Abstract:
Circuits for detecting faults in variable resistive position feedback sensors common in the use of motion control systems, provide a filter coupled to an output signal of the variable resistive position sensor for processing the signal to detect noise indicative of intermittent faults. A memory stores occurrences of signals from the filter. The memory can activate an alarm which indicates that a failure is detected. The circuits are particularly applicable to medical imaging systems such as SPECT, PET or MRI systems which contain a multiplicity of moving components that require accurate motion control and positioning.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates generally to a mechanism for detecting faults in resistive position feedback sensors, which are used in many different applications where motion of devices is required, and in particular in medical imaging systems such as SPECT, PET, and MRI imaging systems. More particularly, the present invention detects faults in resistive position feedback sensors by detecting intermittent noise in an output signal of such sensors. 
       BACKGROUND OF THE INVENTION 
       [0002]    Motion control systems typically utilize variable resistive devices, such as potentiometers or linear strip variable resistors, as low-cost, absolute position feedback sensors. Accurate motion control is critical in many system applications, especially in medical imaging systems such as SPECT, PET and MRI imaging systems where accurate positioning of movable components such as patient beds, gantries, detector heads, etc. is essential. 
         [0003]    One disadvantage in the use of such variable resistive position sensors is that the electro-mechanical contact of such device can experience an intermittent or permanent failure. While a permanent failure is readily apparent, the susceptibility of specific resistive feedback sensors to intermittent failures can be difficult to identify when equipment is serviced because the failure may not occur during a service inspection. For example, where a contact is prone to intermittent failure due to a contaminant such as dirt or debris, or gradual deterioration of the contact material, movement of the variable resistive device could affect a temporary cleaning of the electro-mechanical contact and thus preclude detection of the underlying problem during servicing of the system. Accordingly, it would be desirable to have the capability of detecting intermittent failures of resistive feedback devices. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    The present invention solves the existing need in the art by providing mechanisms for detecting faults in resistive position feedback sensors that are common in the use of motion control systems. These mechanisms can be embodied in components separate from the motion control system, or incorporated into the programming of digital signal processors controlling the motion system. 
         [0005]    According to a first embodiment, a filter mechanism is used to detect noise in an electromechanical contact of the variable resistive device. A filter receives an output signal of the resistive device, which is typically coupled to a motion control system through a feedback loop. The filter removes normal system noise from the signal. A memory stores occurrences of a noise signal passing the filter, which is indicative of an intermittent fault in the electromechanical contact of the position sensor. An alarm is activated in response to the noise signal, alerting a user that a failure is detected. A reset device can clear the memory when the fault has been repaired. 
         [0006]    In a second embodiment, the mechanism of the first embodiment can be augmented to provide greater discrimination of noise indicative of intermittent failure. A motion control system includes a controller, a motor, a velocity sensor, and a resistive device. The output of the resistive device is coupled to the controller in a feedback loop, wherein the resistive device produces a feedback signal. The output of the velocity sensor is also coupled to the controller in a feedback loop. The velocity sensor output signal is inputted into an integrator, which develops a position signal. A subtractor circuit receives the resistive feedback signal and the position signal and produces a difference signal by subtracting the resistive feedback signal from the position signal. The difference signal passes through a filter and is further processed for the detection of noise. The memory stores intermittent occurrences of signals that pass through the filter. An alarm is activated by the noise signal, alerting a user that a failure is detected. A reset device can clear the memory when the failure has been repaired. 
         [0007]    In a third embodiment, a fault detection circuit includes upper- and lower-limit comparators. The output signal of the variable resistive position sensor is connected to one of the inputs of each comparator. The second inputs of each comparator are set to either a low limit or a high limit. An error signal is generated by the comparators when the low or high limits are exceeded. A memory receives the error signal, and activates an alarm. A reset device can clear the memory when the sensor fault has been repaired. 
         [0008]    Each component of the embodiments described above may also be used separately or in conjunction to detect failure of the variable resistive feedback device, which is independent of the motion control system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The invention now will be described in greater detail in the following by way of example only and with reference to the attached drawings, in which: 
           [0010]      FIG. 1  is a diagram of a conventional motion control system with a resistive feedback position sensor, which is applicable to the present invention; 
           [0011]      FIG. 2  is a diagram of an intermittent fault detection circuit for a variable resistive position sensor according to a first embodiment of the present invention; 
           [0012]      FIG. 3  is a diagram of an intermittent fault detection circuit for a variable resistive position sensor according to a second embodiment of the present invention; and 
           [0013]      FIG. 4  is a diagram of an intermittent fault detection circuit for a variable resistive position sensor according to a third embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    As required, disclosures herein provide detailed embodiments of the present invention; however, the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. Therefore, there is no intent that specific structural and functional details should be limiting, but rather the intention is that they provide a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention. 
         [0015]      FIG. 1  depicts a typical motion control system with a resistive feedback sensor  100 , which is applicable to the present invention. A variable resistive device  110  (such as a potentiometer, linear strip variable resistor, etc.) measures the absolute position of a mechanical system  120 . The mechanical system  120  (which may be a gantry, patient bed, detector head, collimator tray, etc. of a nuclear imaging system such as a SPECT system, PET system, or a MRI system) is driven by a motor  130  which receives a signal from a motion control system  140 . The output motor drive signal of the motion control system  140  is determined by the difference between a reference set point  150  and a resistive feedback signal  160  derived from the variable resistive device  110 . In the simplest form, the control system  140  drives the motor  130  until the set-point  150  is equal to the feedback signal  160 . In this system  100  the variable resistive device  110  can provide position and velocity information of mechanical system  120  for the control system  140 . The variable resistive device  110  used in this embodiment can be implemented by, but is not limited to: a potentiometer, a digital controlled potentiometer, a variable resistor, and a linear strip variable resistor. Also, the control system  140  can be embodied as an analog controller, digital controller, or software-based controller. 
         [0016]    An optional system could also include a velocity sensor  170  and a velocity feedback signal  180  part of the motion control system. In this alternative system the control system  140  drives the motor  130  until the set-point  150  is equal to a value derived from a combination of the resistive feedback signal  160  and the velocity feedback signal  180 . 
         [0017]      FIG. 2  depicts an embodiment of a circuit  200  for detecting noise in an electro-mechanical contact of a variable resistive position sensor device in accordance with a first embodiment of the present invention. A variable resistive device  210  produces an output feedback position signal  260  in response to motion of a mechanical interface  220  (which may be any of the movable components of a medical imaging system as mentioned above, for example). The circuit  200  receives the resistive feedback signal  260  from variable resistive device  210 , where it is fed to filter  230 , which processes the incoming resistive feedback signal  260 . The filter  230  is either a high-pass filter or band-pass filter that has a lower cutoff frequency above the normal frequency response of the system  100  components, which are listed as follows: mechanical system  120 , motor  130 , control system  140 , and set-point  150 . Electrical energy that passes through the filter  230  thus will correspond to noise produced by the electromechanical contact of the resistive device  210 . Intermittent occurrences of signals passing through the filter  230  will be recorded and stored in a non-volatile memory  240 , such as a solid state memory device, flash memory device, etc. The record of the event will remain in the memory  240  until a reset signal  260  produced by a user or by an external event (such as completion of repairs to the system) is applied to the memory to reset its contents. Upon receiving a signal from the filter  230 , the memory  240  triggers an alarm  250 , which indicates that a failure was detected. The alarm may be an audible alarm, visible alarm, or textual alarm, and also may be coupled if desired to other system control components to initiate appropriate action in response to a fault. The alarm alerts an operator that the system requires servicing. The system can be reset  260  after the repairs are completed. 
         [0018]      FIG. 3  depicts a second embodiment  300  for detecting intermittent failures in variable resistive position sensors, which incorporates a velocity sensor  370  in the fault detection. The embodiment of  FIG. 3  augments the embodiment  200  of  FIG. 2  to provide greater discrimination of noise detection by incorporating a velocity feedback signal  380  from velocity sensor  370 . The embodiment of  FIG. 3  can be used, for example, when intermittent noise from the variable resistive position sensor falls into the same frequency band of “normal” system noise produced by the control system. 
         [0019]    As shown in  FIG. 3 , an integrator  330  receives the velocity feedback signal  380  as an input. The integrator  330  integrates the velocity feedback signal  380  to develop a relative position signal  340 , which is sent to a difference circuit  350 . Difference circuit  350  receives the relative position signal  340  at one of it inputs, and receives the resistive feedback position signal  360  from variable resistive sensor device  310  at another of its inputs. The difference circuit  350  processes the variable resistive feedback position signal  360  and relative position signal  340  by calculating the difference between the two signals. A difference between the resistive feedback signal  360  and relative position signal  340  will produce a difference output signal. This difference output signal represents a difference between the response of the mechanical system  320  to driving by motor  130 , and the response of the resistive device  310  to motion by the mechanical system  320 . The typical noise bandwidth of the mechanical system  320  is much lower than the noise of a failed resistive device  310 . 
         [0020]    A filter  390  receives the difference output signal and filters it. The filter  390  is either a high-pass filter or band-pass filter that has a lower cutoff frequency above the frequency response of the system  300  components. Electrical energy that passes through the filter  390  will represent intermittent noise of the variable resistive device  310 . Intermittent occurrences of signals passing through the filter  390  will be recorded and stored in a non-volatile memory (not shown; see memory  240 ,  FIG. 2 ). The record of this event will remain in the memory until a reset signal is provided by an external event or by a user. The memory triggers an alarm which indicates that a failure was detected. The system can be reset after the repairs of the fault are completed. 
         [0021]      FIG. 4  depicts a third embodiment  400  of the invention, in the form of a circuit for detecting intermittent faults in a variable resistive position sensor. The circuit  400  includes a variable resistive device  410  that produces a resistive feedback position signal  460 . The resistive feedback position signal  460  is coupled to a first input terminal of each of a pair of comparators  440 . A first comparator of the pair has a lower limit threshold value  430  applied to its second input terminal, while the second comparator of the pair has a high limit threshold value  435  applied to its second input terminal. The values of the high and low limits are predetermined by adding or subtracting values for the acceptable operating range of the control system during an evaluation period. At the end of the evaluation period the limits are recalculated and reassigned. The recalculation and reassignment of limits creates a dynamic range of acceptable values expected from the system. When the limit of either comparator  440  is exceeded an error signal  450  is produced. 
         [0022]    The error signal  450  thus indicates a fault in the variable resistive device  410 . As in  FIG. 3 , the velocity signal may be added to the determination of fault where the “normal” noise from other components of the motion control system are in the same frequency band as intermittent noise from the variable resistive position device. A memory  470  further processes the error signal  450  by recording the event. The recorded event that caused the error signal  450  remains in the memory  470  until a reset  490  signal is applied to reset the memory. The memory  470  triggers an alarm  480  which indicates that a failure was detected. This information can be used to alert an operator that the system requires servicing, or in leading an operator to an unnoticeable fault. The system can be reset after the repairs are completed. 
         [0023]    Those skilled in the art will appreciate that embodiments of this invention may be practiced in any motion control environments including manufacturing control systems, electromechanical systems, power systems, etc. 
         [0024]    The invention having been thus described, it will be apparent to those skilled in the art that the same may be varied in many ways without departing from the spirit and scope of the invention. Any and all such modifications are intended to be covered within the scope of the following claims.