Cableless operation of a medical device

A method for cableless transmission of operating signals from a mobile remote control unit to a medical device is provided. The method includes recording a transmission quality measurement, and blocking the transmission of the operating signals if the transmission quality measurement or a distance measurement between the remote control unit and a receiver unit of the device fulfills a predetermined trigger criteria. A step detection process is carried out by the remote control unit, and a change in the transmission quality measurement is taken into account if a step movement is determined in the step detection process.

This application claims the benefit of DE 10 2007 019 529.1 filed Apr. 25, 2007, which is hereby incorporated by reference.

BACKGROUND

The present embodiments relate to cableless operation of a medical device.

Medical diagnostic or treatment systems include one or more medical devices for treating a patient. A medical device is operable via one or more control units. A medical device may be, for example, an x-ray recording device, a computer or magnetic resonance tomograph, or an irradiation system.

Operators may operate the medical device from different spatial positions. Medical devices generally communicate with a remote control unit that transmits operating signals to the medical device. The operating signals are, for example, commands for moving equipment, such as adjusting a patient support, or for triggering radiation in the course of an image-recording or irradiation session.

A cableless operating system, such as one based on radio transmission, may be easier to manage and safer than a hard-wired operating system.

In a radio-based operating system, the remote control unit may be intentionally or unintentionally removed from the area surrounding the medical device, such as an examination room. Despite low transmission power, the radio link may still exist outside the room and that a critical system function (e.g. a device movement or a triggering of radiation) will be triggered by accidental actuation of an operating key.

Remote control units with safety-relevant operating functions are generally hard-wired because of the safety concerns, despite the comparatively low ease of operation and the inconvenience caused by the cable.

Because of the safety concerns, cableless remote controls are based on infrared transmission of the operating signals. Infrared transmission requires a visual link for transmitting the operating signals. Accordingly, the possibility of unintentional operating error through walls and closed doors is eliminated. However, only a limited signal transmission range can be achieved with infrared transmission. Moreover, the infrared transmission of operating signals may be obstructed by obstacles in the beam path between the remote control unit and the device.

SUMMARY AND DESCRIPTION

The present embodiments may obviate one or more of the problems or drawbacks inherent in the related art. For example, one embodiment may provide safe and reliable cableless operation of a device, such as a medical device.

As will be discussed below, a “step” refers to the lifting and the putting down of a foot.

In one embodiment, a method for transmission of operating signals from a mobile remote control unit to a device is provided. The method may include recording a transmission quality measure and blocking the transmission of operating signals if the transmission quality measure and/or a measure derived herefrom for the distance between the remote control unit and the receiver unit of the device fulfill a predetermined trigger criteria. Exceeding or falling below a predetermined threshold value of the transmission quality measure or distance measure is provided as a trigger criterion. A change in the transmission quality measure is taken into account when checking the trigger criteria and/or determining the distance measure if a step movement is identified.

The distance of the remote control unit from the receiver unit, and consequently from the device, may be used as a decision-making variable. The decision-making variable may be used to determine whether safe transmission of operating signals is possible. The distance may be estimated from the transmission quality of the operating signals, since transmission quality declines as distance increases. Transmission quality depends on other factors apart from the simple distance, for example, on scattering and reflecting of the signal-transmitting field or on field-attenuating obstacles. The distance of the remote control unit from the device may be determined via the displacement of the remote control unit in space. The movement may be determined by step detection, since a remote control unit is normally displaced by a user. Accordingly, the remote control unit undergoes the step movement of the operating person. Reliable distance information may be drawn from a correlation of the transmission quality with the step detection. In the correlation, only a radial step movement that is linked to a change in the transmission quality is namely taken into account, while an equidistant step movement is ignored. Through correlation of the transmission quality with the step detection, a displacement-determined change in the transmission quality can be distinguished from an impairment of the transmission quality by persons, objects and other obstacles in the room. By correlating the connection quality with the step detection, distance information may be obtained. The distance information may be used to determine whether or not it is safe to transmit operating signals.

Correlation may include correlating of the transmission quality measure and the step detection that results in only such changes in the transmission quality having an impact on the enabling or blocking of operating signal transmission as occur at the same time as a detected step movement.

The received field strength and/or the bit error rate of the transmitted operating signals may be used as a transmission quality measurement. The blocking of operating signal transmission may be carried out at the remote control unit end so that when transmission is blocked, the triggering of an operating signal and/or its cableless transmission to the device is prevented. Alternatively, the blocking may be carried out at the device end so that, while in this case an operating signal is transmitted to the device, the execution of an assigned action is refused. Either the distance itself or any variable, in particular a proportional variable, correlated herewith may be used as a distance measure.

In one embodiment, for step detection, the acceleration acting upon the remote control unit is recorded and evaluated. The evaluation is based on the recognition that a step movement is associated with a characteristic periodic change in (vertical) acceleration. A step movement may be detected by recording at least one high point and one low point of the recorded acceleration. To differentiate a step movement from other accelerations acting upon the remote control, for example, vibrations, an additional check is made as to whether the acceleration changes sufficiently before the high point or low point is reached. A step movement is detected when the change in the acceleration exceeds a predetermined threshold value before the high point or the low point is reached. One or more variables, which are characteristic for the step duration and/or the step frequency, may be additionally determined by analyzing the recorded acceleration. Step duration refers to the time span between the lifting and the putting down of the foot during a step. The step duration corresponds to the time span between a high point and a low point of the recorded acceleration. Step frequency refers to the number of steps detected per unit of time. Its reciprocal value (hereinafter also referred to as the step sequence duration) is used as a particularly precise measurement of step frequency. The step sequence duration is the time span between two consecutive high points or low points of the recorded acceleration.

Using the variable or variables described above, a plausibility value is determined that represents a measure of the error probability of the detected step movement. Determination of the plausibility value is influenced by stored empirical values relating to the expected step duration and/or step frequency and the number of steps detected in succession. An error in step detection is more improbable the closer the detected values for step duration and step frequency match the stored empirical values and the more individual steps in direct succession have been detected.

The method may be executed only when the remote control is located at a defined distance from the device and/or when the transmission quality measure has already fallen below a predetermined threshold value. As long as the remote control unit is located within close range of the device, in which the transmission quality measure exceeds the threshold value, the transmission of operating signals is enabled irrespective of changes in the transmission quality measure and irrespective of step detection.

Complete blocking of operating signal transmission is preferably preceded by a critical distance range in which the triggering of an operating signal is permitted only if an enabling signal is triggered in combination, for example, simultaneously or within a predetermined time period. The enabling mode is activated if the transmission quality measurement and/or the distance measurement are in a critical range with respect to predetermined threshold values.

In one embodiment, an optical or acoustic alarm signal may be emitted if an attempt is made to trigger an operating signal even though the transmission of operating signals is completely blocked or is permitted only in enabled mode.

In one embodiment, an apparatus (system) for cableless operation of a device includes a mobile remote control unit for the cableless transmission of operating signals to the device. The apparatus includes a receiver unit assigned to the device for receiving the operating signals and a control unit, which is optionally assigned to the remote control unit or to the device. The control module interacts with the remote control unit. The control module records the above-described transmission quality measurement and optionally determines based on the measurement the distance measurement and initiates (instigates) blocking of the transmission of operating signals if the transmission quality measurement and/or the distance measurement fulfill the trigger criteria. The remote control unit includes a step detection unit for the detection of steps according to the method. The control module correlates the transmission quality measurement and a step movement detected by the step detection unit.

In one embodiment, the step detection unit includes an acceleration sensor for recording the acceleration acting upon the remote control unit and an evaluation module that detects, in accordance with the method described above, a step movement by evaluating the recorded acceleration.

The control module and the evaluation module may be software modules, which are implemented so as to able to run in corresponding hardware modules of the remote control unit and/or of the device.

DETAILED DESCRIPTION

FIG. 1shows an apparatus (system)1for the cableless operation of a medical device2. The device2is, for example, a computer tomograph.

The apparatus1includes a mobile remote control unit3. The remote control unit3includes an externally accessible operator panel4. The operator panel4includes a keypad5having a number of operator keys6, which may emit operating signals3for controlling the device2. The operator panel4includes an enabling key7, which may generate an enabling signal F, and a light-emitting diode (LED)8.

The remote control unit3may include a keyboard control9, a remote control10, a radio unit11, a loudspeaker12and a three-dimensional acceleration sensor13.

At the medical device2end, the apparatus1includes a radio unit14.

The keyboard control9serves to digitalize the operating signals B generated by the operator keys6. In normal operating mode of the remote control unit3, the keyboard control9routes the digitalized operating signals B to the radio unit11. The radio unit11transmits the operating signals B over a radio path15(e.g., cablelessly) to the radio unit14acting as the receiver unit of the device2.

The radio unit14forwards (transmits) the operating signals B to a device control16of the device2. The device control16executes the operating signals B, for example, triggers a device movement or a radiation emission.

The remote control10, which may be a microcontroller with assigned storage, monitors whether the transmission quality of the signal transmission between the radio unit11and the radio unit14is sufficient for safe signal transmission. As a measure of the transmission quality, the signal strength S and the bit error rate (BFR) are fed to the remote control10as an input signal by the radio unit11.

For outputting a blocking signal L, the remote control10may be connected to the keyboard control9so as to block the keyboard control9if, pursuant to checking, safe signal transmission is not guaranteed. If there is a blocking signal L, the keyboard control9does not forward triggered operating signals B to the radio unit11, so that no transmission of these operating signals B to the device2takes place.

The enabling signal F may be transmitted (fed) to the remote control10as input signals. The enabling signal F may be generated by the enabling key7and an acceleration signal A measured by the acceleration sensor13. The remote control10may control the light-emitting diode8and the loudspeaker12for outputting optical alarm signals W1or acoustic alarm signals W2.

A control module17and a step-detection module (evaluation module)18are implemented in the remote control10in the form of software modules. With the acceleration sensor13, the step-detection module18may be used as a step-detection unit19.

FIG. 2shows an examination room20with a surrounding wall21, in which a door opening22and a radiation-protection window23are inset. The medical device2and the apparatus (system)1provided for operation are disposed in the examination room20. For reasons of simplification, of the medical device2, only a patient support24is shown inFIG. 2. Of the apparatus1, the remote control unit3and the radio unit14, which functions as a device-end receiver unit, are shown inFIG. 2.

With regard to the quality of the radio link between the remote control unit3and the radio unit14, three areas are defined. An inner area25is enclosed by the wall21of the examination room20. An intermediate area27is outside the examination room20, but inside a predetermined outer limit26. An outer area28is outside the outer limit26.

The inner area25and the intermediate area27differ from one another significantly in the transmission quality of the radio link. As along as the remote control unit3is disposed in the inner area25, the variables used as a measure of the transmission quality, signal strength S and bit error rate BFR, will respectively exceed or fall below predetermined threshold values S1and BFR1. If the remote control unit3is taken out of the examination room20, into the intermediate area27, then the signal strength S declines, as a consequence of the radio shielding caused by the wall21, to a value below the threshold value S1, while the bit error rate BFR increases and exceeds the assigned threshold value BFR1. The outer limit26separating the intermediate area27from the outer area28is defined by the distance r of the remote control unit3from the radio unit14. The remote control unit3is located in the intermediate area27if it is disposed within a critical distance range, namely outside the examination room20but inside a threshold distance r0from the radio unit14. The remote control3is located in the outer area28if it is disposed at a distance r from the radio unit14that exceeds the threshold distance r0.

The control module17may determine the position of the remote control unit3by evaluating the signal strength S and the bit error rate BFR.

The control module17may compare the signal strength S and the bit error rate BFR with the assigned threshold values S1and BFR1. Where the signal strength S and the bit error rate BFR exceed or fall below the respective threshold values S1and BFR1, the control module17infers (determines) a position of the remote control unit3inside the inner area25and consequently enables the keyboard control9(by not generating the blocking signal L) without further conditions.

If the control module17detects that the signal strength S or the bit error rate BFR fall below or exceed the respectively assigned threshold value S1or BFR1, then the control module17infers (determines) that the remote control unit3is outside of the inner area25and into the intermediate area27. The control module17determines the distance r and determines by comparing the distance r with the stored threshold distance r0whether the remote control3is located within the intermediate area27or within the outer area28.

If the control module17determines that the remote control unit3is in the intermediate area27, then the control module17blocks the keyboard control9by generating the blocking signal L, but permits manual unblocking of the keyboard control9by generating the enabling signal F. The control module17may be implemented such that by pressing on the enabling key7, which generates the enabling signal F, the blocking signal L is cancelled for a predetermined time, for example, 10 seconds. During the predetermined time, operating signals B may be generated effectively and transmitted to the device2via the operator keys6. After expiration of the predetermined time, the control module17blocks the keyboard control9again, such that the enabling key7has to be pressed again to trigger further operating signals B. Alternatively, the predetermined time may be “retriggered” by pressing an operator key6within the predetermined time and the keyboard control9consequently remains unblocked for a further amount time. The keyboard control9is not blocked again until after expiration of the predetermined time following the last effectively triggered operating signal B.

If the control module17determines that the remote control unit3is in the outer area28based on a determination that the trigger criteria (r>r0) is fulfilled, then the control module17blocks the keyboard control9fully, such that the block cannot be overridden, even by pressing the enabling key7.

FIG. 2shows three exemplary movements29a,29band29cof the remote control unit3. During the course of the first movement29a, the remote control unit3remains inside the inner area25. During the first movement29a, operating signals B may be triggered unconditionally. During the course of the second and third movements29band29c, the remote control unit3is carried out of the inner area25into the intermediate area27. Unconditional triggering of operating signals B is possible in a first section of the movements29b,29c. For example, as soon as the remote control unit3is carried out of the examination room20, the enabling key7has to be pressed before operating signals B are triggered. During the course of the movement29c, the remote control unit3is carried over the outer limit26and into the outer area28. During the course of the movement29c, the transmission of operating signals B is blocked as soon as the remote control unit3is carried over the outer limit26.

The control module17may indicate to the operator whether the remote control unit3is located in the inner area25, the intermediate area27or the outer area28. The control module17may activate the light-emitting diode8. For example, the light-emitting diode8shines green continuously if the remote control unit3is located in the inner area25, flashes green if the remote control3is located in the intermediate area, and shines red continuously if the remote control3is located in the outer area28.

The control module17may activate the loudspeaker12, to output an acoustic alarm signal W2if an operating signal B (in particular, a safety-critical operating signal) is triggered despite a block being in place. A further acoustic alarm signal W2may be output when the radio link between the remote control unit3and the radio unit14is interrupted.

To determine the distance r, the control module17may use the signal strength S, the bit error rate BFR, and the result of a step detection. Step detection may be carried out by the step detection unit19, for example, by the acceleration sensor13and the step detection module18.

For step detection, the acceleration sensor13records the acceleration acting upon the remote control unit3and feeds the acceleration signal A resulting from this measurement to the control module10. The step detection module18may evaluate the temporal course of the acceleration signal A to detect a step movement of a user carrying the remote control unit3.

Step detection is based on inertia navigation. While walking, the human body performs a periodic upward and downward movement.

At the beginning of a step, the operator raises the respective stepping leg from the ground and moves it past the standing leg. The body of the operator moves upward so that the body experiences a vertical upward acceleration. The body movement reaches its highest point when both legs are located alongside one another. At the highest point, the body experiences no vertical acceleration. As soon as the stepping leg has been led past the standing leg, the body moves downward so that a downward vertical acceleration acts upon the body. The downward vertical acceleration comes to a halt when the stepping leg is placed on the ground again at the conclusion of the step.

The remote control unit3has no fixed orientation relative to the surrounding space. From the viewpoint of the remote control unit3, the vertical direction is not predetermined in a fixed manner. The three-dimensional (3D) acceleration vector is first defined by the acceleration sensor13in a stationary system of coordinates in relation to the remote control unit3. The 3D acceleration vector allows the vertical acceleration associated with a step movement to be recorded. For the evaluation, the step-detection module18takes into account only the magnitude of this acceleration vector in the form of the acceleration signal A (Equation 1).
A=sqrt(ax2+ay2+az2),  Equation 1
where ax2, ay2and az2are components of the three-dimensional acceleration vector.

The acceleration vector, which is recorded by the acceleration sensor13, is generally oriented in a vertical spatial direction due to the dominant influence of acceleration due to gravity. The magnitude of the acceleration vector is influenced (e.g., almost exclusively only) by vertical acceleration changes, while the influence of horizontal accelerations on the magnitude of the acceleration vector remains negligible.

An acceleration value below the value of acceleration due to gravity (1 g) is an indication of a vertically downwardly directed acceleration of the remote control unit3, while an acceleration magnitude that exceeds the value of acceleration due to gravity is an indication of an upwardly directed vertical acceleration of the remote control unit3.

A step movement consequently results in a temporal course of the acceleration signal A that oscillates about the magnitude of the acceleration due to gravity. To prevent triggering errors caused by vibrations or other body movements, the acceleration signal A may be smoothed by the step-detection module18. The step-detection module18may apply an exponential moving average (EMA) filter to smooth the acceleration signal A.FIG. 3shows an example of a temporal course of the correspondingly smoothed acceleration signal A during a step movement.

As shown inFIG. 3, a high point of the acceleration signal A (i.e. a local maximum of the acceleration value) indicates the start of a step, while a low point (i.e. a local minimum) of the acceleration signal A indicates the termination of a step. The start times ta1, ta2and end times te1, te2of two consecutive steps are plotted inFIG. 3.

To detect a step movement, the step-detection module18searches, in accordance with a method outlined in a simplified manner inFIG. 4, for high points and low points of the smoothed acceleration signal A. After the program start (act40), the current value Aiof the acceleration signal A is recorded cyclically and compared with the respectively preceding value Ai-1(act41). Once the current value Aiexceeds the respectively preceding value Ai-1(J), act41is repeated, with the current value Aibeing read in afresh in each case and the previously current value being stored as the new preceding value Ai-1. Once the current value Aifalls below the preceding value Ai-1(N), this is deemed to be an indication that a high point has occurred. In this case, in act42, the time corresponding to the current value Aiis stored as the start time taiof a step.

In act43, the current value Aiis then again read and compared with the preceding value Ai-1. If it is established that the current value Aifalls below the preceding value Ai-1(J), then act43is repeated. Otherwise (N), the time corresponding to the current value Aiis stored as the end time teiof the step. The step-detection module18feeds the stored start times taiand end times teito the control module17as a pointer to a detected step movement.

The control module17may calculate, based on the recorded signal strength S, the bit error rate BFR, and the information about the step movement, the current distance r of the remote control unit3from the radio unit14.

FIG. 5illustrates a sample embodiment of a method for calculating the distance r. The control module17may initialize a distance variable r (block50) with an initial value r1(block51) if it establishes on the basis of the signal strength S or the bit error rate BFR that the remote control unit3is outside of the inner area25. The initial value r1corresponds to the distance between the radio unit14and the door opening22(seeFIG. 2).

The control module17may determine, at predetermined time intervals, the changes ΔS, ΔBFR in the signal strength S, and the bit error rate BFR, respectively. From the change ΔS, the control module17derives, for example, on the basis of a stored characteristic curve S(r) (block52), a distance change ΔrS. Using a stored characteristic curve BFR(r) (block53), the control module17may derive, for example, from the change ΔBFR, a distance change ΔrBFR. By averaging (blocks54and55), the control module17derives, for example, from the distance changes ΔrSand ΔrBFR, an average distance change Δrm. The average distance change Δrmmay correspond to the distance change that emerges from the evaluation of the transmission quality.

The control module17may use the result of the step detection provided by the step-detection module18, for example, the start times taiand end times teiof the detected step movement recorded within the observed time interval. The control module17may calculate in block56the number of steps detected in the time interval (referred to hereinbelow as the number of steps n). By multiplying the number of steps n with a stored average step length, the control module17determines (block57) a path covered by the step movement Δs.

The average distance change Δrmand the path Δs may be correlated with one another by geometric averaging (blocks58and59). In block59, a sign-retaining root formation is performed. The mathematical operation performed by the blocks58and59corresponds to the formula: sign(Δrm·Δs)·sqrt(Δrm·Δs).

The result of the geometric averaging is the distance change Δr to be calculated in accordance with the method. Based on the distance change Δr, the distance variable r is updated in accordance with the formula r=r+Δr.

Using the method shown inFIG. 5, the transmission quality is a result correlated with the outcome of the step detection to the extent that a change in the transmission quality is taken into account in relation to a change in the distance variable r only if at the same time a step movement has been identified. The distance variable r is not affected by a change in the transmission quality that is not associated with a step movement. A detected step movement, which is not correlated with a change in transmission quality, does not lead to a change in the distance variable r.

FIG. 6shows another embodiment for searching for high points and low points of the smoothed acceleration signal A. An act60or60′ is added ahead of acts41and43respectively. A check is carried out as to whether the smoothed acceleration A undergoes a sufficiently large change (act60or60′), for example, a change that exceeds a threshold value Δy, before reaching a high or low point. If this is the case (J), then in the subsequent act41or43, a check is carried out as to whether a high or a low point is present. Otherwise (N), act60or60′ is repeated. By adding the act60,60′ in advance, the risk of an error in detecting a high or low point, for example, as a result of noise effects or short-term vibrations, is significantly reduced.

FIG. 7illustrates an exemplary extension of the method according toFIG. 6. After the detection of a high point or low point, a step duration d and, as a measure of the step frequency of consecutive steps, a step sequence duration D are determined on the basis of the stored start time taiand end time teiof a detected step. The step duration d is defined by the time interval between the detected high point taiand the subsequent low point tei. The step sequence duration D is determined by the time interval between two consecutive start times tai-1and tai(seeFIG. 3).

In the method according toFIG. 7, the step-detection module18checks whether the step duration d lies within an expected interval [dmin; dmax] which is predetermined by stored threshold values dminand dmax(act61). If this is the case (J), then the step-detection module60flags (act62) that a valid step has been detected. Otherwise (N), the method flow goes back to act60.

If a valid step has been detected, then the step-detection module18checks in act63whether the step sequence duration D lies within an expected range [Dmin; Dmax] with threshold values Dminand Dmax. If this is the case (J), then the step-detection module18flags that a valid subsequent step of a step sequence has been detected (act64) and goes back to act60. The program flow otherwise goes back directly from act63to act60.

The step-detection module18determines from the results of act62and64how many valid acts have been detected consecutively in a valid sequence and derives from the result, on the basis of a stored characteristic curve, a plausibility value for the detected step movement. The derivation of the plausibility value is based on the recognition that a detected step movement, which consists solely of a single step in isolation, is marked by a comparatively high degree of uncertainty. Vibrations of the remote control unit3may, with a comparatively high degree of probability, also give rise to an acceleration pattern that is detected according to the method as a single step. The probability of such a detection error occurring decreases with increasing regularity of the acceleration signal A, as produced by a longer step sequence. The plausibility value is made available by the step-detection module18to the control module17and is utilized by the control module17.

The control module17may calculate, on the basis of the plausibility value, an error value for the distance variable r. The error value may be taken into account when deciding whether the transmission of operating signals should be permitted or blocked. For example, the control module17may prompt an operator via the light-emitting diode8and/or the loudspeaker12through suitable alarm signals W1,W2to bring the remote control3back into the inner area25if the error value exceeds a predetermined maximum value.

The distance between the remote control unit3and the radio unit14may be estimated exclusively based on the transmission quality, for example, on the basis of the signal strength S and the bit error rate BFR. No distance measurement is explicitly calculated in this variant of the method.

The control module17may check first in accordance withFIG. 2by comparing the signal strength S and the bit error rate BFR with the assigned threshold values S1and BFR1whether the remote control unit3is located in the inner area25and in this case enables the keyboard control9without further conditions.

If the control module17detects, for example, in accordance withFIG. 2, that the remote control unit3is outside of the inner area25and in the intermediate area27, then the control module17, deviating from the method variant described above, checks at predetermined time intervals whether the signal strength S or the bit error rate BFR falls below or exceeds a respectively assigned second threshold value S2or BFR2. With respect to the second threshold values, the relations S2<S1and BFR2>BFR1apply, where the threshold values S2and BFR2respectively correspond approximately to the value of the signal strength S or bit error rate BFR that is to be expected on average when the remote control unit3is located at a distance r=r0from the radio unit14. If the trigger criteria (S<S2)(BFR>BFR2) is fulfilled, then the control unit17pinpoints the remote control unit3in the outer area28, if simultaneously, for example, in the preceding time interval, a step movement has been detected by the step detection unit19in the manner described above, and in this case blocks the emission of operating signals.

Otherwise, in the absence of a detected step movement, the control unit17pinpoints the remote control unit3in the intermediate area27and enables the emission of operating signals B in enabled mode, even if the signal quality would fulfill the trigger criteria stated above.

In order to achieve a sufficiently good temporal resolution, but be able to detect a step movement reliably, the above-mentioned time interval is preferably of the order of a few seconds, for example, between about 15 and 30 seconds.

In one embodiment, the control unit17checks continuously over time the trigger criteria (S<S2)(BFR>BFR2), and if this criteria is fulfilled, blocks the emission of operating signals if a step movement has been detected in a moving amount of time prior to fulfillment of the trigger criteria. The amount of time is preferably several seconds, for example, between 15 and 30 seconds.

The control unit17takes into account, when deciding about the blocking or enabling of operating signal transmission, the plausibility of the detected step movement. Operating signal transmission may be blocked only when at least one predetermined number of steps has been detected in sequence within the time interval or time window.