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Single fault safe user interface Disclosure Number: IPCOM000239595D
Publication Date: 2014-Nov-18
Document File: 2 page(s) / 41K

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The Prior Art Database



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Single fault safe user interface

Multi spatial axis control by humans of motorized movement like e.g. the stands of an X-ray system is typically implemented using joystick like user interfaces. Where uncontrolled movement of objects poses an unacceptable safety risk, international basic safety standards like e.g. the IEC60601-1 for medical electrical equipment, requires the system implementation to be single fault safe. A well-known system level design implementation for single fault safety is the use of an electrical guard signal that enables or prohibits activation of the movement on top of the movement control signal. In order for the guard signal to act single fault safe, its status needs to be driven by a means sufficiently independent (i.e. uncorrelated) from the means used to derive the movement control signal.

Current state-of-the-art implementations for deriving the movement control signal from a joystick like user interface are:

 Use of two orthogonal located potentiometers that change resistance value as function of the angular diversion of the joystick axis from its mechanically zero external force position.

 Use of two orthogonal located optical shaft encoders read-out by a small piece of firmware on a microcontroller that calculates the angular diversion of the joystick axis from its reference position.

 Use of a hall sensor measuring the displacement of a small permanent magnet located at the bottom of the joystick axis and a microcontroller calculating the diversion from the reference position of the joystick axis based on the hall-sensor signal.

Current state-of-the-art implementation for deriving the guard signal for enable move is:

 Use of an optical sensor (e.g. light emitting diode in combination with a light sensitive diode) that is used to activate or deactivate the guard whenever the joystick axis angular position is sufficiently (say > 5 degrees) diverting from the mechanical zero external force position.

A drawback of the referenced optical sensor for deriving the guard signal is that it requires a significant displacement to alter the response of the photo sensitive detector (e.g. photo-diode). This is generally implemented by positioning the detection sufficiently far away from the joystick axis point of rotation. This is getting more cumbersome the smaller the dimensions of the joystick are. It also may add added complication to the electronic and mechanical design because the control sensor and guard sens...