Patent Publication Number: US-6342879-B1

Title: Joystick actuators

Description:
INTRODUCTION 
     This invention relates to joystick actuators. 
     Joystick actuators are used in a number of applications in which accurate manual control of an electrical, mechanical or hydraulic system is required, generally in two dimensions. Such joystick actuators have a handle adapted to be grasped by the operator and to be manipulated in order to vary an electrical output signal from the actuator in such a manner as to control the system in a manner determined by operator actuation. It is known for such joystick actuators to incorporate potentiometers or sensing coils for providing an output signal dependent on the position, or the rate of movement, of the handle. However such known arrangements suffer from the fact that they incorporate wearing parts and/or are costly to manufacture. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an improved joystick actuator which can be produced in a straightforward manner and which is capable of a long service life. 
     The invention is defined by the accompanying claims. 
     It should be understood that the term “light” is used in this specification to denote electromagnetic radiation within a wide range of wavelengths and is not limited to wavelengths within the visible spectrum. 
     In order that the invention may be more fully understood, a preferred embodiment of joystick actuator in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 are vertical sections through the actuator in two different positions; 
     FIGS. 3 and 4 are diagrammatic perspective view of the joystick member of the actuator in the two positions; 
     FIG. 5 is a diagram of the detection circuit of the actuator, 
     FIG. 6 is a graph showing the output voltages of the position sensing light detectors as the joystick member moves along the X and Y axis, 
     FIG. 7 is a graph showing the output voltage of the further light detector, and 
     FIG. 8 is a graph showing a proportion of the output voltage of the further light detector summed with the output voltages of the position sensing light detectors. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1 and 2, the joystick actuator  1  comprises a detection circuit  2  within a housing  3  (shown only partially in the drawings), and a joystick member  4  which is pivotally mounted by a ball and socket joint  5  so as to be pivotable in two mutually transverse directions relative to the housing  3 . The joystick member  4  has a handle  6  projecting upwardly and outwardly of the housing  3  and adapted to be grasped and manipulated by the operator, and an end part  7  within the housing  3  bearing a light transmitter in the form of a light-emitting diode  8  thereon. The detection circuit  2  within the housing  3  comprises a printed circuit board  9 , four position sensing light detectors (only two of which are shown in FIGS. 1 and 2) in the form of photosensitive diodes  10 , a further light detector in the form of a photosensitive diode  11  and associated processing circuitry. The diodes  10  are symmetrically positioned in relation to a mean position of the joystick member and the diode  11  is positioned centrally within the diodes  10 . 
     As will more readily be appreciated by referring to FIGS. 3 and 4, manipulation of the joystick member  4  between a central position (FIGS. 1 and 3) and an extreme position (FIGS. 2 and 4) results in movement of the light-emitting diode  8  relative to the diodes  10  in such a manner as to decrease the light level received by at least one of the diodes  10  and to increase the light level received by at least one other of the diodes  10 . Thus, in the illustrated example, movement of the joystick member  4  from the central position to the extreme position along the direction X—X will result in the supply of signals dependent on the received light levels from the diodes  10  to the processing circuitry to produce an electrical output signal indicative of both the direction and the degree of movement of the joystick member  4 . Similarly electrical output signals indicative of the direction and degree of movement of the joystick member  4  will be given when movement of the joystick actuator  4  is in the direction Y—Y, or when X movements and Y movements are combined so as to move the joystick member  4  along a direction intermediate the X—X and Y—Y directions. 
     As the joystick member  4  moves so that the light emitted by the diode  8  approaches one of the photodiodes  10 , the output of the photodiode  10  will increase and will peak when the diode  10  receives maximum light intensity from the diode  8 . Further movement of the diode  8  in the same direction will result in the output of the photodiode  10  falling again. FIG. 6 shows two curves, one representing the combined outputs of the photodiodes  10  (with one output inverted with respect to the other) aligned with the X—X direction and the other representing the combined outputs of the photodiodes  10  (with one output inverted with respect to the other) aligned with the Y—Y direction. It will be noted that these curves peak and then fall off as the light from the diode  8  passes over the then moves away from each photodiode  10 . For example, a voltage of 6.5 volts will indicate two possible positions of the joystick member  4 . In one of these the axis of the joystick member  4  intersects a plane containing the four diodes  10  within a circle passing through the diodes  10 . In the other, the axis of the joystick member  4  intersects said plane outside the said circle. 
     The further diode  11  is located within the said circle and at the center of the said circle so that it will lie directly below the diode  8  when the actuator is at rest, spring centered and equidistant from the four photodiodes  10 . 
     The output voltage of the further photodiode  11  follows a curve of the type shown in FIG.  7  and this is used to modify the outputs of the position sensing photodiodes  10  such that the latter outputs are substantially linearized as shown in FIG.  8 . This is generally achieved by summing a proportion of the output from the photodiode  11  with each of the outputs of the position sensing photodiodes  10  although ideally some additional processing may also be carried out on the outputs of the photodiodes  10  by performing an algorithm on these outputs. 
     Referring to FIG. 5, the signals outputted by the photodiodes  10  and  11  are supplied to a transimpedance circuit  13  comprising a respective amplifier with gain for converting the current output signal from each photodiode  10 ,  11  to a voltage signal, the five resulting voltage signals being supplied to a multiplexer  15  of a microprocessor  14 . The multiplexer  15  supplies an analogue output signal indicative of its five inputs to an analogue-to-digital converter which converts the signal to an 8-bit binary signal. An arithmetic logic unit  17  is provided with a read-only memory  18  containing a program code and a random-access memory  19  for temporarily storing program values. The binary values indicative of the light levels received by the five photodiodes  10  are processed in the arithmetic logic unit  17 . The arithmetic logic unit  17  performs an algorithm on the binary values and provides outputs X and Y indicative of the X and Y positions of the joystick member. 
     The microprocessor  14  can also compare the signal from the photodiode  11  with the signals from the photodiodes  10  and use the signal from the photodiode  11  to validate the signals from the photodiodes  10 . If any of the signals from the photodiodes  10  change but the signal from the photodiode  11  does not change, this will indicate that the signal from the photodiode  10  is not valid. This safety feature will prevent unexpected outputs from the joystick actuator in the event that one or more components develop a fault condition. 
     The above embodiment is given by way of example only and various modifications will be apparent to persons skilled in the art without departing from the scope of the invention. For example, the light emitting diode  8  could be replaced by a light reflector mounted on the joystick member and arranged to reflect light from a fixed light source within the housing.