Abstract:
A control system comprising a control input device having a movable magnet, a pole-piece frame arrangement positioned about the magnet and positioned therein at least two magnetic flux sensors for sensing movement of the magnet in a given direction. The control system further comprises a monitoring arrangement for monitoring the output signal of each of the sensors and permits the input device to control the system only when the output of the sensors are within a predefined range. This multiple sensing provides a fail-safe in the event that one of the sensors generates an erroneous signal.

Description:
BACKGROUND 
   This application claims priority to Great Britain Application No. 0417668.1, filed Aug. 6, 2004. The above-listed application is hereby incorporated in its entirety herein by reference for all purposes. 
   The present invention relates to a control system and more particularly to a joystick type control system, and particularly to such systems utilizing magnetic positional sensing used in safety critical human/machine control interfaces. 
   Various uses for joystick control systems, such as the present invention, include wheelchairs, forklift trucks or other man-carrying vehicles, and control of machines such as cranes, robots or other industrial equipment where a dangerous situation could exist in the event of a control system failure. In such a system, dual joystick position sensor channels may be used, and the outputs compared to one another continuously. This ensures that if there is a problem with one of the sensor channels, the error is picked up due to a mismatch in the outputs at the 2 channels. If a discrepant output (differential beyond a predetermined threshold) occurs, the control system rapidly and safely disables the system. 
   The force with which a user operates the controller and, to a lesser extent, manufacturing tolerances, can result in the joystick shaft shifting in position translationally in the three orthogonal directions (x,y,z). Due to such tolerances and the fact that the primary and back up sensor in each fail-safe pair cannot occupy exactly the same position in space, the outputs from the sensors in the pair will differ slightly and allowance must be made for this when setting the tolerance threshold. The sensors are typically programmable, allowing each pair to be calibrated to provide a zero difference in output from each sensor of the pair, under normal operating conditions. However, if the threshold is too small then the monitoring system may indicate a malfunction, creating false errors referred to as nuisance trips in the art. 
   Alternatively, the sensors in each pair could be arranged to provide outputs having opposite sense. In such an implementation, the output of one sensor of the pair could be arranged to provide a positive output, and the other sensor of the pair could be arranged to provide a negative output. However, in both arrangements, the sum of the outputs of the sensors in a given pair, or their mean, is required to be a constant to within the tolerance threshold. 
   For joystick systems of the magnetic sensing type, it is necessary to measure the angular position of the joystick shaft (and therefore the magnet) without introducing errors due to the linear motion of the magnet in the three orthogonal directions. There is thus a need for an improved control system. 
   BRIEF SUMMARY 
   Various apparatus and method embodiments of the invention are described herein. For example, in one embodiment of the invention, a control system comprising a control input device having a movable magnet, a pole-piece frame arrangement positioned about the magnet, and positioned therein at least two first magnetic flux sensors for sensing movement of the magnet along a first axis, a monitoring arrangement for monitoring the output signal of each of the at least two first sensors, wherein a process can be implemented dependant upon the monitored output signals of the at least two first sensors. This and other embodiments are disclosed herein. The preferred embodiments described herein do not limit the scope of this disclosure. 
   In various illustrative embodiments of the present invention, the monitoring arrangement processes together the output signals of the at least two first sensors, to generate a first check value, and wherein a fail-safe process can be implemented dependent upon the first check value. 
   In accordance with various embodiments of the present invention, the primary delivery route for magnetic flux to the sensors in respective pairs is via the pole-piece frame arrangement. Thus, the gap between the sensors and the magnet is greater than the gap between the magnet and specific portions of the pole-piece frame arrangement. The pole-pieces of the frame arrangement are manufactured of highly magnetically permeable, soft material, such as radiometal, mumetal or other similar material with low hysteresis. The pole-piece frame may comprise pole-piece elements in contact or spaced by small gaps. 
   In various embodiments, the pole-piece frame arrangement includes a first pair of gaps diametrically arranged about the magnet. The pole-piece frame may be spatially arranged to shield the sensors from, or minimize the influence of, unwanted components of flux which would generate unwanted differences between the outputs of each sensor of a given pair. 
   In still other embodiments, the control system further comprises at least two second magnetic flux sensors positioned in the pole-piece frame arrangement for sensing movement of the magnet about a second axis, a monitoring arrangement for monitoring the output signal of each of the at least two second sensors to generate a second check value, wherein a process can be implemented dependant upon the monitored output signals of the at least two second sensors. 
   In a control system according to embodiments of the present invention, the first sensor pair is used to monitor angular movement of the control input device in a first axis, and the second sensor pair is used to monitor angular movement in a second axis. In various embodiments, the first and second sensor pairs are spaced at ninety degrees (90°) about the magnet. 
   A fail-safe control output may be provided dependent upon the monitored difference in output between the sensors in each pair. The fail-safe control output may be dependent upon the monitored difference in output between the sensors in each pair reaching or exceeding a predetermined threshold value. 
   The monitoring arrangement monitors the difference in output between sensors in different pairs, to ascertain the angular position of the magnet with respect to the pole-piece frame. 
   For each sensor pair, Hall effect sensors are mounted in side-against-side configuration in respective first and second gaps in the pole-piece frame arrangement. The sensors may be sandwiched between spaced facing flanges of the pole-piece frame. The spaced facing flanges may be more extensive than the sensors, reducing the risk of magnetic field distortion at the sensors which may otherwise be present due to, for example, edge effects. 
   The pole-piece frame may include specific flux collector elements disposed nearer to the magnet than the sensors are disposed to the magnet. The flux collector elements may be substantially planar panels. In one embodiment, the planar panel flux collector elements may be supported by narrower connection arms of the pole-piece frame arrangement. 
   In various embodiments, the pole-piece frame arrangement includes pole piece lengths extending substantially perpendicularly with respect to one another. In this arrangement the lengths beneficially extend at forty five degrees (45°) to the axis through an intermediate sensor pair and the magnet. A sensor pair may be therefore positioned in a gap between the mutually perpendicularly extending pole-piece lengths. 
   In various embodiments, the pole-piece frame arrangement includes a pole-piece element positioned intermediate to one or both sensor pairs and the magnet. This pole piece element is therefore provided forwardly (magnet-side) of a sensor pair, and acts to shield the behind positioned sensor from direct flux from the magnet. This shield collector pole-piece carries flux to pass through the alternative pair of sensors. 
   The control input device may comprise a joystick shaft. The joystick shaft has a ball mount, the magnet being embedded within the ball. The ball is mounted on a bearing socket, comprising the controller. 
   In various illustrative embodiments, the invention comprises a joystick control device comprising a movable magnet, and a pole-piece frame arrangement positioned about the magnet, the pole-piece frame arrangement including at least one pair of gaps diametrically arranged about the magnet, and positioned therein at least two magnetic flux sensors. 
   The monitoring arrangement comprises a processing system for receiving, processing and producing output control signals in response to sensor input. 
   In still further embodiments, there is provided a control system comprising a control input device having a movable magnet, a pole-piece frame arrangement positioned about the magnet, and positioned therein at least one magnetic flux sensor, wherein the at least one magnetic flux sensor is housed in a screening can arrangement to direct magnetic flux away from the at least one sensor when the control input device is in the null position. 
   The screening can ensures that when the joystick is in the zero, upright position, any flux flowing from the pole piece to the screening can does not pass through the sensors (or at least is minimized). In addition, the screening can provides mechanical stability and preferably reduces any magnetic flux external to the cans from entering the magnetic flux sensors and affecting their outputs. In various embodiments, the screening can arrangement is symmetric. 
   In other illustrative embodiments, there is provided a control system comprising a control input device having a movable magnet, a pole-piece frame arrangement positioned about the magnet, and positioned therein at least one magnetic flux sensor, wherein the pole-piece frame includes flux collector elements disposed more closely to the magnet than the sensors are disposed to the magnet. 
   NOTATION AND NOMENCLATURE 
   Certain terms are used throughout the following description and claims to refer to particular system components. Persons skilled in the art will appreciate that components may be denoted in the art by different names. The present invention includes within its scope all components, however denoted in the art, that achieve the same function. In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to.” Also, the terms “couple,” “couples” or “coupled” are intended to refer to either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect connection via other devices and connections. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be further described in specific embodiments by way of example only, and with reference to the accompanying drawings in which: 
       FIG. 1  is a cut-away section of an exemplary device used in the control system of the invention; 
       FIG. 2  is a perspective view of a first embodiment of an exemplary control device in accordance with the invention; and 
       FIG. 3  is a perspective view of a second embodiment of an exemplary control device in accordance with the invention. 
   

   DETAILED DESCRIPTION 
   The present invention is amenable to implementation in various embodiments. The disclosure of specific embodiments, including preferred embodiments, is not intended to limit the scope of the invention as claimed unless expressly specified. In addition, persons skilled in the art will understand that the invention has broad application. Accordingly, the discussion of particular embodiments is meant only to be exemplary, and does not imply that the scope of the disclosure, including the claims, is limited to specifically disclosed embodiments. 
   Referring to  FIG. 1  of the drawings, the control input device  10  comprises a shaft  11 , one end of which is attached to a ball  12 , in which there is a molded magnet  13 . The molded magnet may comprise neodynium-iron-boron (NdFeB), samarium cobalt (SmCo), ferrite or other permanent magnetic material. The ball  12  is situated in a socket (not shown) and the shaft  11  is biased to the central upright position by means of a spring  14  and sliding bush  15 , which may be conical or flat. 
   The magnet  13  is oriented within the ball  12  such that the axis of magnetization is along the axis of the shaft  11 . The ball  12  further comprises two diametrically opposite recesses  16 A for accommodating a stirrup clip  16 . The clip  16  fits into matching groove  16 B formed on the main body  17  of the input device  10  to prevent the rotation of the shaft  11  about its long axis. 
   Referring to  FIG. 2 , in accordance with a first embodiment of the invention, the ball  12  is surrounded by a pole-piece frame arrangement which lies in the plane that is substantially perpendicular to the axis of the shaft  11 . The pole-piece frame arrangement is formed of a material with a high magnetic permeability and comprises four collector plates  18 A,  18 B,  18 C,  18 D, equally spaced around the magnet supported by four pole-piece arms  19 A,  19 B,  19 C,  19 D which have a comparatively smaller frame area than the plates  18 . The collector plates  18  and arms  19  are oriented such that their plane is substantially parallel to the axis of the shaft  11  in its un-deflected upright position. In various embodiments, the pole-piece frame arrangement may be square with the corners of the arms turned outwardly from the magnet  13  with four pairs of plates  20 A,  20 B,  20 C,  20 D, along a parallel to the square diagonal, forming gaps  21 A,  21 B,  21 C,  21 D, there between. 
   In two of the gaps  21  that have a common adjoining side of the pole-piece frame arrangement (i.e.  21 A and  21 D), there may be placed a pair of identical Hall effect sensors  22 , aligned side to side, to sense the flux component in the direction perpendicular to the pole faces forming the gap. The sensors are separately used to detect either right and left, or forward and aft movement of the shaft  11  and generate the appropriate signal to the controlled device. However, the input conveyed by the user on the shaft  11  is only enabled if the difference in flux measured in each sensor of the pair is within a tolerance threshold. The tolerance threshold takes into account any unintentional translational (x,y,z) movement of the ball  12  within the socket  13 , any flux distortions within the gap, remanent flux within the pole piece, any misalignment of the sensors, non-homogeneity of the magnet and any external magnetic fields which could influence the sensing. The sensors (arranged as a pair, triplet, quadruplet and so on) ensure that in the event of a failure of one of the sensors, or an erroneous signal output from one of the sensors  22 , the difference between the sensor outputs is greater than the tolerance threshold. A fail-safe process may then be implemented and no control signal will be generated. The system controlled by the input device will then be disabled. 
   The relative dimensions of the sensing element of the Hall effect sensors  22  and the pairs of plates  20 A,  20 B,  20 C,  20 D ensure that the flux passing from one plate of the gap  21  to the opposite plate of the same gap passes through both sensing elements of the Hall effect sensors  22 . To enable the flux to pass through both sensing elements of the Hall effect sensors, the smaller area sensing elements housed within the Hall effect sensors  22  may be placed central to the larger area plates  20 A,  20 B,  20 C,  20 D to avoid the distorted flux trajectory near the plate edges. 
   The pole-piece frame arrangement may be configured such that the collector plates  18 A,  18 B,  18 C,  18 D, are the closest parts of the frame arrangement to the magnet  13 . The collector plates  18 A,  18 B,  18 C, and  18 D may be arranged to pick up a change in magnetic flux, as opposed to the smaller area arms  19 , in accordance with the angular disposition of the shaft  11  from the upright position or a flux change directly influencing the sensor pairs  22 . 
   In use, the angular movement of the shaft  11  toward a first gap creates a magnetic potential difference within the pole-piece frame which causes flux to flow symmetrically around the circuit to the diagonally opposite gap of the pole-piece arrangement. For example, the angular movement of the shaft in the direction of gap  21 A will cause collector plates  18 A and  18 B to experience more “North-pole” than collector plates  18 C and  18 D, which both experience more “South-pole”. In this manner, a flux will pass across the gaps  21 B and  21 D. Since plate pairs  20 A and  20 C are at the same magnetic potential separately, no flux will pass across gaps  21 A and  21 C. However, a pair of sensors located within gap  21 D will experience a flux change and thus generate an electrical signal due to the Hall effect, thereby indicating the desired input control. 
   Referring to  FIG. 3  of the drawings, in accordance with another embodiment of this invention, the magnet  13  is surrounded by a pole-piece frame arrangement which lies in a plane that is substantially perpendicular to the axis of the shaft  11 . The pole-piece frame arrangement is formed of a material with a high magnetic permeability and comprises four magnetic shields/collector plates  180 A,  180 B,  180 C,  180 D, equally spaced around the magnet. 
   In various embodiments, the pole-piece frame arrangement may be circular and split into four quadrants by four pole-piece arms  190 A,  190 B,  190 C,  190 D which have a comparatively smaller frame area than the plates  180 . The end of each arm  190  is turned inwardly toward the magnet  13  but is shielded from the magnet  13  by the plates  180 . 
   The inward protuberance at the ends of the pole-piece arms  190  form four gaps  210 A,  210 B,  210 C,  210 D there between, equally spaced around the magnet. Within each gap is placed a Hall effect sensor  22  such that opposing pairs are arranged to detect either forward/aft or left/right deflection of the shaft  11 . 
   In use, the angular movement of the shaft  11  toward a first gap creates a magnetic potential difference within the pole-piece frame, which causes flux to flow symmetrically around the circuit to the diagonally opposite gap of the pole-piece arrangement. For example, the deflection of the shaft  11  in the direction of the gap  210 A will cause the magnetic potential at the protuberances of arms  190 A and  190 D forming gap  210 A to become more “North-pole” than the protuberances of arms  190 B and  190 C forming gap  210 C, which experience more “South-pole”. In this manner the flux lines will flow around the pole-piece frame arrangement from gap  210 A to  210 C, passing through the Hall sensor in gap  210 B and  210 D, thereby generating a signal to activate the desired control. The plates  180  placed between the magnet  13  and gaps  210  prevent the flux of the magnet from directly reaching the sensors  22  within the gaps  210  and thereby ensure that the flux in the gaps  210  is uniform and independent of the flux from the magnet. The plates collect the flux from the magnet and channel the flux toward each protuberance of the respective arm  190  thereby prevent the flux from penetrating the gap directly from the magnet. 
   The input conveyed by the user on the shaft  11  is only enabled, however, if the flux measured in one sensor of the opposing pair is within a threshold tolerance of that measured in the second sensor of the same pair. The tolerance threshold takes into account any unintentional translational (x,y,z) movement of the ball  12  within the socket  13 , any flux distortions within the gap, remnant flux within the pole piece, any misalignment of the sensors, non-homogeneity of the magnet and any external magnetic fields which could influence the sensing. The sensors (arranged as a pair, triplet, quadruplet, and so on) ensure that in the event of a failure of one of the sensors, or an erroneous signal as the output from one of the sensors  22 , the difference between the sensor outputs is greater than the tolerance threshold. A fail-safe process is then implemented and no control signal will be generated. The system controlled by the input device will then be disabled. 
   In these embodiments described, the pole-piece frame arrangement acts as the primary conduit to pick up and divert magnetic flux across the respective pairs of Hall effect sensors  22 . This ensures that, as far as practicable, the individual sensors in each pair experience the same flux and therefore, in the absence of system failure, substantially the same output is generated for each of the sensors in a respective pair. This occurs irrespective of translational movement of the shaft  11  and magnet  13  in x, y or z directions relative to the positioning of the collectors  18  on the pole-piece frame. In various embodiments, movement in the x, y and z directions may be compensated for by the square frame nature of the pole-piece frame arrangement (since the collector plates  18  are at forty-five degree (45°) angles from the shaft sensor sensitive axis, and therefore two plates  18  simultaneously pick up the flux components). In various other embodiments, translational movement in the x, y and z direction may be compensated for by the shield/collector plates  180  which are at ninety degrees (90°) about the shaft axis. 
   In all of the above embodiments, the magnetic sensing arrangement may be enclosed within symmetric screening cans  23 . The cans  23  ensure that when the joystick is in the zero, upright position, any flux flowing from the pole-piece to the screening cans does not pass through the sensors (or at least, is minimized). Once the upper and lower cans are introduced into an effective proximity to the magnetic pole-piece arrangement, the pole-pieces which deliver the flux to the sensors all remain at the same magnetic potential with respect to each other. As a result, when the joystick is in the upright position, the flux circulating through the sensors is minimized. In addition, the cans  23  provide mechanical stability and help to reduce any magnetic flux external to the cans  23  from entering the magnetic sensing arrangement and affecting the sensor outputs. 
   While the preferred embodiments of the present invention have been shown and described, modifications thereof can be made by persons skilled in the art without departing from the spirit and teachings of the invention. The embodiments described herein are exemplary only, and are not intended to limit the scope of protection provided herein. For example, it should be appreciated that whilst the embodiments described here refer to control system input devices having a pair of sensors  22  for safety critical control in a given direction, more than two sensors could equally be used for “fail-safe” redundant operation.