausing an application is provided by any prior art haptic control device. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an electrical circuit is defined for controlling a force applied by a device to a user through a member of the device that is gripped by the user. The electrical circuit includes a switch having a first state and a second state. The switch is actuated when the user grips the member, causing the switch to change to the first state. Conversely, the switch changes to the second state when the user releases the member. A processor that is coupled to said switch responds to the state of the switch to control the application of the force to the member. When the switch changes to the second state, indicating that the user has released the member, the processor immediately disables application of the force. Further, after the switch changes to the first state, indicating that the user has just gripped the member, the processor enables a slow increase in the force applied immediately. By immediately disabling the application of force to the member when it is released, the user protected from possible injury. And, by requiring that the force applied to the member slowly increase when the user has just gripped the member, the electrical circuit ensures that the user has sufficient time to fully grip the member before the full magnitude of the force acts on the member. 
     Another aspect of the present invention is directed to a sensor circuit for determining whether a user is gripping a member of a haptic feedback device to control a force applied to the member. The sensor circuit includes a source of a light signal, and a sensor for detecting the light signal. When the user grips the member, a path of the light signal from the source to the sensor is affected. The sensor produces an output signal indicative of the light signal detected by the sensor, and the output signal changes due to the affect of the user gripping the member. A processor is coupled to the source of the light signal and to the sensor. The processor enables the source to produce the light signal for a first time interval, monitors the output signal of the sensor during the first time interval, disables the source during a second time interval so that the source does not produce the light signal, and monitors the output signal of the sensor during the second time interval. As a function of the output signal during the first time interval and the output signal during the second time interval, the processor determines whether the user is gripping the member. The processor only enables the force to be applied to the member when the user is gripping the member, but not when the user is not gripping the member. 
     Preferably, the source and the sensor are disposed proximate the member and positioned relative to each other so that the user gripping the member interrupts the path of the light signal between the source and the sensor. The processor preferably monitors the source to determine that the source is operational when enabled. Alternatively, the source and the sensor are disposed proximate the member and positioned relative to each other so that the user gripping the member reflects the light signal from the source toward the sensor. 
     The processor periodically enables the source, alternating the first time interval with the second time interval. In addition, the processor determines a first average value of the output signal when the source is enabled and a second average value of the output signal when the source is disabled, finds the difference between said average values, and as a function of the difference, determines whether the user is gripping the member. 
     Preferably, the light signal is within a non-visible waveband. The invention can be applied to a number of different devices. For example, in regard to game control devices, the member can comprise either a control handle, a yoke, a steering wheel, or a digit cradle. 
     Yet another aspect of the present invention is directed to an electrical circuit for controlling application of a feedback force to a user by a member of a device. The electrical circuit has a source that emits light when energized and a light sensor that provides an output signal indicative of whether light from the source is incident on the sensor. The source and the sensor are disposed proximate the member and spaced apart from each other so that a light path between the source and the sensor is obstructed when the user grips the member, preventing light emitted by the source from reaching the sensor. A processor that is coupled to the sensor receives the output signal and thereby determines when the user is gripping the member. When the user is not -ripping the member, the processor disables generation of the feedback force. As an additional feature, the processor is coupled to the source to determine whether the source has failed. If so, the processor also disables the feedback force. 
     Still further aspects of the present invention are directed to methods that are generally consistent with the apparatus discussed above. Specifically, the steps of each method implement functions that are com-parable to those of the components of the apparatus. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     The foregoing aspects and m-any, of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is an electrical schematic of the preferred embodiment employing an optical switch; 
     FIG. 2 is a block diagram of the logic flow that determines the slow turn on and fast turn off m-odes of operation; 
     FIG. 3 is a block diagram of the logic flow that determines whether the optical switch is functioning properly; 
     FIG. 3A is a block diagram of the logic flow that determines the processing and storing of digitized values that are employed to ensure the correct operation of the sensor in the preferred embodiment; 
     FIG. 3B is a block diagram of the logic flow that determines the reject of false triggering of the sensor that is caused by ambient light sources; 
     FIG. 4 is a side elevational view of a haptic (force feedback) joystick to which the present invention is applied as a preferred embodiment; 
     FIG. 5A is a front elevational view of a force feedback yoke (support and base not shown) to which the present invention is alternatively applied; 
     FIG. 5B is a front elevational view of a force feedback steering wheel game control (support and base not shown) to which the present invention is alternatively applied; and 
     FIG. 5C is a side elevational view of a force feedback digit cradle to which the present invention is alternatively applied. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An electrical circuit  100 , which is used for controlling feedback force and other aspects of a haptic joystick, is shown in the schematic block diagram of FIG.  1 . With reference to this Figure, the anode of a light emitting diode (LED)  102  is connected to terminal I of a connector  108   b , and the cathode is coupled to terminal  2  of the connector. The collector of a phototransistor  104  is coupled to the anode of LED  102 , and the emitter is coupled to terminal  4  of connector  108   b . One end of a resistor  106  is coupled to the emitter of transistor  104 , and the other end of the resistor is connected to terminal  3  of connector  108   b . In the preferred embodiment, the light emitted by LED  102  lies within the infrared waveband, and phototransistor  104  conducts when infrared light is incident upon its base. Thus, an optical switch is formed by LED  102  and phototransistor  104 , which thus respectively comprise the source and light sensor of the optical switch. The optical switch has two states. When the infrared light from LED  102  strikes the base of phototransistor  104 , the optical switch is in a conductive state, and when the light path between the phototransistor and the LED is affected by an object, e.g., the presence of the hand of the user grasping the control handle of the joystick interrupting the light path, the phototransistor changes to a non-conductive state. 
     As shown in FIG. 4, a control handle  402  of a haptic joystick  400  to which the present invention is applied includes an orifice  404  disposed on the front surface of the joystick&#39;s handle (below the trigger switch). Phototransistor  104  is disposed within orifice  404 . Similarly, an orifice  406  is disposed below the front surface of control handle  402  that is gripped by the user, and LED  102  is disposed within this orifice so that the light it emits is directed toward phototransistor  104 . It will be noted that the positions of LED  102  and phototransistor  104  can be interchanged without affecting the operation of the optical switch. With this configuration, an uninterrupted light path is provided between the source (LED  102 ) and light sensor (phototransistor  104 ) until the user grips the joystick control handle. When the user grips control handle  402 , the user&#39;s hand interrupts the light path between orifices  404  and  406 , which stops the conduction of the optical switch and changes its output signal. This change in the output signal caused by the user gripping the control handle enables the generation of force feedback, but the magnitude of the force applied to the control handle ramps up relatively slowly after the user&#39;s hand first wraps around the control handle. Limiting the initial force applied to the control handle in this manner avoids the possibility that user will be surprised by the application of an unexpectedly strong force before firmly gripping the control handle. 
     Referring back to FIG. 1, a connector  108   a  having numbered terminals corresponding to those of connector  108   b  is mated to connector  108   b  to link the optical switch (which is disposed in the control handle of the joystick) to the remaining components of electrical circuit  100 . Significantly, connectors  108   a  and  108   b  enable ready replacement of the optical switch components (phototransistor  104  and LED  102 ) without disturbing the other components in the electrical circuit. Most of the other components of electrical circuit  100  are preferably disposed in the more spacious base of the joystick. 
     One end of a capacitor  110  is connected to earth ground, and the other end is coupled to VCC (+5 volts) and to terminal  1  of connector  108   a . A capacitor  116  is connected between earth ground and terminal  2  of connector  108   a . One end of a resistor  118  is also coupled to terminal  2  of connector  108   a , and the other end of the resistor is coupled to the collector of an NPN transistor  114 . The emitter of NPN transistor  114  is connected to earth ground, and its base is connected to one side of a resistor  112 . The other side of resistor  112  is connected to an OUT1 output port of a microcontroller unit (MCU)  136 . MCU  136  includes both random access memory (RAM) and read only memory (ROM), although neither are separately shown. The ROM is used for storing machine instructions that control the operation of MCU  136 , both in regard to the operation of the haptic joystick, and more importantly, the function of the present invention. Although MCU  136  preferably comprises a microcontroller unit, it may alternatively comprise a microprocessor or central processing unit with integrated circuits for memory, an analog-to-digital converter (ADC) chip, and other functions that are included in the single integrated circuit employed for the MCU. 
     An IN1 input port of MCU  136  is coupled to the other end of resistor  118  and to the collector of transistor  114 . When MCU  136  sets the LED DRIVE signal at the OUT1 output port high, the signal is provided at the base of transistor  114 , which causes the transistor to conduct. The conduction of transistor  114  enables an electrical current to flow from VCC through LED  102  to ground, through transistor  114 . The current flowing through LED  102  whenever transistor  114  conducts causes the LED to emit infrared light. 
     When LED  102  is conducting, the LED SENSE signal applied to the IN1 input port of MCU  136  should be equal to the voltage drop across the collector and emitter of NPN transistor  114 , unless the LED has failed, creating an open circuit back to VCC. Such a failure of LED  102  would cause the LED SENSE signal to instead be equal to zero volts (once capacitor  116  is discharged). However, MCU  136  does not provide a continuous LED DRIVE signal to the base of transistor  114 . Instead, the LED DRIVE signal is a series of periodic pulses, so that the LED DRIVE signal is high for 250 microseconds and low for 1.75 milliseconds. Although the conduction of LED  102  is pulsed on and off, MCU  136  monitors and stores the LED SENSE signal values produced at the IN1 input port during both conditions, i.e., during the discrete times that the LED is energized as well as when it is not energized. In this manner, the LED SENSE signal produced at the IN1 input port enables MCU  136  to determine the conduction for LED  102  and the proper operation of NPN transistor  114 , and thus, to determine if either the LED or the NPN transistor has failed. 
     A capacitor  130  is connected between earth ground and the non-inverting input of an operational amplifier (op amp)  120 . An inductor  138  is optionally connected in series between a terminal  4  of connector  108   a  and the non-inverting input of op amp  120 . The inductor and capacitor  130  comprise a filter. Typically, inductor  138  takes the form of a ferrite bead or a core encircled by a few turns of wire. The disposition of inductor  138  makes the connection to the non-inverting input of op amp  120  slightly inductive, so that the input impedance is raised relative to high frequency signals. In this way, oscillation of electrical circuit  100  is prevented, which might cause the emission of, or susceptibility to, radio frequency interference (RFI) by the circuit. 
     Also, terminal  3  of connector  108   a  is coupled to earth ground. One end of each of resistors  122  and  124  is connected in common to the inverting input of the op amp. The other end of resistor  122  is connected to the common ground, and the other end of resistor  124  is connected to an IN2 input port of MCU  136 . Also connected to the IN2 input port is an end of each of resistors  126  and  128 . The other end of resistor  126  is connected to the output of op amp  120 , while the other end of resistor  128  is connected to the common ground. The configuration of op amp  120  and resistors  122 ,  124 ,  126 , and  128  define a fixed gain amplifier, where the gain is controlled by the resistance values. The signal produced at the emitter of phototransistor  104  is sensed by the non-inverting input of op amp  120 . The fixed gain amplifier increases the magnitude of the sensed signal, so that the RCVR signal output by op amp  120  and applied to the IN2 input port of MCU  136  is nearly equal to VCC when phototransistor  104  conducts. As described above, MCU  136  causes LED  102  to be periodically energized with LED DRIVE signal pulses that cause the LED to emit infrared light pulses in a corresponding periodic manner. So long as the user is not gripping the control handle of the joystick, the light pulses incident on the base of phototransistor  104  cause a further corresponding periodic conduction of the phototransistor. The periodic conduction of phototransistor  104  is reflected in the pulsing of the RCVR signal presented at the IN2 input port of MCU  136 . 
     An X-axis actuator  132  is coupled to the OUT2 output port of MCU  136  and is controlled by a pulse width modulated (PWM) X signal provided by the MCU. X-axis actuator  132  generates a feedback force directed along the X-axis of the joystick device that is applied to the control handle of the joystick. The magnitude of the X-axis feedback force is in proportion to the magnitude of the PWM X signal. The OUT3 output port of MCU  136  is coupled to a Y-axis actuator  134 , which employs the PWM Y signal to control the generation of a force along the Y-axis that is applied to the control handle of the joystick. If the user is gripping the control handle, the user will experience the feedback force that is transmitted to the user&#39;s hand through the control handle. 
     Significantly, if the program executed by MCU  136  determines that the values of the LED SENSE signals monitored during the times that LED  102  is (should be) either energized or de-energized are inconsistent with the proper operation of the LED, i.e., determines that the LED has failed, then the MCU will immediately disable output of the signals PWM X and PWM Y that would otherwise be provided to X-axis and Y-axis actuators  132  and  134 . 
     The values of the RCVR signal during the times that LED  102  is periodically energized and de-energized are independently averaged, and their difference is employed to determine whether the light path between the LED and the phototransistor has been broken by the user gripping the control handle of the joystick. When it is determined that the average values of the RCVR signal during the times that the LED is energized and de-energized are nearly the same, then the beam is considered broken by a user&#39;s hand gripping the handle. This is an important feature of the present invention. Using a more conventional mode of operation for the optical switch would be less reliable. Specifically, if the LED were continuously energized and if the level of the RCVR signal were monitored to detect a change in magnitude caused by the user gripping or releasing the control handle, it could be difficult to differentiate between that cause and changes in the magnitude due to variations in ambient light level. 
     The following discussion of a flow chart  200  shown in FIG. 2 addresses the logic of a program executed by MCU  136 . The present invention virtually eliminates false triggers of the optical switch caused by ambient light sources and enables a slow increase (or ramping up) of feedback force to occur when the user grips the control handle, but an immediate disabling of the feedback force when the user releases the control handle. Beginning at a start block, the logic moves to a block  202 , which provides for initializing variables and storing the current value of the servo clock. Initialization includes setting the values of an “on” accumulator, “off” accumulator, and fade-in accumulator to zero, and a counter&#39;s value to eight. Further, an LED “on” error bit, LED “off” error bit, sensor error bit, and sensor active bit are set to false. Like most processing devices, MCU  136  includes a time base that is used for all timing functions. In the present invention, the time base is employed as a servo clock that periodically generates a servo clock “tick” when looping through the main body of the program. The logic moves to a decision block  204 , and a determination is made as to whether the difference between a current servo clock value and the stored clock value is greater than a servo clock tick interval, which in the preferred embodiment, is two milliseconds. If the difference is not greater than this time interval, then the logic flow continues to loop through decision block  204  until an affirmative determination is made. Once a servo clock tick interval is determined to have elapsed, and the current value of the servo clock is stored, the logic jumps to a block  206  and the value of the LED DRIVE signal is set low. It will be recalled that LED  102  conducts and emits infrared light only when the LED DRIVE signal is high. Further, the infrared light must be incident upon the base of phototransistor  104  to cause the phototransistor to turn “on,” and pull the value of the RCVR signal high. Thus, a low value for the LED DRIVE signal will turn “off” LED  102 . 
     The logic moves to a block  208 , and an ADC circuit that is internally disposed within MCU  136  is employed to digitize the values of the LED SENSE and RCVR signals. A software interrupt is provided by the ADC to the program when the analog-to-digital (A-D) conversion is completed. The logic advances to a block  210  where the digitized values for the LED SENSE and RCVR signals are stored, and the LED SENSE signals are processed to determine if LED  102  is operating correctly. In the discussion of FIG. 3A below, a detailed description of the logic performed within block  210  is provided. Next, the logic moves to a block  212 , which indicates that logical steps are performed to reject false triggering of phototransistor  104  by ambient light. A more complete description of the steps referenced by block  212  is provided below in FIG.  3 B. 
     Moving now to a decision block  214 , a determination is made as to whether a sensor active bit is true. If the determination is negative, the logic advances to a block  216  and the fade-in accumulator value is set equal to zero. The logic flows to a block  218 , and the motor drive signal is multiplied by the fade-in accumulator value yielding a product equal to the new motor drive signal. The logic advances to a block  220 , and MCU  136  outputs the new motor drive signal as pulse width modulated signals (PWM Y and PWM X) to X axis actuator  132  and Y axis actuator  134 , respectively. Next, the logic loops back to block  204  to repeat the logic flow. It is important to note that multiplying the motor signal by a fade-in accumulator value of zero will also create a new motor drive signal value of zero. In this way, the generation of force feedback by the actuators will be stopped whenever the sensor active bit is set to false, thereby disabling the application of force to the control handle by the actuators. 
     However, if the determination at decision block  214  was affirmative, the logic advances to a decision block  222  to determine whether the value stored in the fade-in accumulator is less than  256 . If not, the logic jumps to a block  218 , and the logic flow continues as discussed above. If the determination in decision block  222  is true, the logic proceeds to a block  224 , and the fade-in accumulator is incremented. Next, the logic steps to block  218 , continuing as discussed above. 
     The program steps implemented with the digitized values of the LED SENSE and RCVR signals are shown in a flow chart of block  210  in FIG.  3 A. Beginning at a start block, the logic advances to a block  226 , in which the software interrupt generated by the ADC of MCU  136  is received. Once the software interrupt is received, the logic moves to a decision block  228 , and a determination is made as to whether the LED DRIVE signal is high. As discussed in detail above, when the LED DRIVE signal is high, LED  102  is “on” (conducting), and when the signal is low the LED is “off” (non-conducting). 
     If the determination at decision block  228  is false, the logic advances to a block  230 , which provides for storing the digitized “off” values for the RCVR signal, and the LED SENSE signal. The logic steps to a decision block  232  and determines whether the “off” LED SENSE value is low. Significantly, the digitized value of the “off” LED SENSE signal is only employed to determine if the range of the digitized value is low. Although the precise value of the “off” LED SENSE signal is not considered in the determination, it is envisioned that a sense resistor could be employed in another embodiment to provide the actual flow of current through LED  102 . In this way, the conductivity of LED  102  may be monitored. If the determination at decision block  232  is true, the logic advances to a block  234 , and the LED “off” error bit is set to true. The logic proceeds to a block  236 , and the LED DRIVE signal is set high so that LED  102  will turn “on” and begin conducting. If the determination at decision block  232  is false, the logic skips block  234  and jumps to block  236 . After block  236 , the logic moves to a block  238 , and the A-D conversion of the RCVR and LED SENSE signals by the ADC is restarted. The logic loops back to block  226  and the waits to receive the software interrupt from the ADC that signifies the A-D conversion is complete. 
     Once the software interrupt signal is received, the logic returns to decision block  228  again to determine if the LED DRIVE signal is set high. If true, the logic advances to a block  240 , and the digitized “on” values of the RCVR and LED SENSE signals are stored. The logic then proceeds to a decision block  242  to determine if the LED SENSE signal is high. If affirmative, the logic advances to a block  244 , and the LED “on” error bit is set to true. The logic steps to a block  246 , and the LED DRIVE signal is driven low. If the determination at decision block  242  is negative, the logic jumps to block  246 . An exit block follows block  246 , indicating a return to the main program in FIG.  2 . 
     As described in the discussion of FIG. 3A above, the conduction/non-conduction of LED  102  is cycled by the logic flow of the program, so that for each tick (two milliseconds) of the servo clock, LED  102  is “on” (conducting) for 0.250 milliseconds and “off” (non-conducting) for 1.75 milliseconds. The relatively brief “on” time interval versus the substantially longer “off” interval is provided to insure that both states of LED  102  are sampled during each servo tick and to reduce power dissipation. 
     Turning now to FIG. 3B, a flow chart that addresses the logical steps within block  212  are shown. Beginning at a start block, the logic moves to a decision block  250  to determine if either the LED “on” or the LED “off” error bits are set to true. If either bit is true, the logic moves to a block  266  and the sensor error bit is set to true. The logic steps to a block  268  to reinitialize the error and control bits, i.e., the LED “on” error, LED “off” error, and sensor active bits are set to false. The logic moves to a block  270  in which the counter is re-initialized to a value of eight. The logic then advances to a block  272 , and the “on” accumulator value is zeroed. The logic proceeds to a block  274 , and the “off” accumulator value is zeroed. The logic jumps back to the main body of the program and exits the at block  212 . 
     However, if the determination at decision block  250  was false, the program advances to a block  252  in which the stored digitized value of the “on” RCVR signal is accumulated in the “on” accumulator. The logic then flows to a block  254  where the stored “off” RCVR signal is accumulated in the “off” accumulator. Thereafter, in a block  256 , the value of the counter is decremented. After decrementing the counter, the next step occurs in a decision block  258 . In this decision block, a determination is made as to whether the counter value is equal to zero. If false, the procedure concludes in block  212  and returns to the main flow of the program. 
     The logic continues to loop back through the main body of the program to decision block  258  until eight values of the RCVR values have been accumulated in both the “on” accumulator and the “off” accumulator. Accumulation of these values represents an implicit sliding window average of the most recent eight “on” and eight “off” RCVR values. 
     Once the value of the counter is determined to be zero at decision block  258 , the logic moves to a decision block  260  to determine whether the averaged “off” RCVR value is greater than a predetermined threshold value. If true, the next step is block  268 , and the program proceeds as described above. If false, the logic moves to a block  262 , and the difference or delta between the average “on” RCVR value and the average “off” RCVR value is computed. The logic moves to a decision block  264 , and a determination is made as to whether the delta is greater than another predetermined threshold value. If true, the logic jumps to block  268  and follows the logic flow as detailed above. However, if the determination is false, the logic steps to a block  276 , and the sensor active bit is set to true. Finally, the logic jumps to block  270  and proceeds as described above, finally returning to the main flow of the program. 
     The actuators of the preferred embodiment are electrical motors having linkages that are coupled to the handle of the joystick, and the motors are disposed within the base of the joystick. Another embodiment of the claimed invention could employ alternative types of actuators, such as pneumatic, hydraulic, or mechanical. Furthermore, it will be apparent that the present invention is applicable to devices that include more than the two actuators (X-axis and Y-axis) employed in the preferred embodiment described above. For example, an additional actuator could be used to generate linear or rotational movement relative to the vertical axis (Z-axis) of the joystick control handle. 
     It should be noted that the present invention can be practiced using other types of switches besides the optical switch described above. For example, a mechanical switch that is actuated by the user gripping the control handle may be employed instead of the optical switch. Further alternatives include a capacitive or proximity switch that responds to the inherent capacitance of a user&#39;s body when applied to grip a control member of the device, or a pressure sensitive switch that responds to the pressure applied by a user gripping the control member. Details of these alternative switches are not shown, but suitable switches of these types are readily available or easily fabricated by one of ordinary skill in the art. 
     It should also noted that although the present invention is specifically disclosed for controlling the feedback force applied to a haptic feedback joystick  400 , as shown in FIG. 4, it is also applicable to other types of gaming control devices in which haptic feedback is used, such as a force feedback yoke  420 , as shown in FIG. 5A, and a digit cradle  440  as shown in FIG.  5 C. Yoke  420  includes a pair of light sources  422 , one of each being mounted at the top of each hand grip portion of the yoke, and a corresponding pair of light sensors  424 , mounted below the hand grip portion of the yoke. The digit cradle includes a cradle  422  for receiving the user&#39;s forefinger. On one side of the cradle is disposed a light source, and on the opposite side is disposed a light sensor for sensing the presence of the user&#39;s finger in the cradle. 
     A force feedback steering wheel  430 , which is shown in FIG. 5B, includes a pressure switch or a capacitive (proximity) switch  432  around the rim of the steering wheel. Either of these types of switches is suitable for detecting the grip of the user on the steering wheel control member to implement the present invention. 
     The preferred embodiment discussed above controls the magnitude of the feedback force generated by a joystick and applied to its control handle when the user initially grips the handle or releases the control handle. It is also envisioned that the present invention might be applied to devices that generate force independent of the user&#39;s control actions, e.g., to tools or other devices that are intended to be held by the user. The physical disposition of the switch for detecting the grip of the user may be altered, but the functionality of the present invention can be applied in much the same manner disclosed above. 
     In addition to responding to the removal of the user&#39;s hand from the control handle of the joystick by disabling the application of force, the host computer can be programmed to respond to the RCVR signal by placing an application that is executing on the host computer (and which is controlling the haptic joystick) in a pause mode or placing the operating system and host computer in a “sleep” mode in which it powers down to a standby state. Either the application or the operating system may be programmed to respond to the status of the RCVR signal and thus, to the presence of the user&#39;s grip on the control handle. 
     As yet another alternative, an application may send a signal to the MCU that causes it to disable the application of the force to the control handle in respond to the user entering a pause command in an application, in the same manner disclosed above when the MCU responds to the user releasing the control handle. Since the ability to immediately disable the application of force when the control handle is released is included in the haptic joystick, the same response can readily be applied to the user pausing a game or other application that is normally controlling the haptic joystick. 
     Although the present invention has been described in connection with the preferred form of practicing it and modifications thereto, those of ordinary skill in the art will understand that many other modifications can be made to the invention within the scope of the claims that follow. Accordingly, it is not intended that the scope of the invention in any way be limited by the above description, but instead be determined entirely by reference to the claims that follow.