Abstract:
The present invention relates to a remote control ( 160 ) operates fail-safe. The remote control comprises a safety filter ( 150 ) in order to provide a fail save operation. The present invention also relates to an adjustable patient table ( 100 ) comprising a fail-safe wireless remote control for controlling an actuator to adjust the table.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a fail-safe interface for in an environment with strict safety requirements. 
     The present invention also relates to a remote control that requires a fail-safe interface. 
     The present invention is particularly relevant for a wireless remote control that can control movements of in a patient table in a medical environment but can be used in any system where adjustments of articles need to be made while safety concerns due to erroneous conditions are high. Such an environment includes a hospital operation and investigation room, which have adjustable patient tables for use with a wide variety of equipment such as X-ray machines, MRI scanners etc. 
     BACKGROUND OF THE INVENTION 
     In 2000, IEC accepted the IEC 61508 standard (“Functional Safety of Electrical/Electronic/Programmable Electronic Safety-Related System”) to support companies that use Safety Instrumented Systems (SIS) to protect persons and facilities from hazardous events. Four safety integrity levels (SIL, level 1˜4) are defined by IEC 61508 to statistically represent the integrity of the SIS when a process demand occurs. The SIL takes into account device integrity, architecture, voting, diagnostics, systematic and common cause failures, testing, operation, and maintenance. A SIL establishes an order of magnitude targets for risk reduction. This target failure measure is the intended probability of dangerous mode failures to be achieved in respect of the safety integrity requirements, specified in terms of either the average probability of failure to perform the safety function on demand (for a low demand of operation); or the probability of a dangerous failure per hour (for a high demand or continuous mode of operation). The higher the SIL, the greater the impact of a failure and therefore the lower the failure rate that is acceptable. The method used to derive the SIL designation, must be carefully documented using well-established methods. The first step to determine or designate a SIL, is to conduct a process hazards analysis (PHA). The methodology of IEC 61508 focuses most of the actual evaluation on the potential injury, fatality, or other risk to individual persons. 
     In an environment where articles need to be adjusted using a wired or wireless remote control, actuators may be used switched by relays. Especially when the articles comprise hospital patient table, safety concerns take a high priority. 
     In a typical embodiment, a micro-controller scans the inputs of a keyboard of a wireless remote control and/or a footswitch that is used to control a movable patient table. When a person controls the remote an output of its microprocessor will control a relay. Typically, a micro-controller in a remote control could get in an error situation. When an output would get stuck the patient table could then move uncontrollable something that can be very dangerous and must be prevented. 
     When a micro-controller is connected to a relay an error analysis is difficult to perform as semiconductors and software are involved. For this reason an error or a fault condition is not always detectable. In case of safety relevant applications, this is a serious problem. When an error condition would occur however, it is desirable that any movement will stop immediately to prevent any damage, or worse, patient harm. 
     It is also desirable to use components in a fail-safe system that are known from the past and that have a well known (reliable) behaviour. 
     SUMMARY OF THE INVENTION 
     It is, accordingly, an object of the present invention to provide a safety interface for a remote control. 
     It is another object of the invention to provide the safety filter for the remote control to adjust a patient table that will cause any movement to halt in case an error condition has been detected. 
     In one embodiment, a micro-controller is controlling a relay using a safety filter. 
     In another embodiment, the safety filter comprises a band-filter that outputs a relay control activation signal only when the micro-controller outputs a signal with a main component that lies within a certain frequency range. In a preferred embodiment, the safety filter comprises a transformer that is known from the past to be reliable enough to be used in fail-safe systems. 
     In another embodiment present invention provides an adjustable patient table ( 100 ) comprising a fail-safe wireless remote control for controlling an actuator to adjust the table. 
     In a preferred embodiment the invention provides a remote control ( 160 ) for controlling a safety relevant function. It comprises a first user input signal path and a second user input signal path, a transmitter ( 110 ) for transmitting a first signal from the first signal path and a second signal from the second signal path, a receiver ( 120 ) for receiving the first signal and the second signal, a first switch control that in response of a validated first signal can output a first switch control output signal, a second switch control comprising a safety filter ( 150 ) that in response of a validated second signal can output a second switch control output signal. The safety relevant function can only be activated when the first switch control outputs a validated first switch control output signal and the safety filter outputs a validated second switch control output signal. 
     These and other aspects of the invention will be apparent from and will be elucidated with reference to the embodiments described hereinafter. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described in more detail, by way of example, with reference to the accompanying drawings, wherein: 
         FIG. 1  shows a block-diagram of a remote control system  160  in a safety relevant system  100  in accordance with the invention; 
         FIG. 2  shows a best mode implementation of a safety filter  150  for use in the remote control system  100  in a safety relevant system  100  in accordance with the invention; 
         FIG. 3  shows the frequency response of safety filter  150  and the relay  170  sensitivity with respect to the safety filter response in accordance of a best mode embodiment in accordance with the invention. 
     
    
    
     Throughout the drawings, the same reference numeral refers to the same element, or an element that performs substantially the same function. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a block-diagram of a remote control system  160  in a safety relevant system  100  in accordance with the invention. System  100  comprises a keyboard scanner  102 , a footswitch  104  (with open contacts on off-state), remote control system  160 , and safety relevant device  190 . Remote control system  160  comprises a dual executed and redundant signal path (visualized as 1 st    103  and 2 nd    105 ) and comprises microcontroller  106 ,  108 ,  122 ,  124 , transmitter  110 , transmission path  115 , receiver  120 , relay switch driver  144 , amplifier  140  and safety filter  150 . Device  190  comprises relays  170  and  172  and motor  180 . Motor  180  can only be activated when both relays  170  and  172  are activated. Relay  170  can only be activated with a validated first switch control signal  153  and relay  172  can only be activated with a validated second switch control signal  155 . 
     An example of device  190  is a patient table that can, e.g., be moved up and down using motor  180  as part of an actuator. However any movement will abruptly be stopped when system  100  would get into an error condition by at least de-activating relay  170  or relay  172 , which would otherwise pose a hazardous situation for, e.g., a patient lying on the table. 
     To safely control a function in the safety relevant system  100 , a control line is typically be executed double, the second being redundant to the first one. This starts with footswitch  104  and keyboard  102  that have two outputs lines per function. Microcontrollers  106  and  108  check the integrity of the signal on 1 st    103  and the signal on 2 nd    105  and they check each other status as well. In case of any fault condition, microcontroller will not output a signal that could effect an action. Another task of microcontrollers  106  and  108  is to prepare their respective input signals for transmission my transmitter  110 . Receiver  120  will receive a multiplex of the transmitted signals and will separate them in respective output signals to microcontrollers  122  and  124 . Transmission path  115  can be wireless (e.g., radio frequent, RF), infra-red (IR), or cable conducted (e.g., with the 1 st  and the 2 nd  signal modulated on one and the same conducting carrier). Microcontrollers  122  and  124  will check the integrity of each respective input signal and will check each other on a fault condition as well (by preferably a handshake). If again no error condition has been detected microcontroller  122  will output a first control signal  126  for relay switch driver  144 . Also if again no error condition has been detected microcontroller  122  will output a second control signal  128  to amplifier  140 . Second control signal  128  comprises a periodic pulse train signal (with a block form shape) with a ground frequency component between F 1  and F 2 , typically F 0 . The idea is that when microcontroller would be in an error condition, it would not be able to output such a, typically software generated, block form shaped signal. F 1  will be larger than 0 Hz and F 2  will be smaller than the tact frequency of microcontroller  124 . 
     The pulse train signal will be used to control relay  170 . But before the pulse train reaches the relay it will pass amplifier  140  followed by (preferably narrow band) filter  150  Amplifier may be needed as the signal  128  coming from microcontroller  124  will typically not be powerful enough to drive relay  170  after passing filter  150 . This band-pass filter only will output a substantial signal when the pulse-train has a main frequency component that matches the band-pass filter (so with F 1 &lt;frequency&lt;F 2 ). 
       FIG. 2  shows a best mode implementation of a safety filter  150  for use in the remote control system  100  in a safety relevant system  100  in accordance with the invention 
     Filter  150  includes LC Band pass filter  152  comprising capacitor C 0  and transformer T 0 , rectifier diode D 1  and capacitor C 1 . In active operation filter  150  will receive a block wave signal that will be band-pass filtered by a selection transformer comprising capacitor C 0  and transformer T 0  and then substantially rectified by diode D 1  and capacitor C 1 . The resulting DC-output signal will be powerful enough to close the switch by driving relay  170 . 
       FIG. 3  shows the frequency response  310  of safety filter  150  and the relay  170  sensitivity  320  with respect to the safety filter response in accordance of a best mode embodiment in accordance with the invention. LC Band pass filter  152  acts as a narrow band filter with a characteristic as in  310 . When a pulse train with a main frequency component with F 1 &lt;frequency&lt;F 2  is offered to filter  150  a DC signal will appear at the output of filter  150  that is large enough to activate relay switch  170 . The filter rectifies the filtered pulse train by means of D 1  and C 1 . In case of an error condition (Software error or hardware error within the micro-controller), the dedicated frequency will not be transmitted by microcontroller  124 . 
     No Signal of a conspicuously higher or lower frequency than F 0  will substantially pass the (band-) filter and will not activate relay  170 . Components C 0 , T 0 , D 1 , and C 1  should be chosen carefully and known to be highly reliable and predictable. As these components are passive, the number of error conditions that need to be taken into account for failure analysis is much limited and thus an operation in compliance with safety standards such as IEC 61508 can be realized. 
     To ensure a most reliable fail-safe operation, all components used in system  100  should be chosen to be reliable and simple For that reason it is recommended to use as much as possible well-known and tested components. As the behavior of all these components is known, all error cases can be analyzed and simulated in the design phase. 
     The fail-safe wired or wireless remote control is very useful in a medical setting where cables can be hazardous and make operation of medical equipment awkward. The remote control can for instance be used for controlling movements of a patient table. That can for instance be useful when maneuvering the table to an medical system for examining a patient who is lying on the table. The remote control may well be used for controlling a vertical/horizontal movement of the patient table with a remote control in such a system the remote control can be applicated in a much wider area of use where safety concerns play a role. 
     One of ordinary skill in the art will recognize that alternative schemes can be devised to create a fail-safe remote control by making tweaks in the control and/or system described. For instance, pre-programmed state machines could replace microcontrollers  106 ,  108 ,  122  and  124 . Instead of controlling a motor, also other functions could be switches using relays  170  and  172 , such as activating an exposure of an X-ray machine or activating (or de-activating) any other function which use has safety regulations. 
     The foregoing merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are thus within its spirit and scope. 
     For instance a multiple of independent footswitches and/or keyboard signal could be connected to microcontrollers  106  and  108  for controlling different (e.g., safety related) functions. Consequently microcontrollers  122  and  124  could output a multiple of signals for controlling the different functions. Per function a different safety filter  150  could be devised with a different pass band behavior.