WO2016159866A1

WO2016159866A1 - Method and device for determining a position of a piston, which is movable in a cylinder

Info

Publication number: WO2016159866A1
Application number: PCT/SE2016/050271
Authority: WO
Inventors: Christophe Mattei; Robert Risberg
Original assignee: Väderstad Holding Ab
Priority date: 2015-04-02
Filing date: 2016-03-31
Publication date: 2016-10-06
Also published as: SE1550399A1; SE538819C2

Abstract

Method of determining a position of a moving piston (3a) in a cylinder (1), said method comprising emitting with the aid of an emitter (6) placed adjacent to the cylinder, at a first point in time (t_i) an ultrasonic signal so that it propagates inside the cylinder and is reflected on the surface of the piston, receiving with aid of a receiver (7) placed adjacent to the cylinder, at a second point in time (t2), a first resulting ultrasonic signal which contains the result of the ultrasonic signal reflected on the piston, obtaining a filtered ultrasonic signal through comparing the first resulting ultrasonic signal and a reference signal, wherein the reference signal is a resulting ultrasonic signal received by a receiver at a reference time point (t_o) and wherein the reference time point is different from the second point in time (t2), and in a calculating unit calculating the position of the piston based on the filtered ultrasonic signal.

Description

METHOD AND DEVICE FOR DETERMINING A POSITION OF A PISTON

WHICH IS MOVABLE IN A CYLINDER

Technical field

This document relates to a method and device for determining the position of a movable piston in a cylinder with the aid of ultrasound.

The method and the device can be used in agricultural implements such as soil-working agricultural implements.

The method and device also have applications In devices which are not agricultural implements or are not soil-working agricultural implements.

Background

When using cylinders In different contexts, it is desirable to determine the situation the cylinder Is in, in other words to determine the position of a moving piston inside the cylinder. Being able to obtain a cylinder piston's exact position may, for example, be desirable when wanting to place a cylinder in a certain position for any reason, for example when a function is to be connected to the piston when it is in a predetermined position. Examples of such functions could be that the dispensing of granules such as seeds, fertilisers or pesticides begins as soon as a raising or lowering operation is carried out, or that the letting in or out of sections of an agricultural implement is carried out in a predetermined order where a later stage is triggered by a predetermined position being reached.

A cylinder usually comprises a tubular casing and a piston with a piston rod and two cylinder ends. In one cylinder end is a hole in which the piston rod moves. Such piston cylinders are used is piston engines, piston compressors, piston pumps and as actuating cylinders in various applications. Examples of actuating cylinders are hydraulic cylinders and pneumatic cylinders.

When determining the position of a cylinder it is desirable that there is as little damage to the cylinder itself as possible. In addition, the position determination should be precise and reliable. The use of ultrasound for position determining in a cylinder has been known for a long time. The principle is based on an ultrasonic pulse being sent out from an emitter, propagating through the cylinder, reflecting on the surface of the cylinder piston and being read by a receiver. Based on the time it takes for the ultrasonic pulse to propagate into the cylinder, the position of the piston in the cylinder is determined as the time is proportional to the length of the area the signal travels through. Examples of position

determination with the help of ultrasound are found, for example, in

WO8501800 and US5463596.

In this field there are a number of drawbacks of the prior art. For each obstacle the emitted ultrasonic signal encounters interference to the signal occurs. This occurs for example when an ultrasonic pulse passes through a wall. To counteract such interference one or more openings have been made in the cylinder's outer casing and the emitter(s) connected to the openings so that the ultrasonic signals can propagate in the cylinder without disruption. However, the effect on the cylinder is great as the casing has to be adapted which is laborious and time-consuming. Attempts have also been made to place one on more emitters on the outside of the cylinder which results in interference to the signal when it propagates through the cylinder's casing.

Interference and noise also occur inside the cylinder, for example due to unevenness, vibration and suchlike. This means that a receiver of the ultrasonic signals receives signals containing noise and interference which makes it very difficult to determine which part of the signal really is the part reflected by the piston. The signal may have been reflected on other surfaces or objects inside the cylinder, causing interference to the signal and its dispersion.

In hydraulic cylinders where hydraulic fluid fills the entire or parts of the cylinder the signal is also affected by the medium it travels through.

Temperature changes in the hydraulic fluid cause the propagation speed of the signal to change and it is therefore difficult to determine the position of the piston as the propagation time at one temperature and at a certain position is not the same as at another temperature. This also applies to pneumatic cylinders where temperature and pressure changes can cause alternations to the ultrasonic signal's propagation properties.

Position determination with ultrasound in accordance with the previously known methods is therefore inexact and in principle unusable due to the interference that occurs on the propagated ultrasonic signals if the ultrasound emitter is placed on the cylinder. If, instead, the ultrasound emitter is to be placed inside the cylinder, adaptation of the cylinder is required which is laborious and time-consuming and also costly. The aim of the present application is to overcome the problems that are associated with the prior art as has been set out above. It is desirable to achieve precise, reliable position measurement with as little effect on existing cylinders as possible.

Summary

One aim of this document is therefore to bring about an improved method of determining a position of a moving piston in a cylinder and, in particular, one which fully or partially meets the requirements set out in the introduction.

The invention is defined by the attached independent claim. Forms of embodiment are evident from the dependent claims, the following description and the drawings.

Brought about in accordance with a first aspect is a method of determining a position of a moving piston in a cylinder, said method comprising emitting with the aid of an emitter placed adjacent to the cylinder, an ultrasonic signal at a first point in time so that it propagates inside the cylinder and is reflected on the surface of the piston, receiving with aid of a receiver placed adjacent to the cylinder, a first resulting ultrasonic signal at a second point in time which contains the result of the ultrasonic signal reflected on the piston, obtaining a filtered ultrasonic signal through comparing the first resulting ultrasonic signal and a reference signal, wherein the reference signal is a resulting ultrasonic signal received by a receiver at a reference time point and wherein the reference time point is different from the second point in time, and in a calculating unit calculating the position of the piston based on the filtered ultrasonic signal.

As the resulting ultrasonic signal is filtered by way of comparison with the reference signal which is obtained in the same way, the signal interference can be effectively filtered out. In this manner the filtered ultrasonic signal can then be used to calculate an exact value of the piston's position.

The comparison can involve subtracting the reference signal from the resulting ultrasonic signal and thereby obtaining the filtered ultrasonic signal as a difference between the first resulting ultrasonic signal and the reference signal.

Alternatively the reference signal can be processed before being used for subtracting so that the signal which is subtracted is a first signal derived from the reference signal.

As a further alternative, or addition, the resulting ultrasonic signal can be processed before subtraction takes place so that the signal subject to subtraction is a derived second signal.

In the event that the piston has exactly the same position in the resulting ultrasonic signal as in the reference signal, the reflection from the piston is also filtered out. But if the piston has moved between the resulting ultrasonic signal and the reference signal, the reflection from the piston will appear as a clear difference between the resulting ultrasonic signal and the reference signal.

The difference value between the resulting ultrasonic signal and the reference signal can be calculated at several, preferably every, points in time as of the first point in time up to and including the second point in time. Each difference value between the first point in time up to and including the second point in time can be compared with a previously defined threshold value wherein a difference value can be seen as representing a reflection of the ultrasonic signal on the piston only if it exhibits a predetermined relation to the threshold value. A propagation time for the resulting signal can be given as the time from the first point in time up to and including the point in time for a difference value of a predetermined order which shows the predetermined relation to the threshold value.

The difference then contains a clear picture of the reflection from the piston as noise has been reduced through eliminating the interference present both at the time of the reference signal and the time of reception of the resulting signal.

The calculation of the position of the piston can involve determining the distance between the emitter and the piston surface based on the resulting signal's propagation time and its propagation speed. As the propagation speed varies between different media, the signal's propagation speed can vary during the propagation time, which can thereby be taken into account when determining the distance.

The calculating unit can determine the propagation speed based on a measured temperature inside or adjacent to the cylinder or based on a previously defined expected cylinder temperature distribution.

The reference time point can be within 1 second from the second time point t₂, preferably within 10 ms, 5 ms, 2 ms or 1 ms from the second time point. As the times of reception of the reference signal and the resulting signal are very close to each other the surrounding conditions are very alike. This provides a good precondition for reducing noise and calculating an exact a position as possible.

The ultrasonic signal is emitted into the cylinder at an angle greater than 30°, preferably greater than 45°, greater than 75°, greater than 85° or around 90° relative to a direction parallel to the surface of the piston against which the signal is reflected. The signal should preferably be emitted into the cylinder in a direction as close to perpendicular to the cylinder piston's surface as possible. In this way interference caused by the signal being reflected and dispersed inside the cylinder is minimised. At least one of the ultrasonic signal and the resulting ultrasonic signal can propagate through a fluid, such as a gas or a liquid during a part of the time between the first and the second points in time.

At least one of the ultrasonic signal and the resulting ultrasonic signal can propagate through a material, which forms part of a wall which delimits a space in which the piston moves, during a part of the time between the first and the second points in time.

Ultrasonic signals can be generated by the emitter through a piezoelectric component, such as a ceramic disk, being brought into an active state in which ultrasound waves are created, and ultrasonic signals are recorded by the receiver through the piezoelectric component, which has been brought into a passive state, being activated by incoming ultrasound waves. The fact that the emitter and receiver are at least partially made of the same components makes assembly simpler and the emitter and receiver take up less room.

The method of determining a position of a moving piston in a cylinder can be used as a method of determining a position of a moving piston in a cylinder in an agricultural implement such as a soil-working agricultural implement.

Alternatively the method can be used as a method of determining a position of a moving piston in a cylinder in apparatus which is not an agricultural implement or is not as a soil-working agricultural implement.

Brought about in accordance with a second aspect is a device for determining a position of a moving piston in a cylinder, wherein the device comprises: emitting with the aid of an emitter placed adjacent to the cylinder, at a first point in time an ultrasonic signal so that it propagates inside the cylinder and is reflected on the surface of the piston, receiving with the aid of a receiver placed adjacent to the cylinder, at a second point in time, a first resulting ultrasonic signal which contains the result of the ultrasonic signal reflected on the piston, obtaining a filtered ultrasonic signal through comparing the first resulting ultrasonic signal and a reference signal, wherein the reference signal is a resulting ultrasonic signal received by a receiver at a reference time point and wherein the reference time point is different from the second point in time, and in a calculating unit calculating the position of the piston based on the filtered ultrasonic signal.

At least a part of the cylinder can contain a fluid, such as a gas or a liquid, through which at least one of the ultrasonic signal and the resulting ultrasonic signal propagates during a part of the time between the first and the second points in time.

The device can comprises a material which forms part of a wall which delimits a space in which the piston moves, through which at least one of the ultrasonic signal and the resulting ultrasonic signal propagates during a part of the time between the first and the second points in time. The wall can face the surface of the piston and have a normal which has an angle of greater than 30°, preferably greater than 45°, greater than 75°, greater than 85° or around 90° relative to the surface of the piston.

The ultrasound emitter and receiver can be arranged in a joint casing and/or have joint control electronics and/or a joint power supply. They can also be made at least in parts of the same components, that is to say components which in the active state generate ultrasonic signals and in the passive state record ultrasonic signals.

In accordance with a third aspect an agricultural implement is brought about comprising a device for determining a position of a moving piston in a cylinder on the agricultural implement. Preferably such an agricultural implement can be a soil-working agricultural implement such as a plough, a harrow, a cultivator, a distributing machine for pesticides or fertiliser, a sowing machine and/or a precision sowing machine.

In accordance with a fourth aspect an apparatus, which is not an agricultural implement or which is not a soil-working agricultural implement, is brought about wherein the apparatus comprises a device for determining a position of a moving piston in a cylinder on the apparatus.

Brief description of the drawings

Fig. 1 shows a schematic view of a piston cylinder in cross-section Fig. 2 shows an example of an ultrasound signal received in a receiver. Fig. 3 shows an example of a filtered version of the ultrasound signal received in Fig. 2.

Fig. 4 shows an example of an ultrasound signal received in a receiver. Fig. 5 shows an example of a filtered version of the ultrasound signal received in Fig. 4.

Detailed description

Fig. 1 shows a cylinder 1 comprising a case 2 and a movable piston 3a with a piston rod 3b and two cylinder ends 5. The case 2 encloses a chamber 2 in which the piston 3a moves. At the end of the chamber, seen in a direction parallel to the cylinder's direction of movement there is a hole in which the piston rod 3b moves. An ultrasound emitter 6 and ultrasound receiver 7 are connected to the cylinder 1. "Connected to" is taken to mean that the emitter and/or the receiver is/are placed on the cylinder, either directly on the cylinder case, in a recess in the cylinder case or via an adapter.

In the case of placing in a recess it can, but does not have to be a through recess. Preferably the emitter 6 and/or the receiver 7 is/are placed on the cylinder, either directly on the cylinder case, in a non-through recess in the cylinder case or via an adapter. Such an adapter can be designed to ensure good coupling of the signals to/from the emitter/receiver. In this way no damage is done to the cylinder which makes for a simple application which is also cost effective and can be used for many different types and models of cylinder.

The emitter 6 and receiver 7 can be separate units or can be integrated into one and the same unit. They can therefore be arranged in a joint casing and/or have joint control electronics and/or a joint power supply. The control electronics can, for example, comprise a processor unit, a control unit, amplifier, signal processing unit etc. The emitter and receiver can also include at least in parts the same components, that is to say components which in the active state generate ultrasonic signals and in the passive state record ultrasonic signals. A piezoelectric component such as an external disk can through being actively made to vibrate produce ultrasound waves. When the piezoelectric component is passive it can be made to vibrate by incoming ultrasound waves and thus record incoming signals. It is therefore possible to use conventional ultrasonic emitters for both producing and recording ultrasound waves.

The emitter 6 can be positioned so that an ultrasonic signs is emitted into the cylinder at an angle greater than 30°, preferably greater than 45°, greater than 75°, greater than 85° or around 90° relative to a direction parallel to the surface of the piston the position of which is intended to be determined. By emitting signals into the cylinder at an angle as close to 90° as possible, signal interference occurring as a result of reflection, divergence etc. is kept to a minimum. For practical and design reasons it may be the case that the emitter cannot be positioned to generate signals at exactly 90°, but positioned so that the signal is emitted at an angle differing from 90°.

The cylinder 1 can comprise one or more walls and/or parts of walls.

These walls/parts of walls can for example be positioned so that they define the space in which the piston 3a moves. Such a wall can face the piston surface 3c, for example through a wall or a wall part the normal of which has an angle of greater than 30°, preferably greater than 45°, greater than 75°, greater than 85° or around 90° relative to the surface 3c of the piston.

A calculating unit for calculating the piston position in the cylinder can be placed in direct connection with the emitter and/or the receiver or placed separately from the emitter and/or the receiver. The calculating unit can communicate with the emitter and the receiver either in a wired or wireless manner depending on the circumstances.

The cylinder 1 can for example be an actuator, such as a hydraulic cylinder or a pneumatic cylinder, that is to say a cylinder which partially contains a fluid, such as a liquid or gas. The conditions in such cylinders are proportionally very much affected by changes in temperature and pressure which can occur during use of the cylinder as a result of the fact that they contain a fluid or gas. Properties, such as the propagation speed, of ultrasonic signals which are sent through a medium such as a fluid or gas are thus changed considerably when properties of the medium change. The speed of sound depends on the temperature, which means that changes in temperature considerably affect the precision and accuracy of position measurements in such cylinders. The change in speed can be or the order of +/-0.1-0.2 %/K, which is a significant temperature drift.

Cylinder 1 can also be spring cylinder, such as a pneumatic spring. As a further alternative the cylinder 1 can be a damper cylinder or a measuring cylinder for measuring a linear position.

The position determination of a moving piston in a cylinder will be described below with reference to Figs. 2-5.

The emitter 6 emits, at a first point in time ti an ultrasonic signal so that it propagates inside the cylinder 1 and is reflected on the surface of the piston 3c, the position of which is to be determined. The receiver 7, at a second point in time t₂, receives a resulting ultrasonic signal which is the result of the ultrasonic signal reflected on the piston. Other things can also have happened to the signal between the first and the second point in time. It may, for example, have been reflected on other surfaces in the cylinder, propagated through different media and through objects such as walls etc. In this way a resulting ultrasonic signal is generated with various interference, noise and reflection echoes from diverse objects. The signal is recorded by the receiver 7 and forwarded to the calculating unit. An example of a resulting signal is shown in Fig. 2. A number of reflections are seen in this signal but from this representation it cannot be determined which of these reflections originates from the piston surface 3c.

The sampling frequency can lie between 10 and 00 MHz, preferably

25 - 100 MHz which means that time sampling for ultrasonic signals is preferably carried out at the tenths and hundredths of a microsecond level, i.e. there is preferably 0.01 - 0.1 ps between two samples, though time sampling with both longer and shorter intervals can be used depending on the circumstances. The repetition frequency, i.e. the frequency at which an ultrasonic signal is emitted and received is preferably around 500 Hz, but can be both greater or lesser than this.

To find out what in the resulting signal is actually the reflection from the piston surface, the resulting signal is compared with a reference signal in the calculating unit. The reference signal is a signal received in a receiver at a reference time to which falls either before or after the time of receiving the resulting signal t₂. The shortest possible time between the reference time to and the time of receiving the resulting signal t₂ depends on the repetition frequency which thereby affects the maximum resolution for position determination.

By comparing the resulting signal with the reference signal a filtered ultrasonic signal is brought about which can be used for calculating the position of the piston. A comparison between the resulting signal and the reference signal can, for example, be carried out by subtracting the value of the reference signal at each point in time from the value of the resulting signal at the corresponding point in time. Comparison between the resulting signal and the reference signal can preferably be carried out every time a value for the signals is generated, or can also be carried out less often. In order to facilitate the determination of the propagation time the envelope of

discrepancy can be calculated. An example of a filtered signal, as

represented by the envelope of discrepancy at point in time do, d₀₊i, do+2- . do+n, between the two signals is shown in Fig. 3. Here it can be seen how interference and noise are reduced and on the basis of the filtered signal it can therefore be determined when reflection on the piston surface 3c has occurred.

Further examples of a resulting signal and the filtered signal are shown in Figs. 4 and 5

It is advantageous to use a reference signal recorded at the receiver 7 as closely as possible to the resulting signal. The reference time point to can be within 1 second from the second time point t₂, preferably within 0 ms, 5 ms, 2 ms or 1 ms from the second time point t₂. As the times of reception of the reference signal, to and the resulting signal t₂ are very close to each other the surrounding conditions are very alike. This provides a good precondition for reducing noise and calculating as exact a position as possible. When the temperature and/or the pressure around or in a cylinder changes the propagation speed of an ultrasonic signal is affected considerably, especially if the cylinder is hydraulic or pneumatic. Through filtering based on a recently received signal or a closely following signal the conditions for propagation of the signals are largely unchanged. It is also probable that signals following closely one behind the other are reflected at the same point in the cylinder and are affected in a similar way when they propagate through, for example, a wall or a part of a wall in the cylinder. However the signals should be separated in terms of time so that a difference in the piston's position has occurred for there to be a possibility of detecting the piston's position.

On the basis of the filtered signal the propagation time t_p from when the signal is sent from the emitter, ti until it is reflected on the piston surface can be determined. This can be done, for example, by using a predefined threshold value t_tr. A difference value can be seen as representing a reflection of the ultrasonic signal on the piston only if it exhibits a predetermined relation to the threshold value t_tr. For example, the difference value can be seen as being a reflection from the piston if it exceeds the threshold value t_tr. In this way small difference values lying close to zero and probably not representing a reflection can be disregarded. The predetermined relation should also be able to mean that the difference value lies within a certain interval in relation to the threshold value t_tr, but there can also be other possibilities depending on the prevailing circumstances.

The propagation time tp starts when the signal is sent from the emitter, i.e. at point in time t|. It preferably ends when the signal is reflected against the surface of the piston, which, as described above, is shown in that a difference value exhibits a predetermined relation to a threshold value t_tr. The last point in time of the propagation time, i.e. the time that stops the propagation time, can be the time of a difference value of a predetermined order exhibiting the predetermined relation to the threshold value. A difference value of a predetermined order can, for example, be the first difference value that exhibits the predetermined relation. This means that the first difference value that exceeds a threshold value is seen to represent reflection on the piston surface. It can also be the n^,h difference value in a number of successive difference values which all exhibit the predetermined relation, when n is a positive whole number. In this way it can be assured that a reflection on the piston surface has taken place in that a number of consecutive difference values indicated this and that it not just one single difference value that fulfils the predetermined relation. In this way error sources are avoided which may cause an individual erroneous value.

As the propagation time has been calculated the distance s the signal has travelled, which represents the position of the piston 3a in the cylinder 1 , can be obtained based on the propagation time t_p and the propagation speed v_p of the signal. The propagation speed of a signal propagating in the cylinder can vary between different media such the cylinder wall, air, fluid etc. The propagation speed can be predefined for different types of media in the cylinder and/or determined in real time. For example, this can be done through two surfaces on the piston generating reflections where the distance between these surfaces is known. Alternatively calibration can be carried out through measuring propagation times at the piston's end positions and utilising a ratio between these, which, for example, is linear. Further methods of determining the propagation speed may be possible depending on the type of cylinder, the design of the cylinder and/or other influencing parameters.

Something which affects the propagation speed of an ultrasonic signal is the temperature of the medium through which the signal is propagating. The calculating unit can therefore determine the propagation speed based on a measured temperature (T) inside or adjacent to the cylinder or based on a previously defined expected cylinder temperature distribution. As the propagation time of the signal is proportional to the distance between the emitter and the piston, this distance can be calculated through the

propagation speed of the signal being known. However, the propagation speed is dependent on which medium the signal travels through and the calculations can be adapted to accordingly. As stated, the speed also depends of the temperature in the medium in which the signal is propagating. The temperatures on, or in the vicinity of the cylinder may therefore have to be measured while the cylinder is in operation. Alternatively an expected temperature distribution can be used, which is based, for example, on previous tests of the cylinder while in operation.

All the calculations described above are preferably carried out by a calculating unit in real time or as close to real time as possible, but can also be take place afterwards. The result of the position calculation can be forwarded from the calculating unit to other units. For example, the piston positions can be sent as input data to a control and/or regulating device, for displaying on a screen, for storage on a server or to a mobile unit such as a mobile telephone, computer etc. In the same way, other data can be forwarded from the calculating unit.

The calculating unit can be connected to just one emitter and receiver, or, alternatively one and the same calculating unit can be used for receiving data from several emitters/receivers.

Determination of the position of a moving piston in cylinder as described above can be advantageously used in agricultural implements, such as soil-working agricultural implements for example. Through the position of one or more cylinders in the agricultural implement being generated, it is possible, for example, to choose to place a cylinder in a given position before further measures are taken, such as starting a lowering sequence for parts in the implement. The term soil-working agricultural implements refers to, for example, ploughs, harrows, cultivators, sowing machines and precision sowing machines.

The environment around the agricultural implement and the way in which its tools are used are tough and the component parts of the implement should be wear-resistant and robust. Furthermore, external conditions such as the temperature can change considerably during use. Particularly important in such implements are durable, reliable position measurements which not require existing parts such as cylinders to be adjusted before position determination can take place.

In an agricultural implement position measurement can be carried out in one, several or all of the cylinders. The results of the position determination can be sent from one or more calculating units to a central unit on the agricultural implement. Also conceivable is to connect one single calculating unit to all the emitters/receiver units in the agricultural implement thus functioning as a central unit. From a central unit the information can then be forwarded, for example to a display for presentation of the information to a user of the agricultural implement. In this way an operator of the agricultural implement receives information about the positions of the piston cylinders in the implement in real time, or very close to real time. Alternatively further calculations can be carried before the information is presented to the user.

Measures consequent to the received information about cylinder positions can also be carried out automatically, i.e. without the operator giving any instructions. Control signals can be generated based on position information and sent to different parts of the agricultural implement. In this way, measures such as lowering sequences, start/stop of measurements etc. can be carried automatically or at the request of a user.

It is clear that there are very many devices which are not agricultural implements. Cited as examples can be devices for setting a relative position between at least two components, devices for measuring a relative position between two components, pressing devices, damping devices, cushioning devices etc. Devices which are agricultural implement but which are not soil- working agricultural implement can be, for example, reaping machines, machines for distributing pesticides and/or fertiliser, traction vehicles such a tractors, transporting devices such as trailers or waggons as well as lifting devices such as scoops, pallet trucks, bale lifters.

Claims

1. Method of determining a position of a moving piston (3a) in a cylinder (1), said method comprising:

emitting with the aid of an emitter (6) placed adjacent to the cylinder, at a first point in time (ti) an ultrasonic signal so that it propagates inside the cylinder and is reflected on the surface of the piston (3c),

receiving with aid of a receiver (7) adjacent to the cylinder, at a second point in time (t₂), a first resulting ultrasonic signal which contains the result of the ultrasonic signal reflected on the piston (3a),

obtaining a filtered ultrasonic signal through comparing the first resulting ultrasonic signal and a reference signal,

wherein the reference signal is a resulting ultrasonic signal received by a receiver at a reference time point (t₀) and wherein the reference time point is different from the second point in time (t₂), and

a calculating unit calculating the position of the piston based on the filtered ultrasonic signal.

2. Method according to claim 1 , wherein the comparison involves subtracting the reference signal or a first signal derived therefrom from the resulting ultrasonic signal or a second signal derived therefrom and thereby obtaining the filtered ultrasonic signal as a difference between the first resulting ultrasonic signal and the reference signal.

3. Method according to claim 2 wherein difference values (d₀, d₀₊i , do+2...do+n) between the resulting ultrasonic signal and the reference signal are calculated at several, preferably every, points in time as of the first point in time (ti) up to and including the second point in time (t₂).

4. Method according to claim 3, also involving each difference value (d₀, do+i, do+2 - do+n) between the first point in time (ti) up to and including the second point in time (t₂) being compared with a previously defined threshold value (t_tr), wherein a difference value can be seen as representing a reflection of the ultrasonic signal on the piston (3a) only if it exhibits a predetermined relation to the threshold value.

5. Method according to claim 4 wherein a propagation time (tp) for the resulting signal is the time as of the first point in time (t-i) up to and including the point in time for a difference value of a predetermined order which shows the predetermined relation to the threshold value (t_tr).

6. Method according to claim 5 wherein the calculation of the position of the piston (3a) involves determining the distance (s) between the emitter (6) and surface of the piston (3c) based on the resulting signal's propagation time (t_p) and its propagation speed (v_p).

7. Method according to claim 6 wherein the calculating unit determines the propagation speed (v_p) based on a measured temperature (T) inside or adjacent to the cylinder (1 ) or based on a previously defined expected temperature distribution for the cylinder.

8. Method according to any one of the preceding claims wherein the reference time (to) is within 1 second of the second point in time (t₂), preferably within 10 ms, 5 ms, 2 ms or 1 ms of the second point in time (t₂).

9. Method according to any one of the preceding claims wherein the ultrasonic signal is emitted into the cylinder (1 ) at an angle greater than 30°, preferably greater than 45°, greater than 75°, greater than 85° or around 90° relative to a direction parallel to the surface of the piston against which the signal is reflected.

10. Method according to any one of the preceding claims wherein at least one of the ultrasonic signal and the resulting ultrasonic signal propagates through a fluid, such as a gas or a liquid during a part of the time between the first and the second points in time (ti, t₂).

11. Method according to any one of the preceding claims wherein at least one of the ultrasonic signal and the resulting ultrasonic signal propagate through a material, which forms part of a wall which delimits a space in which the piston moves, during a part of the time between the first and the second points in time (ti, t₂).

12. Method according to any one of the preceding claims wherein ultrasonic signals are generated by the emitter (6) through a piezoelectric component being brought into an active state in which ultrasound waves are created, and ultrasonic signals are recorded by the receiver (7) through the piezoelectric component, which has been brought into a passive state, being activated by incoming ultrasound waves.

13. Method of determining a position of a moving piston (3a) in a cylinder (1 ) in an agricultural implement, comprising a method according to any one of previous claims.

14. Method according to claim 13 wherein the agricultural implement is a soil-working agricultural implement.

15. Method of determining a position of a moving piston (3a) in a cylinder (1 ) in a device which is not an agricultural implement, comprising a method according to any one of claims 1-12.

16. Device for determining a position of a moving piston (3a) in a cylinder (1 ), wherein the device comprises:

an emitter (6) placed adjacent to the cylinder, arranged at a first point in time (ti) to send an ultrasonic signal so that it propagates inside the cylinder and is reflected on the surface of the piston (3c), a receiver (7) placed adjacent to the cylinder, arranged at a second point in time (t₂), to receive a first resulting ultrasonic signal which contains the result of the ultrasonic signal reflected on the piston (3a),

a calculating unit set up to bring about a filtered ultrasonic signal through comparing the first resulting ultrasonic signal and a reference signal, wherein the reference signal is a resulting ultrasonic signal received by a receiver at a reference time point (t₀) and wherein the reference time point is different from the second point in time (t₂), and

to calculate the position of the piston (3a) based on the filtered ultrasonic signal.

17. Device according to claim 16, wherein a part of the cylinder (1 ) contains a fluid, such as a gas or a liquid, through which at least one of the ultrasonic signal and the resulting ultrasonic signal propagates during a part of the time between the first and the second points in time (ti, t₂).

18. Device according to claim 16 or 17, wherein the device comprises a material which forms part of a wall which delimits a space in which the piston moves, through which at least one of the ultrasonic signal and the resulting ultrasonic signal propagates during a part of the time between the first and the second points in time (ti, t₂).

19. Device according to claim 18 wherein the wall faces the surface of the piston (3c) and has a normal which has an angle of greater than 30°, preferably greater than 45°, greater than 75°, greater than 85° or around 90° relative to the surface of the piston.

20. Device according to any one of claims 16-19, wherein the ultrasound emitter (6) and receiver (7) are arranged in a common casing and/or have joint control electronics and/or a joint power supply.

21. Device according to any one of claims 16-20 wherein the emitter (6) and the receiver (7) can also consist, at least in parts, of the same

components, that is to say components which in the active state generate ultrasonic signals and in the passive state record ultrasonic signals.

22. Agricultural implement preferably for soil working, comprising at least one device according to any one of claims 16-21 for determining a position of a moving piston (3a) in a cylinder (1 ) on the agricultural implement.

23. Apparatus comprising at least one device according to any one of claims 16-21 for determining a position of a moving piston (3a) in a cylinder (1 ) on the apparatus, wherein the apparatus is not an agricultural implement.