EP1595055A1

EP1595055A1 - Control device for motor torque reduction of winding/unwinding units for flexible elements and method of controlling said device

Info

Publication number: EP1595055A1
Application number: EP04706687A
Authority: EP
Inventors: Giorgia Berengo
Original assignee: Fitem SRL
Current assignee: Fitem SRL
Priority date: 2003-02-17
Filing date: 2004-01-30
Publication date: 2005-11-16
Anticipated expiration: 2024-01-30
Also published as: WO2004072429A1; EP1595055B1; ES2303050T3; ATE389776T1; DE602004012533D1; DE602004012533T2; ITVI20030032A1

Abstract

A control device for motor torque reduction of a winding/unwinding unit for flexible elements comprising: an electric motor (2) electrically connected to a first condenser (9) operable in both rotation direction, coupled with a reduction unit (12) connected to a winding/unwinding roll (11) of a flexible element, an encoder (16) mechanically connected to said reduction unit, a logic unit (26) comprising memory means (26a) and counting means for the number of impulses coming from said encoder, able to elaborate said impulses and to send control signals to said motor torque reduction means, said logic unit programming means (13, 14) and actuator means (13) to activate said motor. The device is characterized in that said torque reduction means are operated by said logic unit when the number of impulses computed by said counting means is equal to a number previously set in said logic unit memory means (26).

Description

CONTROL DEVICE FOR MOTOR TORQUE REDUCTION OF WINDING/UNWINDING UNITS FOR FLEXIBLE ELEMENTS AND METHOD OF CONTROLLING SAID DEVICE.

The invention is about a control device for motor torque reduction of winding/unwinding units for flexible elements and the method of controlling said device.

It is known that for awnings or roller shutters winding and unwinding, proper winding/unwinding units comprising electrical operated gearmotors are used, provided with adjustable limit stop devices and braking devices. Driving units usually available on the market comprise a winding roll externally placed on a mainly longitudinally arranged tubular body in which an electric motor is placed, connected to a reduction unit provided with a protruding output shaft on one side, and to braking devices on the opposite side. The braking devices are placed inside the tubular body and they comprise an electric brake, while the limit stop devices usually comprise electromechanical microswitches.

The electric motor is typically of asynchronous monophasic reversible type, suitable for working in both rotation directions, clockwise and anticlockwise. This allows to obtain the winding and the unwinding in the awnings or roller shutter ascent and descent.

A problem of winding/unwinding units of known type is that, because of the awnings and roller shutters intrinsic mechanic and elastic characteristics variation with time, the winding and/or the unwinding deteriorate from the initial conditions after the assembly. It is known indeed that the stop positions in ascent or in descent during the winding and the unwinding are adjusted in the assembly stage by a limit stop devices mechanic manual calibration, provided by the assembler. The limit stop position in this way realized remains fixed during the winding/unwinding unit normal operations while the awnings dimensions, especially their length, changes with time as explained, causing the awnings to stop in different positions from the desired ones.

To prevent such problem, control devices are used which allow, in the winding or unwinding final stage, the electric motor working with reduced torque or speed in respect of the normal operation for at least an interval starting immediately after the mechanic limit stops intervention. In such manner a condition referred to as "excess stroke" is determined, which allows a further winding/unwinding stage after the limit stops intervention. A problem of such technique is that said control devices comprise limit stops of mechanic type which are subject to wear and thermal expansion phenomena with time, compromising their precision, response speed and reliability.

To overcome such problem, devices recently appeared on the market, in which the mechanic limit stops have been replaced with limit stops of electronic type so that the motor stop in the winding or the unwinding direction is controlled by a logic unit which computes the motor revolutions number by an encoder connected to the motor itself.

The logic unit provides for stopping the motor feeding when a number of revolutions corresponding to the upper limit stop or the lower limit stop is reached.

However, such kind of devices does not provide for a torque choking before reaching the upper or lower stop condition.

This lack of torque choking represents a limit of the device because the motor stop at upper or lower limit stop without preventive choking of the torque and thus of motor speed can involve a limit stops position sliding after several movements. The object of the present invention is to solve the aforesaid problems.

Is thus a first object of the invention to provide for a torque reducing device for motors belonging to winding/unwinding units for flexible elements which allows to electronically set the interval, and consequently the revolutions number, in which the motor torque reduction is required before the upper or lower stop. Another object is to obtain that said torque reduction interval is easily programmable and changeable with time, if required.

A further object to be obtained is to perform a constant control of the revolutions number actually reached by the motor in respect of the ones computed by the revolutions counting device. Another object to be obtained is that the device according to the invention controls the motor stop during the winding or the unwinding if the flexible element connected to said motor finds obstacles during its movement. A further object is to perform in easy way both the electronic limit stops setting and the interval setting, before the limit stop, in which the motor torque choking has to be obtained. Still another object is to obtain that the device according to the invention can be remotely controlled and is thus fit for receiving radio impulses coming from external sensors.

These and other objects, which will be better highlighted in the following description, are reached by a device for motor torque reduction of winding/unwinding units for flexible elements which, according to the first claim contents, comprises:

- an electric motor electrically connected to a first condenser operable in both rotation directions, coupled with - a reduction unit connected to a winding/unwinding roll of a flexible element;

- an encoder mechanically connected to said reduction unit;

- a logic unit comprising memory means and counting means for the number of impulses coming from said encoder, adapted to process said impulses and to send control signals to - motor torque reduction means;

- logic unit programming means;

- actuator means to activate said motor, and it is characterized in that said torque reduction means are operated by said logic unit when the number of impulses computed by said counting means is equal to a number previously set in said logic unit memory means.

The invention comprises also the method of controlling the torque reduction of the device according to the invention.

According to a preferred embodiment of the invention, the device provides for torque reduction means working by the connection, at a predetermined time depending on the motor revolutions, of a condenser of different capacity from the normally connected condenser associated with the motor electric circuit. According to a different embodiment of the invention, the torque reduction means provides for the choked voltage feeding of the condenser normally connected to the motor. In such case it is possible to program the nominal torque choking as needed, being able to modify at will the feeding parameters of the condenser connected to the motor by a proper motor control logic unit programming. Advantageously, in both cases the device logic unit provides for computing the number of impulses coming from the motor connected encoder and for comparing said number with a number previously set in the memory unit so that, when the number of computed impulses reaches the predetermined number, the logic unit controls the torque reduction means connected to the motor until the motor energy cut-off when the upper or lower stop is reached. Further characteristics and peculiarity of the invention will be better highlighted in the description of two preferred embodiments of the invention given in an explanatory but not limiting way, and shown in the figures of the annexed drawings, wherein:

- Figure 1 shows a schematic way of a winding/unwinding unit with the device according to the invention; - Figure 2 shows a flow chart of the device according to the invention;

- Figure 3 shows a functional logic diagram of an embodiment of the device according to the invention;

- Figure 4 shows the electric circuit of Figure 3 during the reduced torque operation; - Figure 5 shows a variation of the diagram of Figure 3;

- Figure 6 shows the device of Figure 5 in its reduced torque operation;

- Figure 7 shows the voltage development versus time across the main condenser of the device of Figure 5 according to the invention;

- Figure 8 shows a perspective view of the device according to the invention comprising the motor and the electronic board;

- Figure 9 is a schematic view of the encoder of the device according to the invention; and

- Figure 10 is a sectional view of Figure 9.

The control device of the invention is schematically shown in Figure 1 and it is generally indicated with numeral 1.

It is provided with an electric motor 2 comprising a reduction unit 12 on one side and an electric brake 19 on the other side.

The motor, the reduction unit and the electric brake are disposed in a tubular body 40 on which is externally placed a winding roll 11 around which the flexible element, not shown in the figure, is wound/unwound.

The electric motor 2 is properly connected to a winding unit 11 through a reduction unit and a pulley P.

With reference to the electric diagram of Figure 3 is noticed that the single phase asynchronous electric motor 2 of reversible type is connected to the power grid, generally indicated with numeral 3, where is visible a line wire 4, a neutral wire 5 and a ground wire 6.

The electric motor 2 is provided with two windings 7 and 8 for the clockwise and anticlockwise rotation and it is also provided with a main condenser 9 placed between the two windings 7, 8 for the nominal torque operation. The two windings 7, 8 comprise a cablehead 7a, 8a in common, connected to the neutral wire 5 by a thermal protection device 20 of known type.

Each cablehead 7b or 8b is selectively connected to the line wire 4 by the shunter 10 which allows the selection of the motor 2 rotation direction.

The selection of the motor rotation direction occurs in fact by actuator means comprising two push-buttons, one for the ascent and the other for the descent, both indicated with numeral 15 in the flow chart of Figure 2.

One of the two push-buttons, for instance the ascent button, connects the terminal 4 with the circuit section 7c while the descent button connects the terminal 4 with the circuit section 8c. Depending on whether the circuit 7c or 8c is respectively active, the switches

22 or 23 remain normally closed. Said switches are thus operated by the winding/unwinding push-buttons 15.

During the electric motor 2 operation the rotation of the encoder, generally indicated with numeral 16, also occurs. Said encoder is visible also in Figure 8 and it is provided, according to a preferred embodiment shown in Figure 3, with a toothed disc, also called phonic wheel 17, and by a detector 18 comprising a receiver 28 of optical type.

The disc 17 receives the motion from the reduction unit 12 slow shaft by interposed toothed wheels which multiply the rotation speed allowing a proper operation of the optical detector.

In other embodiments the encoder may be provided, instead of a phonic wheel, with a magnetic wheel integral with a HALL effect signals detecting device, substantially obtaining the same result.

The device of the invention furthermore comprises a logic unit, indicated with numeral 26 in Figure 2, which is fed at its cableheads 29 and 30 by the voltage taken at the electric brake 19 cableheads.

Said logic unit is provided by memory means 26a which, in a preferred embodiment of the invention, comprise EEPROM type memories, i.e. programmable, readable and erasable ROM memories. During motor 2 operation, the encoder 16 transmitter 27 emits signals received by the receiver 28 and then transmitted to the logic unit 26 which provides for computing the impulses received from the encoder 16.

As highlighted hereinafter, the impulses received by the logic unit 26 are computed and compared with the number of impulses previously stored in the memory unit 26a.

As consequence of said comparison, when reaching a previously set number, the memory unit 26 emits control impulses to the motor as explained below.

With reference to the electric diagram of Figure 3, during the normal motor operation, for instance in the winding mode, which corresponds to the shunter 10 in position 10a, the circuit section 7c is fed, the switch 22 remaining closed.

In such conditions the motor 2 has the condenser 9 working in parallel between the windings 7 and 8.

In fact the relay 32, which controls the connection and the disconnection of the condenser 9 and the condenser 35, is in a position able to connect in parallel the condenser 9 in its normal operation condition.

When the number of impulses indicated by the encoder 16 reaches a certain number previously set in memory 26a, a signal able to control the two contacts

33 and 34 of the relay 32 starts from the output 31 , belonging to the logic unit 26, so that the condenser 9 is disconnected and the condenser 35 having a reduced capacity in respect of the condenser 9 is connected, as shown in

Figure 4.

In such conditions of the circuit, the auxiliary condenser 35 is connected in parallel with the cableheads 7b and 8b of the windings 7 and 8, and the motor 2 operation is thus at reduced torque in respect of the preceding one, because the capacity of the condenser in parallel with the windings is reduced.

This situation remains until the relay 32 contacts 33 and 34 are maintained in the position previously described.

The reduced torque stage of the motor ends when the logic unit 26 computes an impulses number from the encoder equal to a previously set number indicating the limit stop of the motor.

Once reached said impulses number, the motor stops and the relay returns in the initial conditions, namely in the conditions keeping the main condenser 9 connected and the auxiliary condenser 35 disconnected, as shown in Figure 3. The same operative mode occurs when the shunter 10 is arranged in the position 10b in which the circuit feeding the motor section 8c is fed and consequently the switch 23 is closed and the switch 22 is opened.

Figure 5 shows another embodiment of the device according to the invention, generally indicated with numeral 200, which differs from the preceding described ones for the different way to obtain the motor working torque reduction.

As shown in this Figure, the main condenser 209 is series connected to a first diode 250 through its cathode 250a and thus connected to the windings 207, 208 of the motor 202.

The anode 251a of a second diode 251 , whose cathode is connected to a thyristor 252 preferably but non necessarily consisting of a TRIAC, having the control gate 252a connected to the processing unit 226, is connected to the cathode 250a of the diode 250. A two-contacts 233, 234 relay 232 is controlled by the logic unit 226 by the output 231.

In particular, the relay 232 first contact 233 is normally closed and it is connected between the first anode 252b and the second anode 252c of the thyristor 252 while the second contact 234, normally open, is connected to the thyristor 252 second anode 252c and it is in parallel with the limit stop 213.

The operation will be described, as in the preceding embodiment, with the shunter 210 in contact position with the terminal 210a and thus with the winding 207 connected to the mains line wire.

During the nominal torque operative stage, the relay 232 contact 233 is closed while the second contact 234 is open, so the thyristor 252 first 252b and second 252c anode are short-circuited.

In such situation the main condenser 209 is connected in parallel with the windings 207, 208 by the two diodes 250, 251 push-pull arranged in parallel to each other. It is evident that in the positive half wave part the diode 250 is conductive, while the diode 251 is locked.

In the negative half wave part the situation is inverted, with the diode 251 in conduction mode and the diode 250 locked.

The operation is thus equivalent to the situation in which the main condenser 209 is directly connected in parallel with the two windings 207 and 208. The reduced torque operation starting instant is determined in the same manner of the previously described embodiment, that is when the number of impulses of the encoder reaches the revolutions number previously set in the logic unit memory. This event causes the relay 232 first contact 233 to open and its second contact 234 to close, as shown in Figure 6.

The operation of said reduced torque circuit can be subdivided in two half- periods relating to the mains feeding. In the positive half wave part the diode 250 is conductive while the diode 251 is locked and the nominal voltage is applied across the condenser 209, as in the nominal torque operation case.

In the negative half wave part the diode 250 is locked while the diode 251 is conductive so the main condenser 209 is in series with the thyristor 252. By the gate 252a signal modulation, the logic unit 226 controls the thyristor 252 in switch on or in switch off mode, choking the voltage applied to the main condenser 209 and determining a medium value variation in the cycle. It is evident that the motor torque reduction percentage depends on the gate 252a activation, provided by the logic unit 226, interval which could be advantageously set by programming said logic unit 226. In Figure 7 is shown a possible voltage development across the main condenser 209 as a function of time, which is represented on x-coordinate. It is noticed that in the positive half wave part the voltage development corresponds to the nominal torque operation while in the negative half wave part the voltage moves away from the nominal torque value, represented by the hatched line.

It is important to note that in the device according to the invention the encoder 16 and the logic unit 26 are also used to set up both the upper and lower limit stops of the roll movement connected to the motor, as well as to set the reduced torque intervention time, expressed by a number of impulses generated by the encoder before the limit stop.

As described below, and with particular reference to Figure 2, by a proper logic unit programming it is possible to decide the upper and the lower limit stops, equal to a certain revolutions number and to the impulses number recorded by the encoder, as well as the winding roll positions in which the reduced torque until the lower limit is needed. More particularly, by first programming means provided by a push-button 13 upper and lower limit stop positions are set, storing the number of impulses transmitted by the encoder 16 elapsing between the upper and the lower limit stops. Operatively, said kind of storage is carried out through an external push-button accessible by the operator who, in a first stage, enables the memory 26a to receive control impulses.

By ascent or descent actuator means 15, comprising two control push-buttons, the motor 2 is operated until the desired position of the flexible element connected to the roll 11 , and thus to the motor, is reached.

Once the desired position is reached, the programming button 13 is operated for the second time, to store such position in memory 26a. The descent actuator means 15 is subsequently operated and the number of impulses emitted by the encoder 16 is stored in this stage, until the lower limit stop is reached.

Once reached said lower limit stop, a further actuation of the push-button 13 provides for storing the number of impulses corresponding to the complete stroke between the upper and the lover limit stops. When the upper and the lower limit stops programming by the programming means 13 is completed, it is possible to program the reduced torque.

This programming is made by second programming means 14 which can conveniently be provided by a computer connected to the memory unit 26. A first number indicating the number of remaining impulses of said encoder before reaching the upper limit stop position, during which a torque reduction is needed, is stored in memory 26a by the computer 14.

When said number of impulses is reached, the logic unit 26 will emit a signal 31 which changes the relay 32 position, when the electric circuit feeding the motor is that shown in Figure 3. If the device according to the invention comprises the electric circuit shown in Figures 5 and 6, that is when the torque choking is provided by reduced voltage feeding of the condenser 9 connected to the motor, it is possible to store in said second memory means a second number indicating the reduction percentage of an electric parameter proportional to said motor torque which, as previously said, could be the feeding voltage of the condenser connected to the motor. Similarly, as to the torque reduction before the lower limit stop position is reached, a third number, indicating the number of remaining impulses of the encoder before reaching the lower limit stop position, and a fourth number, indicating the reduction percentage of the electric parameter proportional to the desired reduced motor torque, are stored by said second programming means 14, if the device is provided by the circuit shown in Figures 5 and 6. Therefore during normal operation, in the flexible element winding or unwinding, when the number of remaining impulses to reach the upper or the lower limit stop is respectively equal to the first number or to the third number stored in memory 26a, the logic unit 26 emits an impulse which activates the relay 32 to switch on the auxiliary condenser 35 and to switch off the main one 9, if the circuit is that shown in Figure 3, while it activates the TRIAC 252 to choke the voltage feeding across the condenser 209 and thus to obtain the desired torque choking effects, if the circuit is the one shown in Figures 5 and 6.

Independently from the electric diagram used to carry out the device according to the invention, that is independently whether the torque reduction is obtained by switching on a reduced capacity condenser or by the variable and choked voltage feeding across the condenser, the device according to the invention provides for a substantially continuous control of the encoder position and thus of the motor by controlling the impulses sent by the encoder 16. Such control is performed by a further detection means 24 placed in the encoder disc 17, in a different position from the detector 18, as shown in Figures 9 and 10. In that case the disc 17 is provided with reflecting means 172, as shown in Figure 9, which comprise reflecting discs allowing the detection means 24 to transmit a separate impulse to the logic unit 26 when they pass in front of said detection means 24, properly consisting of a photodiode. The logic unit 26 provides itself for a comparison between the impulses emitted by the first actuator 18 and by the detection means 24.

It is possible that the number of impulses emitted by the sender 18 is different from the one emitted by the detection means 24 if the disc 17 is stopped in an intermediate position between two consecutive holes 171. In the memory unit 26 a certain impulses number of the detection means 24 is programmed, for instance by the second programming means 14, and a comparison between said impulses number, once reached, and the number of impulses transmitted by the actuator 18 is performed.

When the previously set number is different from the one reported by the actuator 18, the logic unit provides for changing the stored impulses number of the actuator 18 until it corresponds to that of the detection means 24.

In this way it is certain that counting sliding problems, which can modify and change both the limit stop position and the reduced torque inserting moment at the desired time, are avoided.

Both embodiments of the invention, as shown in Figures 3 and 5, furthermore provide the logic unit for emitting a switching off signal of the electric circuit, and thus a motor stop, when further impulses are not received by the encoder within a predetermined time interval.

This happens when the winding or unwinding flexible element connected to the motor finds obstacles during its motion. In such a case, to protect the motor from overcurrents, the motor feeding disconnection occurs when the encoder does not transmit impulses according to a predetermined sequence.

Advantageously, the device according to the invention is furthermore provided with a radio receiving card able to receive radio signals coming from external sensors, such as light sensors, rain sensors, wind sensors and the like.

Indeed, in case the motor is connected to flexible elements like awnings, depending on atmospheric or light conditions it will be necessary to activate the awning winding or unwinding by controls emitted by the remote sensors which, by radio signals, transmit to the radio card and thus to the logic unit the signals of motor activation in either direction.

Claims

1) A control device for motor torque reduction of a winding/unwinding unit for flexible elements comprising:

- an electric motor (2) electrically connected to a first condenser (9) operable in both rotation directions, coupled with

- a reduction unit (12) connected to a winding/unwinding roll (11 ) of a flexible element;

- an encoder (16) mechanically connected to said reduction unit;

- a logic unit (26) comprising memory means (26a) and counting means for the number of impulses coming from said encoder, adapted to process said impulses and to send control signals to

- motor torque reduction means;

- logic unit programming means (13, 14);

- actuator means (15) to activate said motor, characterized in that said torque reduction means are operated by said logic unit when the number of impulses computed by said counting means is equal to a number previously set in the memory means (26a) of said logic unit.

2) The device according to claim 1 ) characterized in that said torque reduction means comprise a second condenser (35) to be connected to said motor electric circuit in replacement of said first condenser (9) normally connected with said motor, said second condenser having lower capacity than said first condenser.

3) The device according to claim 2) characterized by comprising a relay (32) able to control the disconnection of said first condenser (9) and the connection of said second condenser (35) when the logic unit indicates that a previously set number of impulses from the encoder is reached.

4) The device according to claim 1 ) characterized in that said torque reduction means comprise voltage feeding reduction means across said first condenser. 5) The device according to claim 4) characterized in that the voltage feeding reduction means across said first condenser comprise a thyristor controlled by said logic unit.

6) The device according to claim 5) characterized in that said thyristor consists of a TRIAC (252). 7) The device according to claim 4) characterized in that the logic unit (26) emits control impulses to said TRIAC (252) to choke the feeding voltage of said first condenser (9) when a number of impulses previously set by said programming means (14) in said memory means is reached.

8) The device according to one of the claims from 1 ) to 7) characterized in that said encoder (16) comprises a phonic wheel (17) integral with an optical detector (18), sender/receiver of light signals, electrically connected to said logic unit.

9) The device according to one of the claims from 1 ) to 7) characterized in that said encoder (16) comprises a magnetic wheel integral with a HALL effect signals detecting device.

10) The device according to one or more of the preceding claims, characterized in that said logic unit (26) emits a signal for opening motor feeding switches (22, 23) when the impulses are not received by the encoder beyond a predetermined time interval. 1 1 ) The device according to claim 1 ) characterized in that said programming means (13, 14) comprise first programming means (13) provided by at least a push-button interacting with said logic unit and second programming means (14) provided by a computer.

12) The device according to claim 1 ) characterized in that said logic unit (26) is a microprocessor.

13) The device according to claim 1 ) characterized in that said memory means (26a) comprise an EEPROM type memory.

14) The device according to one or more of the preceding claims, characterized in that it provides for a radio receiving card connected to said logic unit, said radio card being able to receive radio signals coming from external sensors to activate said winding/unwinding roll.

15) The device according to claim 11) characterized in that said push-button (13) transmits to the logic unit the information related to the setting of the limit stop of said winding/unwinding roll. 16) The device according to claim 11 ) characterized in that said computer sets and transmits the parameters related to the torque choking for the storage in the logic unit.

17) The device according to claim 1 ) characterized in that said actuator means (15) for the motor activation are two switches activating the winding and the unwinding movement of the flexible elements respectively. 18) The device according to claims 8) or 9) characterized in that the disc (17) of said encoder (16) provides for one or more reflecting points (172) circumferentially disposed to reflect a signal emitted by a detection means (24), the signals received from said detection means being transmitted to said logic unit.

19) The device according to claim 18) characterized in that said detection means is a photodiode.

20) A method of controlling the torque reduction in a device according to claim 1 ) characterized by comprising the following steps: - setting by first programming means (13) upper and lower limit stop positions of said electric motor and storing said positions in a logic unit;

- storing in said logic unit (26) the number of impulses emitted by an encoder (16) connected to said motor when the motor moves from said upper limit stop to said lower limit stop; - storing in said logic unit, by said second programming means (14), a first number indicating the number of remaining impulses of said encoder before reaching the upper limit stop position, during which a torque reduction is needed;

- storing in said logic unit, by said second programming means (14), a second number indicating the reduction percentage of an electric parameter proportional to said motor torque, said second number being in relationship with said first number;

- storing in said logic unit, by said second programming means (14), a third number indicating the number of remaining impulses of said encoder before reaching the lower limit stop position, during which a torque reduction is needed;

- storing said logic unit, by said second programming means (14), a fourth number indicating the reduction percentage of an electric parameter proportional to said motor torque, said fourth number being in relationship with said third number;

- transmitting a signal from said logic unit to activate torque reduction means when said encoder computes the number of revolutions left for reaching the upper or the lower limit stop, these revolutions being respectively equal to said first or said third number. 21 ) The method according to claim 20) characterized in that the detection of the upper/lower limit stop position comprises the following steps: a) activating a transmission channel with said logic unit by said first programming means (13); b) activating the motor in winding/unwinding stage by an ascent/descent push-button (15) until a desired stop position; c) storing said stop position reached by the motor in the winding/unwinding stage by transferring said set of impulses to said logic unit memory; d) blocking the memory capability to receive instruction.

22) The method according to claim 20) characterized by further comprising the motor feeding stop when the logic unit does not receive signals in sequence from the encoder beyond a predetermined time interval.

23) The method according to claim 20) characterized by further providing for controlling the said encoder (16) disc (17) position, said control comprising the following steps: - connecting to said disc of said encoder a control device (24) able to transmit and to receive different signals from the ones emitted to said encoder;

- sending said signals to a computing memory (26a) integral with said logic unit (26); - comparing the computed number of said signals with the impulses number received by said encoder;

- in case of difference, modifying the computed number of impulses of said encoder to equalize it to the number computed by said control device.