Development and commissioning of a safety facility for the algometer test installation

Status:

completed 12/2011

Aims:

In the newly developed test facility (algometer) for the measurement of human pain thresholds, a special plunger is used to generate a strain upon a discrete body point of the test subject until the pain threshold is reached. The strain is applied by means of a linear motor such that a controlled linear rise in force is applied to the body point up to the pain threshold. In order to monitor this strain method, a safety facility was developed and commissioned; it reliably prevents the test subject from being subjected to excessive strain. The highest safety level possible for machinery must be assured for this function. The tester must determine a limit force for each test subject and body point subjected to strain. This individual limit force, which must not be exceeded under any circumstances, is assigned as the parameter for the safety facility. The limit force is monitored permanently whilst the installation is switched on. Should it be exceeded, de-energization functions must be tripped within the safety facility which immediately relieve the test subject of the strain. The full strain range for pain threshold measurement is to be 500 N. The limit force can be specified within this range. The safety facility is to be used to perform tests of the installation's safety functionality each time the installation is switched on and at intervals of ten hours when it is used for longer periods.

Activities/Methods:

The system for measurement of the force during the pain threshold measurement is of redundant design. The forces are measured continually by means of a safety control, and the instantaneous force value is monitored for violation of the selected limit force. A technical spring module, on which the desired limit force can be set, was adapted between the plunger for application of the mechanical strain to the body point and the thrust rod of the linear motor. The pretension distance is measured by the safety controller by means of a redundant laser distance measurement system. The limit force is thus determined by measurement of the pretension distance and the recorded force-distance characteristic of the spring. It is set mechanically on a scale on the plunger head, and displayed in digital form on the safety controller. If this limit force is exceeded during measurement of the pain threshold, the linear motor is immediately de-energized, and the force coupling of the algometer on a guide rail is released. During use, the pressure algometer is able to adopt certain positions. These are also monitored continually. The entire algometer test facility has been evaluated in accordance with the Machinery Directive. In the process, the safety-related control system was tested to EN ISO 13849.

Results:

A safety facility was developed and commissioned for the algometer test facility. The safety facility reliably prevents the test subject being subjected to excessive strain during pain threshold measurements.This facility attains the highest safety level that can be certified in accordance with the Machinery Directive. The Institute for Occupational Safety and Health of the German Social Accident Insurance (IFA) documented conformity with the Machinery Directive by self-declaration, and assigned the CE mark to the facility. The key safety parameter which must be defined and set by the user prior to a pain threshold measurement is a limit force for the maximum acceptable strain upon the test subject at the body point on which a pain threshold measurement is to be performed. This value is specified by the user and can be determined with adequate precision beforehand, for example by manual testing by means of a hand-held algometer. The limit force can be selected within the range up to 500 N. Strain forces above this level are prevented by the safety facility. The limit force is set by the user on a manual scale and monitored on a digital display. It is monitored permanently, i.e. in all operating modes, when the installation is switched on. A safety controller monitors all safety-related parameters. If the limit force is exceeded during a pain threshold measurement, de-energization functions take effect. These trigger a rapid reverse movement of the plunger away from the test subject and into a safe position, and relieve the subject immediately of the potentially excessive strain. From this safe position, the algometer can easily be returned to the "ready" position from which another pain threshold measurement can be performed. The parameters leading to tripping of the safety response must first be recognized on the display, understood, evaluated, and acknowledged following the performance of any measures. In order to ensure that the de-energization functions are ready at all times, they must be tested whenever the test facility is switched on and thereafter during operation at intervals of ten hours. This test mode is imposed by the safety controller; the test facility is enabled for the performance of pain threshold tests only once the safety test has been passed. All electrical systems are of redundant design. The safety facility cannot be manipulated by the user. The facility must be serviced annually.

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