In previously known technology the driving motors for the travelling movement of the conveyor, and also the driving motors for the carrier device on the conveyor frame are stopped in different stop positions by the action of mechanically mounted limit positions.

The limit positions communicate with contactors, for instance, which activate the driving motors. When the contactors are in ON position the driving motors run at full speed and are stopped when the contactors. influence by the limit positions. are caused to assume OFF position. However, the travelling movement and/or carrier device movement is stopped with a certain time delay which means that precision of the stop in the path of travel or of the carrier device on the conveyor is poor. The stop sometimes occurs"too early"in the actual movement and sometimes"too late". The location of the limit position in the path of travel and/or the limit positions of the carrier device must then be mechanically adjusted so that the stop occurs at the intended location. Adjusting the limit positions in this manner is impractical and time-consuming.

It is often desirable for the conveyor to be stopped at several points along its path of travel and/or for the carrier device to be stopped at several points in relation to the conveyor frame. According to known technology this can be realised by permitting the travelling movement or carrier device movement to continue for a specified time and then be stopped, which is hardlv an exact method. Alternatively, several stop locations are determined by means of limit positions arranged at these locations. The problems encountered with such an arrangement are especially difficult as regards the actual travelling movement. Limit position indicators must be used to provide a number of "preparatory"limit positions before a stop position, to indicate that the speed shall be reduced and the driving motor must be stopped some way before the actual stop position, since the conveyor will continue its movement due to inertia. The load on the conveyor is also of significance here. The conveyor must often be pulled to the actual stop position with the aid of hydraulic devices after the driving motor has been stopped.

Another drawback is that the situation is different when the conveyor is travelling in the opposite direction. The result is that a very large number of limit positions is required.

For five stop positions, for instance. thirteen limit positions are required, including preparatory limit positions.

With current technology it is thus difficult to set correct stop positions for the conveyor movements since these are dependent on the performance of the driving motor, cycle time of the control program, friction on rails and the total weight of the conveyor, which is also dependent on the size of the load.

Great precision ot the conveyor movement is required for many applications, e. g. baling paper pulp. Several bales must often be placed on the carrier device device one at a time. It is then of great importance that the distribution of or the spacing between the bales is always substantially the same. The bales must also be well centred on the convevor during transfer from one conveyor to the next. The greater the precision in this respect. which is thus determined by the carrier device being stopped at the correct positions, the simpler will be subsequent handling with machines, trucks and the like.

The object of the present invention is to eliminate the above-discussed drawbacks currently existing in known conveyors.

This object is achieved with a conveyor of the type described in the introductory portion, having the characterizing features defined in claim 1.

With the conveyor in accordance with the invention, the travelling movement is determined from a zero or reference position and sensors are arranged to contactless detect the position of the conveyor on the track of travel by counting pulses, one pulse corresponding to a certain distance. The direction of movement of the driving motor is also known.

According to advantageous embodiments of the invention the position of the conveyor can be detected by measuring the rotation of the driving motor or the wheels. The position can also be measured with the aid of points along the track of travel of the conveyor, in the form of openinus spaced evenly along a strip, which the sensor detects.

Alternatively the strip may be provided with light and dark lines or bands which are detected one after another by a sensor on the conveyor or carrier device in order to sense the movement in this way. The above-mentioned technology, with fixed points. lines or the like along the path of movement offers a very reliable way of determining the position of the conveyor since the actual position is detected, as opposed to the case where the position is determined from the rotation of the driving motor or wheels. If the wheels were to slip in the latter case, for instance, determination of the position would be incorrect.

The position of the carrier device can be detected by measuring the rotation of the driving motor or of the gear wheels. Only one limit position is thus required at each end of the carrier device in order to obtain starting points to determine the relative position of the carrier device after the load has influenced the limit position.

The conveyor in accordance with the invention offers not only functional improvements but also advantages from the manufacturing and installation aspects. With the conveyor in accordance with the invention the stop positions are determined by means of software instead of physically as with the previously known technology. This means that all conveyors can be made the same, thereby facilitating standardisation and thus manufacture. Installation and starting up the conveyor movements can be performed more quickly since no physical stop positions need be moved and adjusted, and the load on the conveyor carrier device can be taken into consideration when controlling the movements of the conveyor.

The pulse transducer technology in accordance with the invention is also well suited for use at so-called frequency operation of the driving motors. At frequency operation the speed of the driving motor is controlled by means of pulses having varying on and off times. The motor speed can in this way be optionally controlled from standstill up to full speed and smooth starting and stopping movements can be achieved so the load is not displaced. Higher speeds can be used for the movements of the conveyor and the speed can be gradually reduced so that the desired stop position is reached exactly. The capacity of the plant is increased in relation to known technology if conveyors comprising pulse transducer technology are used in a baling line. for instance.

According to an advantageous embodiment of the conveyor in accordance with the invention the sensors comprise inductive pulse transducers arranged to detect inductive changes when the wheels or driving motors are rotating. These pulse transducers are extremely reliable and especially well suited for industrial applications, and since detection occurs contactless. there is no wear or abrasion. The pulse transducers are accurate and are well protected from vibrations, damp and dirt and can therefore advantageously be used in tough environments.

The invention also relates to a plant for baling paper pulp as defined in claim 11.

To further explain the invention. embodiments of the conveyor in accordance with the invention. selected by way of example, will now be described with reference to the accompanving drawings. in which

Figures 1-3 show three different examples of conveyors in accordance with the invention. and <BR> <BR> <BR> <BR> <BR> <BR> Figure 4 illustrates the principle of a sensor suitable for use with the conveyors in accordance with the invention.

The examples of embodiments of the conveyor in accordance with the invention shown in Figures 1-3 are primarily intended for use in a plant for baling paper pulp.

Figure 1 shows a stationary chain conveyor with a carrier device in the form of two conveyor chains 32.34. The chains 32.34 are supported by gear wheels attached to the conveyor frame 8 and are driven by a driving motor 6. Spring limit position indicators 10,12 are arranged at the ends of the conveyor. When a load is passed onto one end of the conveyor it affects the limit position at this end and this is then used as zero or reference position.

Sensors. suitably inductive pulse transducers of a type shown in Figure 4. are arranged to detect the rotation of the gear wheels for establishing the rotation of the conveyor chains 32.34 in relation to the zero or reference position. The zero or reference position on the conveyor in question, where the load at present is on the chain, is used as point of departure and emitted from this zero or reference position the number of pulses is then counted in order to obtain the actual position.

The limit position where the load leaves the conveyor is used as a safety device so that the load does not fall off the conveyor.

The driving motor 6 for the conveyor chains 32.34 is connected, via gear wheels and separate drive chains (not shown in the figure), to a shaft common to both chains 32,34 so that they are all the time driven at the same speed. A separate tensioning device is preferably arranged for each chain to ensure correct individual tension in each chain.

Figure 2 shows a travelling conveyor, carried on wheels enabling it to perform a travelling movement along a predetermined track. The track may suitably be formed by rails 14.16. A drive motor 18 is arranged to drive at least one of the wheels supporting this travelling conveyor in order to propel it along its track 14,16.

Sensors of the type shown in Figure 4 are preferably arranged inside the gearbox of the driving motor 18, so that the position of the conveyor on the track of travel, starting from a zero or reference position, can easily be determined by counting the pulses from the sensor. The sensors may, however, be arranged in any movable part of the motor.

The direction of travel of the conveyor and/or the direction of movement of the carrier device in relation to the conveyor frame can be determined through knowledge of the direction of rotation of the driving motor, or, alternatively, by two phase-shifted pulse trains.

The travelling conveyor in Figure 2 is otherwise a chain conveyor of a type similar to that shown in Figure 1.

Figure 3 shows another example of a conveyor in accordance with the invention. This conveyor is of turning or swivelling type, i. e. similar to the travelling conveyor in Figure 2 except that the track is circular and the movement therefore comprises a turning or swivelling movement. In the same way as in Figure 2 the conveyor is support by wheels on a rail 26 which, in this case, is curved in a circle. The driving motors 6 and 18 are normally asynchronous electric motors, connected to the wheels via toothed transmission gears, an inductive pulse indicator being arranged on the driving motor shaft.

All driving motors in the conveyors described above are reversible for switching the direction of movement for both the travelling movement and the movement of the chains.

The examples of conveyors shown in Figures 1-3 can of course be modified and varied in many ways. Thus, the width of the conveyor and number of chains may be increased, for instance, and the length of the conveyor may be varied. This offers great flexibility in constructing various conveyor systems.

In order to detect the rotation of the driving motors 6,18 sensors are used, for instance, of the type described in the brochure"ifm inductive proximity switches", ifm Catalogue 1997/1998, which emit pulse trains depending on the rotation of the motor. Figure 4 shows the principle structure of such a sensor in the form of an inductive pulse indicator comprising a ferrite core 1 with an inductive coil 2. The core 1 and the coil 2 are enclosed in a casing 3. and the electromagnetic field around the sensor is illustrated at 4.

The coil 2 and ferrite core 1 form the inductive part of a tuned LC circuit that drives an oscillator. The coil 2 and core 1 generate a low-energy electromagnetic field 4 on the sensor side of the pulse transducer. When an electric conductor. such as a metal object. enters this field eddy currents are generated in the conductor. These eddy currents draw energy from the field 4 and when the eddy currents become so large that amplifiers pertaining to the pulse transducer are unable deliver sufficient energy, the oscillator stops and the field collapses. In this way"no object in the active zone of the sensor"or

presence of object in the active zone"can be detected in-the form of oscillations with large amplitude or oscillations with small amplitude, and corresponding pulse trains are delivered.

Thus. the movement of a metal part, e. g. a tooth on a rotating gear wheel, can be detected as it moves through the active zone of the sensor. Since the detection is contactless. no mechanical wear occurs and the sensor can be protected from damp and dirt inside a casing. Switching between high amplitude and low amplitude may be effected with high frequency and detection is therefore accurate.

Other types of sensors can also be used in the conveyor in accordance with the invention, e. g. sensors in the form of capacitive pulse transducers, as well as sensors comprising Hall elements, in order to detect the rotation of the wheel or driving motor in question, and deliver a corresponding pulse train. The sensor may also be of optical type, i. e. comprise a light source which emits a light ray towards a receiver, the light source and the receiver being arranged so that the light ray is interrupted a predetermined number of times per revolution of the wheel or driving motor in question, e. g. as a result of the passage of teeth in a rotating gear wheel. A corresponding output pulse train is then obtained from the receiver. Other types of optical sensors, emitting light and detecting reflected light from dark and light bands, oriented perpendicular to the track of movement, e. g. on a strip along the track, can be used. The sensors may also be in the form of incremental transducers arranged to be stepped forward by the pulses a predetermined number of steps per revolution of the wheel or driving motor in question.

These transducers are often designed with inductive or optical sensors.

In the embodiments described above by way of example the pulse transducers are arranged in the drive motor or at gear wheels on the output shaft of the driving motor.

Naturally the pulse transducers may also be arranged to detect rotation of the gear wheels over which the load chains 32. 34 run. and rotation of the wheels supporting the conveyors. respectively, in the embodiments shown in Figures 2 and 3. In principle the sensors can be arranged on any moving part in the motor.

Members are preferably provided to control the speed of the travelling movement and/or the speed of the carrier device. This is possible since, thanks to the sensor, the location of the conveyor and the carrier device, respectively, is known. The speed can thus <BR> <BR> <BR> g radually be reduced so that a"gentle"stop in exactly the desired position is achieved, preferably with the aid of so-called frequency operation, as described above.