Track calibration method and track calibration system

Abstract

A method for trajectory calibration of a linear motor system, comprising-controlling linear motors to move a transfer element along a track in a calibration operation,-in the calibration operation, moving the transfer element along the track based on a position feedback signal from a position sensor of the respective linear motor and its adjacent linear motor. A position jump at the section boundary of each linear motor is determined, an offset value and / or a gain value are / is generated for each linear motor as a function of the determined position jump, and the offset value and / or the gain value for each linear motor are / is stored in a storage unit.

Description

Technical Field

[0001] The present invention relates to a method for track calibration of a linear motor system, in particular a multi-carrier system, and to a system for track calibration. Background Art

[0002] The multi-carrier system comprises a number of linear motors, also called segments, arranged in a row. The linear motors have a track, for example defined by rails, along which a transport element, also called a carrier, can be moved by applying a driving force by the linear motors. The track can be closed, so that the transport element can theoretically move infinitely in one direction along the track, or open, so that the track has two ends and the transport element can move between these two ends.

[0003] Typically, a PCBA board (printed circuit board) with a position sensor (also called an encoder) is installed in each segment. The position sensor is used to measure the position of the vehicle, and the measuring range of the position sensor is usually greater than the length of the segment, so that the position of the vehicle can be detected at any position along the track, even if there is a gap between adjacent segments.

[0004] Due to the size and complexity of such track systems, the components of the system, such as segments, rails, etc., are usually shipped separately to the end user or customer and assembled at the customer's site. The segments and rails that define the track can be assembled with tolerances. In addition, due to possible errors in manufacturing, the PCBA board of the encoder can be installed in the segment with a certain tolerance. In these cases, there are position jumps of the carrier at the segment boundary between two adjacent segments. These position jumps lead to poor movement of the carrier. Summary of the invention

[0005] The present invention seeks to provide a method and system for track calibration to improve the smoothness of vehicle movement at segment boundaries and maintain repeatability.

[0006] This problem is solved by the subject matter of the independent claims. Advantageous embodiments are defined in the dependent claims and result from the description and the drawings.

[0007] The method according to the invention is used for track calibration of a linear motor system, in particular a multi-carrier system, which has at least one conveying element, a plurality of linear motors arranged in a row and having a track, wherein adjacent linear motors abut each other at a segment boundary, and each linear motor is configured to apply a driving force for moving the conveying element along the track, and comprises at least one position sensor, preferably a plurality of position sensors, for detecting the position of the conveying element when the conveying element travels along the track, and a control unit configured to control the linear motors to apply the driving force to the conveying element, the control unit comprising a processing unit and a storage unit. The method comprises the following steps:

[0008] - controlling the linear motor to move the transport element along the track in a calibration run,

[0009] - in a calibration run, determining position jumps at segment boundaries of at least some of the linear motors based on position feedback signals from position sensors of the respective linear motor and its neighboring linear motors,

[0010] - generating offset values ​​and / or gain values ​​for at least some of the linear motors according to the determined position jumps,

[0011] - Storing the offset value and / or the gain value in a memory unit.

[0012] “At least some of the linear motors” may particularly mean that the position jump is determined and the offset and / or gain values ​​are generated for one linear motor only, for every second linear motor along the track or for every linear motor.

[0013] In a calibration run, a transport element is moved along the track. This can in particular be done at a relatively slow and preferably constant speed, for example of the order of 50 mm / s. Since the dimensions of the individual linear motors are known, the positions of the segment boundaries are also known. When the transport element passes a segment boundary, there will be a sudden or step-like difference between the position feedback signals from the position sensors of the linear motors that abut one another at the segment boundary when there is a gap between these adjacent linear motors (i.e. the linear motor upstream of the segment boundary and the linear motor downstream of the segment boundary). This difference corresponds to a "position jump", so that the position jump can be determined by analyzing the position feedback signals from the position sensors of the adjacent linear motors.

[0014] By means of the method according to the invention, the user can easily calibrate the linear motor himself during commissioning. According to the method, no separate sensors or calibration systems are required for calibrating the linear motor system. A linear motor system including one conveying element is all that is required to implement the method and calibrate the track.

[0015] If after commissioning of the linear motor system, a segment is defective and must be replaced, the defective segment can be replaced and the replacement segment can be individually calibrated on the track without any impact on the other segments. It is only necessary to align the start and end positions of the segment with the end position of the previous linear motor and the start position of the next linear motor.

[0016] In the calibration run, the position jump at the segment boundary of each linear motor is determined based on the position feedback signals from the position sensors of the respective linear motor and its neighboring linear motors, which specifically means that the position value obtained from the respective position feedback signal at the respective segment boundary is subtracted, i.e. when considering a specific linear motor, the end position of the preceding linear motor is subtracted from the start position of the considered linear motor to obtain the position jump at the segment boundary to the preceding linear motor. Likewise, the end position of the considered linear motor is subtracted from the start position of the following linear motor to obtain the position jump at the segment boundary to the following linear motor.

[0017] In this regard, the terms "previous," "subsequent," "after," and "previous" refer to the sequence of the linear motors along the track.

[0018] By considering the position jump at the segment boundary, only each linear motor, including its preceding or preceding and following or succeeding linear motors, and the corresponding position jump at the segment boundary need to be considered individually. In particular, it is not necessary to obtain a complete ideal position signal in advance with an additional measurement system, which represents the ideal or nominal position signal of the entire track.

[0019] In the following, an example of determining a position jump and generating offset and gain values ​​is provided, using the following definitions:

[0020] P start : The starting position of the linear motor

[0021] P end : The end position of the linear motor

[0022] P prev : The end position of the previous linear motor

[0023] P next : The starting position of the next linear motor

[0024] ΔP start : The linear motor starts to jump in position

[0025] ΔP end : The position jump at the end of the linear motor

[0026] AP avrg : The average position jump of the linear motor

[0027] Bias: The offset or deviation of this linear motor

[0028] ΔP start,bias : The position jump at the beginning after applying the offset or deviation

[0029] ΔP end,bias: The position jump at the end after applying the offset or deviation

[0030] It should be noted that the determination of the position jump and the generation of offset and gain values ​​are performed separately for each linear motor, so the following calculations always involve the linear motor under consideration ("the linear motor") and the previous and next linear motors along the track relative to the linear motor under consideration.

[0031] Value P start , P end , P prev and P next corresponds to the position feedback signal of the respective position sensor and is determined during a calibration run.

[0032] For example, the position jump can be determined as follows:

[0033] ΔP start =P start -P prev

[0034] ΔP end =P next -P end

[0035] Furthermore, the following is true for the linear motor with index i:

[0036] P end (i) = P prev (i+1)

[0037] P next (i) = P start (i+1)

[0038] ΔP start (i) = ΔP end (i-1)

[0039] In an advantageous embodiment, generating the offset value of the linear motor comprises calculating an average position jump value of a first position jump at a segment boundary to a preceding linear motor and a second position jump at a segment boundary to a succeeding linear motor, and subtracting the average position jump value from the first position jump or the second position jump. Thus, the offset or deviation of the linear motor can be calculated by using the average of the position jumps at the two segment boundaries of the linear motor under consideration and subtracting the average from the end position jump of the linear motor:

[0040]

[0041] Bias=ΔP end -ΔP avrg

[0042] The offset values ​​can be used to shift the start and end positions of the corresponding linear motors:

[0043] P start,bias =P start +Bias

[0044] P end,bias =P end +Bias

[0045] Using the start and end positions of the shift, the shifted position jump at the segment boundary becomes equal to the average value of the position jump:

[0046] ΔP start,bias =P start,bias -P prev =ΔP start +Bias=ΔP avrg

[0047] ΔP end,bias =P next -P end,bias =ΔP end -Bias=ΔP avrg

[0048] The gain value can be calculated as follows, where L motor Defined as the track length of the linear motor or segment under consideration:

[0049]

[0050] By applying the gain values ​​to the offset start and end positions, the position jumps at the start and end of the considered linear motor can be eliminated. When this method is repeated for the other linear motors throughout the track, a complete track calibration can be achieved.

[0051] Although the track calibration has been described with each linear motor considered individually and including its preceding and succeeding linear motors, another approach could be to evenly distribute the position jumps between the segments over the entire track. However, in this case, it is necessary to consider all the linear motors of the track for calculation in order to achieve a global optimization of the entire track.

[0052] In such an embodiment, generating offset values ​​for the linear motors includes calculating an average position jump value from all position jumps determined at the segment boundary, and for each linear motor, subtracting the average position jump value from the ending position jump value of the corresponding linear motor at the segment boundary to the subsequent linear motor.

[0053] In this case, the average position jump value ΔP of all position jumps avrg,all It is defined as follows, where n is equal to the number of linear motors in the track:

[0054]

[0055] Based on the definitions provided previously, the offset or shift value is defined as follows:

[0056] Bias(i)=ΔP end (i)-ΔP avrg，all

[0057] Therefore, the start and end positions after applying the offset values ​​are defined as follows:

[0058] P end，bias (i) = P end (i)+Bias(i)

[0059] P start,bias (i) = P start (i)+Bias(i)

[0060] If an offset value is applied to any linear motor, the position jump at the beginning of the linear motor will be larger, and the position jump at the end of the linear motor will be smaller (by the same amount), and vice versa. However, the sum of the two position jumps remains the same, and the average position jump value also remains the same.

[0061] An offset calculation can then be performed, starting with the last linear motor in the track sequence, by applying an offset or deviation to that linear motor so that the end position jump of that linear motor is equal to the average position jump value. This means that the position jump at the beginning of that linear motor is also changed due to the application of the offset.

[0062] Accordingly, the position jump of the i-th linear motor becomes:

[0063] ΔP end，bias (i) = P next (i)-P end，bias (i) = ΔP end (i)-Bias(i)=ΔP avrg，all

[0064] ΔP start，bias (i) = P start，bias (i)-P prev (i) = ΔP start (i)+Bias(i)

[0065] Furthermore, for segment i-1, consider that applying an offset to segment "i" has an effect on segment i-1:

[0066] ΔP end (i-1) = ΔP start,bias (i) = ΔP start (i)+ΔPend (i)-ΔP avrg，all

[0067] Using the above definition of offset value:

[0068] Bias(i-1)=ΔP end (i-1)-ΔP avrg，all

[0069] Therefore, the shift position jumps as follows:

[0070] ΔP end,bias (i-1) = ΔP end (i-1)-Bias(i-1)=ΔP avrg,all

[0071] ΔP start，bias (i-1) = ΔP start (i-1)+Bias(i-1)

[0072] For closed track calibration, start from the last (nth) segment, so

[0073] ΔP end,bias (n) = ΔP avrg，all

[0074] This means that the last segment is displaced so that the gap between the last segment and the first segment is equal to ΔP avrg,all .

[0075] For the last (nth) segment:

[0076] ΔP start,bias (n) = ΔP start (n)+Bias(n)=ΔP start (n)+ΔP end (n)-ΔP avrg，all

[0077] For the penultimate (n- 1 ) section:

[0078] ΔP start,bias (n-1) = ΔP start (n-1)+ΔP end (n-1)-ΔP avrg，all

[0079] ΔP start,bias (n-1) = ΔP start (n-1)+ΔP start,bias (n)-ΔP avrg，all

[0080] ΔP start,bias (n-1)

[0081] =ΔP start (n-1)+ΔP start (n)+ΔP end (n)-ΔP avrg，all -ΔP avrg，all

[0082] For the third to last segment (n-2):

[0083] ΔP start,bias (n-2) = ΔP start (n-2)+ΔP end (n-2)-ΔP avrg，all

[0084] ΔP start,bias (n-2) = ΔP start (n-2)+ΔP start，bias (n-1)-ΔP avrg，all

[0085] ΔP start,bias (n-2)

[0086] =ΔP start (n-2)+ΔP start (n-1)+ΔP start (n)+ΔP end (n)-3ΔP avrg，all

[0087] When these calculations are performed from n to 2, the results are as follows:

[0088]

[0089] For a closed track: ΔP en (n) = ΔP start (1)

[0090] The remaining gap between the first section and the second section

[0091]

[0092] For a closed trajectory, the offset or deviation value of the first segment will remain at 0 because

[0093] ΔP end,bias (1) = ΔP start，bias (2) = ΔP avrg，all

[0094] ΔP start,bias (1) = ΔP end，bias (n) = ΔP avrg，all

[0095] For open tracks, the situation is different, since there are n segments but only n-1 gaps. In the above calculation sequence, any value, such as 0, can be used. Then the offset value for the nth segment is ΔP end (n)

[0096] Bias(n)=0-ΔP avrg，all

[0097] In an open track, the offset value or deviation of the first section must also be calculated according to the above calculation, because in an open track, the offset value of the first section is usually not equal to zero.

[0098] Therefore, the first position jump and all position jumps at the segment boundaries of the entire track are equal to the average position jump, ie the position jumps are evenly distributed over the entire track.

[0099] The method may also include balancing the offset values ​​generated for all linear motors by determining an average offset value from a maximum offset value and a minimum offset value among the offset values ​​and subtracting the average offset value from each offset value. We consider "B" to be the average offset value:

[0100]

[0101] Then, the various offset values ​​are redefined as follows in order to limit the maximum and minimum offset values ​​for any segment:

[0102] Bias(i)=Bias(i)-B

[0103] In this way, the maximum value among the offset values ​​can be reduced.

[0104] When position jumps are coordinated and have the same value throughout the trajectory, the position jumps can be completely eliminated by calculating the gains as follows:

[0105]

[0106] With the method described above, it is possible to calibrate the track and eliminate position jumps as much as possible without the need for external encoders or encoding systems. Furthermore, the method is valid for both open and closed tracks and it allows calibration of replacement segments when defects are present. This is particularly advantageous in the case of large linear motor systems with many linear motors and saves a lot of effort.

[0107] According to another aspect, the invention relates to a system for track calibration of a linear motor system, in particular a multi-carrier system. The system comprises at least one transport element, a plurality of linear motors arranged in a row and having a track, wherein adjacent linear motors abut each other at a segment boundary, and each linear motor is configured to apply a driving force for moving the transport element along the track, and comprises at least one position sensor, preferably a plurality of position sensors, for detecting the position of the transport element as it travels along the track, and a control unit configured to control the linear motors to apply the driving force to the transport element, the control unit comprising a processing unit and a storage unit, wherein the control unit is configured to perform the method as described above.

[0108] The storage unit may be a central storage unit for the entire linear motor system, or may be an independent storage unit provided in each linear motor.

[0109] In particular, each linear motor can have six outer surfaces, namely a top side, a bottom side, an outer side, an inner side and two side surfaces. The side surfaces of adjacent linear motors are arranged at "segment boundaries". Tracks for conveying elements can be arranged on the outer side. The inner side is arranged in the region of the interior space of the transport system.

[0110] The conveyor system or multi-carrier system can be designed to be cyclic, so that the linear motor has a closed track, along which the conveyor elements can theoretically move cyclically in the same direction. However, the linear motor can also form an open path with a starting point and an end point.

[0111] In particular, the transport element is magnetically driven. For this purpose, the transport element has one or more permanent magnets, to which the driving force is applied by means of a changing and / or moving magnetic field generated by the linear motor. The driving force causes the transport element to move in a moving direction along the track. In particular, the transport elements can be moved independently and separately from each other.

[0112] The transport element may also be referred to as a carrier, a mover, or a runner, and the linear motor may also be referred to as a stator or a segment.

[0113] It should be noted that the descriptions and comments made about the disclosed method for trajectory calibration also apply to the disclosed system for trajectory calibration after necessary modifications, and vice versa. It should be understood that all features and configurations mentioned herein can be combined with each other unless explicitly stated otherwise. BRIEF DESCRIPTION OF THE DRAWINGS

[0114] In the following, the present invention is described by way of example with reference to the embodiments schematically shown in the accompanying drawings. These figures show the following:

[0115] Figure 1Ais a perspective view of a portion of a linear motor system.

[0116] Figure 1B is a schematic diagram of the control unit.

[0117] Figure 2 is a schematic diagram of a transport element and a linear motor that moves along a track during a calibration run;

[0118] Figure 3 A schematic diagram showing the definition of parameters associated with a segment;

[0119] Figure 4 is a diagram visualizing the effects of track calibration and applied offset and gain values;

[0120] Figure 5 is a diagram showing position jumps along an exemplary track before and after track calibration, considering each second linear motor separately, and

[0121] Figure 6 is a graph showing position jumps along an exemplary track before and after calibration of the track, taking into account evenly distributed position jumps for all linear motors. DETAILED DESCRIPTION

[0122] exist Figure 1A , a portion of a linear motor system 10 implemented as a multi-carrier system is shown having a plurality of linear motors 11 arranged in a row with rails 13 mounted on the linear motors 11. Segment boundaries 15 are defined where adjacent linear motors 11 abut one another.

[0123] Furthermore, the linear motor system 10 includes a transport element 19, and each linear motor 11 is capable of applying a magnetic driving force for moving the transport element 19 along the track 13. Furthermore, each linear motor 11 includes a position sensor 17 that cooperates with a position magnet 29 on the transport element 19 to detect the position of the transport element 19 when the transport element 19 travels along the track 13 and generates a position feedback signal.

[0124] Figure 1B A control unit 21 of the linear motor system 10 is schematically shown, which has a processing unit 23 and a storage unit 25. The control unit 21 is connected to the linear motor 11 via a communication link such as a data bus (not shown). Via the communication link, the control unit 21 receives a position feedback signal indicating the position of the transmission element 19 from the position sensor 17, and can provide a control signal to the linear motor 11 so as to control the linear motor 11 to apply a driving force to the transmission element 19.

[0125] Figure 2Any three linear motors 11 of the plurality of linear motors 11 are schematically shown, and as mentioned above, the three segments are adjacent to each other at the respective segment boundaries 15. In the calibration operation, the conveying element 19 moves along the track ( Figure 2 In the calibration operation, the conveying element moves along the track at a speed of approximately 50 mm / s, for example. Figure 2 There are gaps 31 between the first and second linear motors 11 in the segment, for example due to assembly tolerances. These gaps 31 lead to "position jumps" of the linear motors 11 at the segment boundary 15 between two adjacent linear motors 11, i.e., jumps or differences between the position feedback signals of adjacent linear motors 11, and lead to poor movement performance.

[0126] In order to compensate for these gaps, the control unit 21 determines an offset value and a gain value for each second linear motor 11, stores these values ​​in the storage unit 25, and uses them to control the corresponding linear motor 11 so as to reduce or eliminate the position jump. Generating the offset value of the linear motor 11 includes calculating an average position jump value of a first position jump to the preceding linear motor 11 at the segment boundary 15 and a second position jump to the following linear motor 11 at the segment boundary 15, and subtracting the average position jump value from the second position jump. Since the offset value is generated to eliminate or compensate for the gaps at the beginning and end of the corresponding linear motor 11 under consideration, the gaps of the preceding and following linear motors 11 along the track are also automatically compensated or eliminated. Therefore, the offset value can be generated only for each second linear motor 11.

[0127] Alternatively, if the entire track can be considered to have position jumps eliminated as a whole, generating offset values ​​for the linear motors 11 includes calculating an average position jump value from all position jumps determined at the segment boundaries 15, and for each linear motor 11, subtracting the average position jump value from the end position jump value from the corresponding linear motor 11 at the segment boundary 15 to the subsequent linear motor 11.

[0128] The gain value of each linear motor 11 is generated according to the average position jump value and according to the length of the track along the corresponding linear motor 11 .

[0129] refer to Figure 3 , it can be seen that by applying an offset value or deviation value, the starting position P of the "segment i" 11 can be start and end position P end Transform to offset starting position P start,bias and offset end position P end,bias This corresponds to shifting the entire section 11 so that on the one hand P start,bias and P prev The difference betweenend,bias and P next The difference between them is equal in magnitude, where P prev Corresponding to the end position of the previous linear motor 11, P next corresponds to the starting position of the next linear motor 11. In a second step, by applying the gain value, the "length" of the segment 11 along the track can be "stretched" so that after applying the offset value and the gain value, the corrected starting and ending position P of the considered segment start,bias,gain , P end,bias,gain respectively correspond exactly to the end position of the preceding linear motor 11 and the start position of the succeeding linear motor 11. The invention aims to reliably and precisely determine these values ​​in a manner that can be performed without any external measurement system and without having to calibrate the entire linear motor system again when a single linear motor 11 is replaced due to a fault.

[0130] If the position jumps are eliminated globally for the entire track, i.e. all linear motors 11 of the track 13 are taken into account for calculating or determining the offset and gain values ​​in order to achieve a global optimization of the entire track, it is possible to completely eliminate the position jumps, e.g. Figure 4 As shown. Figure 4 As shown, in a first step, an offset value or a gain value is applied so as to reduce the amount of the position jumps and to align the position jumps so that the position jumps reach the same level, i.e., the position jumps are all the same after applying the respective offset values ​​determined for the respective linear motors 11. In a second step, a gain value is applied so that the slope of the position is changed and the position jumps are completely eliminated, i.e., the position signal becomes a constant line corresponding to the actual movement of the transmission element 19.

[0131] Figure 5 and Figure 6 Graphs indicating the benefits of the present invention in terms of position jumps are shown, respectively, when the position jumps are minimized for each second segment, respectively ( Figure 5 ), or when the position jump is distributed to adjacent segments ( Figure 6 ), however, this requires a global consideration of all segments. Figure 5 and Figure 6 Both show the position jump before (left) and after (right) applying an offset value or an offset value.

[0132] like Figure 5 As shown (right side), the position jumps on the segments are changed so that the position jumps of a segment relative to the preceding linear motor and the following linear motor at its segment boundary are averaged and therefore have the same value. Position jumps with the same value can be eliminated by determining and applying the respective gain values ​​in the subsequent steps.

[0133] exist Figure 6In , a global distribution and consideration of all segments has been performed and all segment jumps have been coordinated to have the same value over the entire track.

[0134] In both cases, the amount of position jumps can be significantly reduced.

[0135] Reference numerals list

[0136] 10 Linear Motor System

[0137] 11 Linear Motor

[0138] 13 Track

[0139] 15 Segment boundaries

[0140] 17 Position Sensor

[0141] 19 Transmission Components

[0142] 21 Control unit

[0143] 23 Processing Units

[0144] 25 Storage Units

[0145] 29 position magnet

[0146] 31 Gap

Claims

1. A method for track calibration of a linear motor system (10), in particular a multi-carrier system, the system having at least one conveying element (19), A plurality of linear motors (11) arranged in a row and having a track (13), wherein adjacent linear motors (11) abut each other at a segment boundary (15), and each linear motor (11) is configured to apply a driving force for moving a conveying element (19) along the track (13), and each linear motor comprises at least one position sensor (17) for detecting a position of the conveying element (19) as the conveying element (19) travels along the track (13), and a control unit (21) configured to control the linear motor (11) to apply a driving force to the conveying element (19), the control unit (21) comprising a processing unit (23) and a storage unit (25), The method comprises the following steps: - controlling the linear motor (11) to move the transport element (19) along the track (13) in a calibration operation, - in a calibration run, determining position jumps at segment boundaries (15) of at least some of the linear motors (11) based on position feedback signals from position sensors (17) of the respective linear motor (11) and its neighboring linear motors (11), - generating offset values ​​and / or gain values ​​for at least part of the linear motor (11) according to the determined position jump, - Storing the offset value and / or the gain value in the storage unit (25).

2. The method according to claim 1, It is characterized in that Generating an offset value for a linear motor (11) includes calculating an average position jump value of a first position jump to a preceding linear motor (11) at the segment boundary (15) and a second position jump to a succeeding linear motor (11) at the segment boundary (15), and subtracting the average position jump value from the first position jump or the second position jump.

3. The method according to claim 1, It is characterized in that Generating offset values ​​for the linear motors (11) includes calculating an average position jump value from all position jumps determined at the segment boundary (15), and for each linear motor (11), subtracting the average position jump value from the end position jump value of the corresponding linear motor (11) at the segment boundary (15) to the subsequent linear motor (11).

4. The method according to claim 2 or 3, It is characterized in that A gain value of the linear motor (11) is generated depending on the average position jump value.

5. The method according to claim 4, It is characterized in that A gain value of the linear motor (11) is further generated depending on the length of the linear motor.

6. The method according to any one of the preceding claims, further comprising - Balancing the offset values ​​generated for all linear motors (11) by determining an average offset value from a maximum offset value and a minimum offset value among the offset values ​​and subtracting the average offset value from each offset value.

7. A system for track calibration of a linear motor system (10), in particular a multi-carrier system, the system comprising at least one conveying element (19), A plurality of linear motors (11) are arranged in a row and have a track (13), wherein adjacent linear motors (11) abut each other at a segment boundary (15), and each linear motor (11) is configured to apply a driving force for moving a conveying element (19) along the track (13), and each linear motor includes at least one position sensor (17) for detecting a position of the conveying element (19) as the conveying element (19) travels along the track (13), and A control unit (21) configured to control the linear motor (11) to apply a driving force to the conveying element (19), the control unit (21) comprising a processing unit (23) and a storage unit (25), wherein the control unit (21) is configured to perform the method according to any one of the preceding claims.

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