EP2586896A2

EP2586896A2 - "A method and a device for opening and subsequently closing a weft brake in a thread feeder"

Info

Publication number: EP2586896A2
Application number: EP12005650.2A
Authority: EP
Inventors: Luca Gotti
Original assignee: LGL Electronics SpA
Current assignee: LGL Electronics SpA
Priority date: 2011-10-27
Filing date: 2012-08-03
Publication date: 2013-05-01
Anticipated expiration: 2032-08-03
Also published as: CN103092091A; EP2586896B1; CN103092091B; ITTO20110977A1; EP2586896A3

Abstract

According to the method of the invention, the step motor (14) of the brake (20, 23, 24, 26) of the thread feeder, upon opening command (to), is driven in the direction of opening the brake, while counting the steps made by the motor until the brake is stopped by abutment against a travel stop (22), and storing the step count reached at the time of blockage (step_to_open). upon a subsequent closing command (t₃), the step motor is driven in a direction of closing the brake by a number of steps equal to the stored count.

Description

This invention is concerned with a method for opening and subsequently closing an electronic weft brake in a thread feeder of a textile machine or a knitting machine, and to a thread feeder adapted to carry out the method.
In the field of textile machines, and more specifically of thread feeders, brake systems are known in which a frusto-conical braking member is elastically supported in an axial position in front of the feeder drum so that it is pushed with more or less pressure against the drum by a motor, thereby modulating the braking action so that the average tension acting on the thread, as measured by a suitable downstream sensor, coincides with a preset desired tension. A system of this sort is disclosed, e.g., in prior document EP 707102 .
In the above regulatory system it is not necessary, for control purposes, to know the absolute position of the motor (i.e. of the slide which determines the braking position), because the system regulates the position in a relative way : if the measured tension is below a set value, the motor must move in a braking direction (increased braking), while the opposite is required whenever the measured tension is higher than the desired tension. It can thereby be avoided to use expensive, absolute position sensors, while obtaining an unexpensive and reliable system. In this sort of applications, a step motor, or stepper, is quite appropriate, because the motor can be moved in relative steps that are sufficiently accurate, without needing a very sophisticated electronics.
However, as a person skilled in the field will understand, it is sometimes necessary to fully open the brake, such as during the final threading of the feeder, or for ordinary maintenance (cleaning, etc.). However, the above discussed systems, for the sake of simplicity, include no facility for manually opening the brake: consequently, complete opening is achieved electronically, by directing the control system (via a manual pushbutton or the control console of the textile machine) to rapidly move away the brake slide : the control system then operates the motor so that the slide will traverse in a short time the full travel in the direction of brake release, thereby ensuring a complete opening of the brake. Obviously, since the travel length is set at its maximum value, the slide supporting the brake member will eventually abut against a mechanical stop, and will cause the motor, still under supply voltage, to become stalled as it vainly tries to move the brake.
When the user has completed work with open brake, the slide needs to be closed again, so that it can again carry out its brake regulating task in order to maintain a constant tension in the thread delivery. However, the control system is unable to take the slide back to the position it had before the brake was opened, because that position is unknown.
The system, lacking this information, may use one of several strategies, the most obvious one being to bring the slide back to a standard, predefined initial position, trusting that the control system will subsequently take the slide to the proper position. However, in ceertain applications it is important that the slide is immediately taken back to the braking position from which it had started at the time of brake opening, in order to avoid that, during the transition interval before the optimum tension regulation is attained, a number of problems arise: these range from the formation of defects in the manufactured article (cloth, stockings, etc.) to actual, immediate stop of the textile machine.
It is the main object of the invention to overcome the above limits of the prior control systems and make it possible to open the brake and then quickly return it to its original position, without changing the requirements of structural simplicity and high reliability of the control system.
More particularly, it is an object of the invention to provide a method and a device for opening and closing the brake in a thread feeder, by which the brake can be brought back immediately to its original braking position, without requiring complex and expensive arrangements for detecting the brake position.
The invention achieves the above and other objects and advantages, such as will appear from the following disclosure, with a method for opening and subsequently closing a weft brake in a thread feeder of a textile machine, as recited in claim 1.
The invention is also concerned with a thread feeder which incorporates the features of the above method, as recited in claim 4.
Other advantageous features of the invention are set forth in the dependent claims.
The concept of the invention substantially consists in counting the number of steps made by the motor starting from the beginiing of the opening travel, while monitoring the motor's turning speed or other parameter useful for detecting when the slide has reached the end of its opening travel and abutted against a limit stop. As soon as a blockage is detected, the totalized count is stored in a suitable variable or memory register (step_to_open). In the subsequent braking closing operation, the motor is driven in the opposite direction for a number of steps exactly equal to the stored count step_to_open. By this procedure the slide, and therefore the brake, will be placed in the original position.
A preferred embodiment of the invention will now be disclosed, with reference to the attached drawings, wherein:

Fig. 1 is a view in longitudinal cross-section of a thread feeder of a textile machine on which the method of the invention can be applied;
Fig. 2 is a general circuit diagram of a control system for a step motor which is part of the thread feeder of Fig. 1;
Fig. 3 is a partial block diagram showing the mode of motor regulation by the system; and
Fig. 4 is a collection of signal diagrams useful for understanding the operation of the preferred embodiment of the invention.

With reference to Fig. 1, a drum 10 of a weft feeder, such as disclosed in the above-mentioned EP 707102 , is carried on a stationary support 12, on which is also mounted a step motor 14; motor 14 drives, via a traveling-nut linear actuator 16, 18, a slide 20 which is slidably mounted on stationary support 12, so that it will move in a direction parallel to the axis of drum 10. A limit stop 22 limits the slide travel in a direction away from the drum.
Slide 20 has a ring 23 that is coaxial to drum 10, in which a frustoconical braking member 26 is elastically mounted, by means of radial springs 24, in a position that is frontal and coaxial to drum 10. Braking member 26 is known per se and is adapted to cooperate with the drum, by contacting its rounded front edge, so that a braking action is exerted on a thread 28 which unwinds from the drum and proceeds toward a textile machine (not shown) while moving along a tension detector 30.
A stepper control system (not shown on Fig. 1, and described below) controls motor 14, depending on the tension signal received from detector 30, so that slide 20 and therefore braking member 26 are positioned to apply a braking pressure on the drum such that the average thread tension is kept a a desired pre-set value. The manner of implementing this regulation is disclosed in prior documents EP 2031106 and EP 1314806 .
Referring now to Fig. 2, Pha is one of two phase windings of a bipolar stepper motor driving the brake of Fig. 1.
Phase Pha of the stepper is supplied through an H bridge PDa, comprising four MOSFETs Q₁, Q₂, Q₃, Q₄ that are driven, in a way known per se, by respective signals G₁, G₂, G₃, G₄ provided by a gate driver GDa, which in turn is driven by control signals Pwma1, Pwma2, Pwma₃, Pwma₄. These are generated, according to conventional methods known in the art, by a microcontroller MC. H bridge PDa is connected between a supply voltage Vbus and earth, in order to drive, as known per se, the phase winding so that the full current reversibility in the winding is ensured.
The stepper, as will be obvious for persons skilled in the art, also has a second phase winding Phb, which is not shown on Fig. 2, and which is driven, similarly as described above, by a second H bridge PDb (not shown). This is ultimately governed by a second set of signals Pwmb1, Pwmb2, Pwmb₃, Pwmb₄ (which also are not shown in Fig. 2), generated by microcontroller MC.
Microcontroller MC is also capable of receiving, via inputs Open and Close that are commanded by manual pushbuttons or from a console, respective brake opening and fast closing commands. For the purposes of controlling the motor and determining when it has stopped, according to the preferred embodiment, two signals Iameas and Ibmeas are derived from H-bridges PDa and PDb, respectively, thru respective resistors such as Ra. The signals Iameas and Ibmeas, together with the bus voltage Vbus, are input to microcontroller MC, after being converted to digital signals in an analog-digital converter embedded in the microcontroller or in another A/D converter not shown.
With reference to Fig. 3, it is now explained how the above signals Iameas and Ibmeas are used for controlling the stepper. Again, only phase Pha of the step motor will be considered for simplicity. The current Iameas as measured on the bridged is subtracted from the current reference Iref, thereby obtaining an error current Ierr, which is processed in a preferably PI (proportional + integral) compensator Ca, producing a voltage signal Va, i.e. the voltage which, when applied to phase Pha of the motor, will cancel said error. Signal Va is then applied to block Diva, where it is divided by the supply voltage Vbus of the H-bridge PDa, thereby obtaining the duty cycle duty which determines the pulse width of the PWM signals driving the motor.
In a similar manner, not requiring a separate description, is generated a signal Vb for phase Phb of the motor.
As is well known to a person skilled in the art, signal Iaref is changed by the control system in order to cause two sinusoidal currents that are 90° out of phase and have a frequency proportional to the desired step to flow the phases Pha e Phb of the motor (according to the so-called microstepping driving of the motor). Therefore, during the movement of motor 14 the two voltage signals Va and Vb will also be two sinusoidal voltages that are 90° out of phase and have substantially equal magnitudes, since the electric parameters (Resistance, Inductance and Constant of Counter-electromotive Force) of the two phases of the motor are constructively identical. It is therefore possible to compute, in any instant, the total magnitude of voltage Vamp applied to the motor as quadratic sum of phase voltages Va and Vb : $Vamp = \sqrt ({Va}^{2} + {Vb}^{2})$
It should be appreciated that, as motor 14 displaces brake slide 20, the latter voltage value will be higher than the voltage value applied by the control system when the motor is blocked because it has reached the end of travel. In the former case Va has to overcome, in addition to the resistive and inductive drop, also the motor's counter-electromotive force; in the latter case, when the motor is blocked, this contribution is reduced to zero, so that the above value will be lower.
Assuming that at instant to the control system receives the command of full brake opening, motor 14 is actuated and slide 20 starts moving away from drum 10, i.e. a direction of unbraking. Simultaneously, the system continuously computes the following signals, whose time diagrams are shown on Fig. 4:

Step_to_open : number of steps made by the motor and stored in microcontroller MC;
Vamp: equivalent voltage applied to the motore, according to formula (1) shown above;
dVamp/dt: time-derivative of Vamp.

In order to ascertain that the motor has stopped, or has stalled, the time derivative of Vamp (dVamp/dt) is compared with a predetermined negative threshold DerThr. Whenever the derivative exceeds such threshold in the negative direction, as is shown at instant t1 in the diagram, that indicates that the counter-electromotive force has vanished, because the motor has been blocked. The microcontroller then ceases to drive the motor, at instant t2, and the system waits for a subsequent re-positioning command, which will take place at instant t3. Starting at instant t3 the motor will be driven in the opposite direction (i.e. the brake moving towards the drum) for a number of steps equal to the stored value step_to_open. In other words, as shown on Fig. 4, the variable step_to_open is decremented at each step gained in the opposite direction, until said variable reaches zero, at instant t4.
Programming the microcontroller for implementing in practice the sensorless brake-opening/closing technique which has been disclosed above does not require any special stratagems, but rather falls within the average ability of one skilled in the art, based on the the teachings given above. Persons skilled in the art will also appreciate that the above sensorless technique, in spite of its simple implementation, is capable of providing effective and repeatable results. Also, it should be apparent to a person skilled in the art that the above disclosed technique is not the only feasible embodiment, but rather that in practice any technique of sensorless control of stepper motors can be used, or, by analogy, permanent-magnet motors generally (the model of the step motor is identical to the model of the brushless motor, for instance). Accordingly, by adopting said techniques, detection of rotor speed and/or position is always available, and it is therefore possible to deduce the motor stall simply by providing that the speed detected is zero, or less than a predetermined value. However, the preferred embodiment disclosed above is in general cheaper than other techniques, which are usually adopted in order to obtain the position and/or speed of the motor as accurately as possible and which, consequently, use more refined algorithm and more powerful and expensive microcontrollers.

Claims

A method for opening and subsequently closing a weft brake in a thread feeder of a textile machine, in which the brake is driven by a step motor, characterized in that, upon a brake opening command (to), the step motor (14) is stepped in a brake opening direction while counting the steps until the brake (20, 23, 24, 26) is stopped by abutment against a limit stop (22), and the step count (step_to_open) reached at the time of the abutment (t1) is stored; and in that, upon a subsequent closing command (t3), the step motor is stepped in a brake closing direction for a number of steps equal to the stored step count.
The method of claim 1, characterized in that the stopping of the brake is detected by sensing that the turning speed of the step motor (14) has become zero.
The method of claim 2, characterized in that the motor speed is determined to be zero when the time derivative of the overall voltage (Vamp), computed as the quadratic sum of the sinusoidal voltages (Va, Vb) that are applied to the respective phase windings (Pha, Phb) of the step motor (14), exceeds, in a negative direction, a predetermined threshold value (DerThr).
A braking system for a thread feeder of a textile machine, comprising a thread-delivery drum (10) and a brake (20, 23, 24, 26) driven by a step motor (14) controlled by a microcontroller (MC) for traveling between a closed position of contrast against the drum and an open position of abutment against a limit stop (22), characterized in that the microcontroller (MC) is programmed for:
a) upon reception of a brake opening command (Open):
- stepping the motor (14) indefinitely in a brake opening direction;

- detecting when the motor is stopped by abutment of the brake against said limit stop (22);

- counting the number of motor steps and store the count reached when the motor is detected to be stopped (step_to_open);

b) upon subsequent reception of a brake closing command (Close):
- stepping the motor in a brake closing direction for a number of steps equal to said stored step count.
The braking system of claim 4, characterized in that, in order to detect when the motor is stopped, the microcontroller (MC) is programmed for :
- continuously computing the magnitude of the overall voltage (Vamp) that is applied to the motor by the formula Vamp = √ (Va² + Vb²), where Va and Vb are the respective sinusoidal voltages applied to the respective phase windings (Pha, Phb) of the step motor (14);

- continuously computing the time derivative (dVamp/dt) of said overall voltage magnitude;

- determining that the motor has stopped when said derivative exceeds, in a negative direction, a predetermined threshold value (DerThr).
The braking system of claim 4 or 5, characterized in that the microcontroller (MC) is also programmed for ceasing to drive the motor (14) after detecting that the latter has stopped.