WO1995015221A1

WO1995015221A1 - Method and system for controlling vibration amplitude

Info

Publication number: WO1995015221A1
Application number: PCT/DK1994/000451
Authority: WO
Inventors: Steen Hjorth Hansen
Original assignee: Skako A/S
Priority date: 1993-12-02
Filing date: 1994-12-02
Publication date: 1995-06-08
Also published as: AU1106795A; DK134693A; DK134693D0

Abstract

A vibration system is excited in operation by a vibrator (6) which is driven by an electric current. The vibrator (6) comprises a stationary electromagnet (8) and an oscillatorily suspended armature (10) as well as a vibration regulation circuit for adjusting the oscillation amplitude of the armature (10). The vibrator regulation circuit (13) is adapted to successively regulate this amplitude upwards. The system moreover includes an accelerometer (17) for detecting the size of the momentary acceleration of the system. The accelerometer (17) applies a voltage signal in response to the detected acceleration, said voltage signal ordering the vibrator regulation circuit (13) via an auxiliary circuit (16) to regulate the amplitude of the armature (10) downwards upon a bottom stroke.

Description

METHOD AND SYSTEM FOR CONTROLLING VIBRATION AMPLITUDE

The invention concerns a method of regulating amplitudes in a vibration system which is excited in operation by a vibrator, which is driven by an electric current and which comprises a stationary electromagnet and a oscillatorily suspended arma¬ ture, as well as a vibrator regulation circuit for setting the oscillation amplitude of the armature, and a vibration system for performing the method.

Vibration systems are extensively used in vibration conveyors which are employed for conveying many different types of mate¬ rials within industry, construction and agriculture. Such a conveyor substantially consists of a trough which is oscillato- rily supported by a plurality of springs and of an electromag¬ netic vibrator serving to impart cyclic oscillations to the trough in such a manner that a material present in the trough is thrown upwardly and forwardly in each cycle, causing the ma¬ terial to be successively conveyed from one end of the trough to the other.

The vibrator operates by passing a pulsating current through the windings of the electromagnet, thereby generating a corre¬ sponding pulsating magnetic field which oscillates the oscilla- torily suspended armature with associated oscillation mass to and fro with respect to the electromagnet.

The material to be transported and the conveyor in combination constitute a usually subcritically tuned operating oscillation system. To achieve optimum operation, the operating mass and the counter-oscillating mass should desirably be tuned close to the natural oscillation range of the system. However, it is also necessary to prevent bottom strokes which may occur in such a subcritically tuned conveyor if the working mass is in¬ creased beyond the expected magnitude. This may occur e.g. if some of the material sticks to the trough.

Bottom strokes can destroy the electromagnet, and an air gap having a sufficiently great safety margin between the electro¬ magnet and the armature is therefore provided in conventional electromagnetic vibrators by adjusting the system somewhat from the natural oscillation period, which reduces the length of stroke of the armature and thereby the performance of the vi¬ brator correspondingly.

It has been attempted to overcome this problem by means of a vibration conveyor which is described in EP patent application 0 432 881 Al. In this case, the momentary oscillation state is detected in operation by means of an accelerometer which ap¬ plies signals to the electric control circuit of the vibrator with a view to achieving a oscillation state which is deter¬ mined in advance or is optimum. For this purpose, the circuit contains a memory which stores the value last measured, and ad¬ justment is performed by comparison with the actual value until resonance is obtained. However, this structure, too, requires a certain safety distance between armature and electromagnet to avoid bottom strokes, and the system must therefore likewise be adjusted somewhat from the natural oscillation range, so that the theoretical capacity of this known vibration conveyor is not fully utilized either.

Accordingly, there is need for a vibration system or vibration conveyor of the type mentioned in the opening paragraph, wherein the theoretical capacity of the vibrator is utilized better than in the past. This is achieved by the method defined in claim 1 and by the use of the vibration system of claim 2.

The novel and unique features causing this to be achieved are that the vibration regulation circuit is adapted for successive upward regulation of the oscillation amplitude of the armature, and that the system moreover includes a sensor for detecting bottom strokes between the armature and the electromagnet and to apply a signal, representative of this size, via an auxil- iary circuit to the vibrator regulation circuit, instructing said circuit to regulate the amplitude downwards when the arma¬ ture hits the electromagnet with a bottom stroke after a prede¬ termined period of time.

This structure ensures that the safety distance between the ar¬ mature and the electromagnet is minimized and is eliminated upon a bottom stroke, and that the system may be adjusted close to the natural oscillation range, so that the vibrator will have an optimum operation performance. The distance is in- creased slightly upon a bottom stroke, but immediately thereaf¬ ter the distance again gradually diminishes and is reduced to zero after e.g. an hour, because the armature then hits the electromagnet with a bottom stroke. However, this stroke is al¬ most in the nature of a slight touch, which is not capable of damaging the electromagnet since the state has been reached through a relatively long period of time in very small stepwise increases in the amplitude of the armature.

Till now, all regulation approaches for optimizing the perform- ance of such vibration systems have been focused on avoiding bottom strokes. The structure of the invention overcomes this technical prejudice, because bottom strokes are precisely in¬ tended as an essential part of the regulation technique. More particularly, the vibrator regulation circuit is adapted for successive upward regulation of the oscillation amplitude of the armature, a positive voltage of some volts being applied between two set points in the circuit. The armature will thereby eventually hit the electromagnet, causing the sensor to apply a special overshoot signal which is converted via the auxiliary circuit into a negative voltage difference across the set points. Thereby, the vibration amplitude of the armature is immediately regulated downwards.

The sensor is typically an accelerometer which normally applies sine-shaped signals. This signal is distorted by the overshoot signal upon a bottom stroke. The auxiliary circuit is designed such that it is just this overshoot signal which is allowed to pass through the auxiliary circuit to activate the vibrator regulation circuit.

The auxiliary circuit moreover includes a timer for determining the period of time that has to elapse before a new bottom stroke occurs.

The invention will be explained more fully by the following de- scription of an embodiment, which just serves as an example, with reference to the drawing, in which

fig. 1 is a lateral view of a typical vibration conveyor having an electromagnetic vibrator,

fig. 2 is a schematic cross-sectional view through the vibrator of fig. 1 having a sensor for controlling the vibrator via an electric auxiliary circuit and a vibrator regulation circuit, fig. 3 is a block diagram of the auxiliary circuit,

fig. 4 is a curve showing the signals applied by the sensor when the armature oscillates freely, and

fig. 5 shows the corresponding signal upon a bottom stroke.

Fig 1. shows a typical vibration system in the form of a vibra- tion conveyor which is generally designated 1. This conveyor consists of a trough 2 which is supported via springs 3 by a frame 4 standing on a floor 5. A vibrator 6 serves to excite the conveyor so that materials placed in the trough 2 are con¬ veyed forwardly.

The structure of this vibrator is shown in fig 2. The vibrator has a housing 7 in which a stationary electromagnet 8 having a coil 9 is secured. When an electric current is passed through this coil, a magnetic field is formed, attracting an armature 10 having an oscillating mass 11 which is oscillatorily sus¬ pended in the housing 6 by means of springs 12. When the cur¬ rent through the coil 9 of the electromagnet 8 is allowed to pulsate with a suitable frequency, the armature 10 with the os¬ cillating mass 11 is caused to oscillate with the same fre- quency to and fro with respect to the electromagnet 8. The os¬ cillations are transferred as vibrations to the housing 7 which thereby excites the trough 2, as mentioned before.

The amplitude of the armature is determined by the voltage across the coil 9. The amplitude increases with increasing voltage. The armature is arranged at a distance a from the electromagnet to permit the armature to oscillate freely with respect to the electromagnet. Of course, this distance changes constantly during each oscillation cycle.

The voltage across the coil 9 is controlled by a thyristor con- trol 13 having two symbolically shown set points 14a, 14b. These two set points are connected via a wire 15 to an auxil¬ iary circuit 16, whose importance will be described more fully below. The vibrator moreover accommodates an accelerometer 17 in the form of e.g. a piezoelectric crystal which is connected to the auxiliary circuit 16 by means of wires 18.

Fig. 3 is a block diagram showing how the auxiliary circuit 16 is designed. The set points 14a, 14b of the thyristor control are visible in the diagram. In operation, there is a control voltage of e.g. 1-3.5 volts between these two points, causing the thyristor control to gradually increase the voltage across the coil 9, whereby the amplitude of the armature 10 succes¬ sively increases. The acceleration of the system is measured by the accelerometer 17, which thereby supplies a current of e.g. between 4 and 20 milliamperes which is converted into a voltage signal in a I/U converter 18. When the armature 10 oscillates freely, e.g. without touching the electromagnet 8, the voltage signal will be in the form of first harmonic oscillations or sine-shaped oscillations, as illustrated in fig. 4. A 50/60 Hz notch filter 20, which dampens these first harmonic oscilla¬ tions by about 50 dB, is inserted in the circuit after the I/U converter 19. The rest is removed by a subsequent 400 Hz second order high-pass filter 21. A Schmitt-trigger 22 is moreover provided in the circuit after these filters 20, 21, and the in- put of the Schmitt-trigger thus does not receive any voltage signal when the armature oscillates freely. The signal changes its nature upon a bottom stroke, as a typi¬ cal overshoot signal occurs, which is illustrated in fig. 5. However, this overshoot signal can pass the filters 20, 21, so that the Schmitt-trigger 22 charges a capacitor 24 via a first resistor 23. The voltage hereby generated on the capacitor 24 controls the current in a constant current generator 27 via a transistor 26. As mentioned before, a control voltage exists across the set points 14a, 14b, which causes the thyristor con¬ trol 13 to successively increase the voltage across the coil 9 of the electromagnet 8, thereby increasing the amplitude of the armature 10 correspondingly. A diode 28 and a third resistor 29 are inserted between the set points 14a,b. When, as mentioned, the current of the constant current generator 27 is regulated by the voltage on the capacitor 24 upon a bottom stroke, the voltage across the set points 14a,b is regulated, whereby the thyristor control 13 regulates the current intensity through the coil 9 and thereby the oscillation amplitude of the arma¬ ture downwards. If, in spite of the downward regulation, one or more new bottom strokes should occur, the process is repeated until the armature oscillates freely, and the voltage signal from the accelerator 17 only consists of first harmonic oscil¬ lations which do not arrive at the input of the Schmitt-trigger 22. Then the thyristor control again begins to regulate the am¬ plitude of the armature upwards.

At this time the capacitor 24 is still charged and affects the constant current generator 27 with a voltage causing the con¬ stant current generator to reduce the control voltage across the set points 14a,b, so that this control voltage does not im- mediately reach the value where the amplitude of the armature is greatest and bottom stroke occurs. This situation does not occur until the capacitor 24 has been discharged via the resis¬ tor 25. The amount of time this takes is determined by the ca- pacity of the capacitor 24 and the size of the resistor 25. Thus, the capacitor 24 and the resistor 25 in combination con¬ stitute a timer which decides how long it takes before the ar¬ mature again hits the electromagnet. For example one hour may elapse, during which period the armature approaches the elec¬ tromagnet very slightly for each oscillation. When finally the armature touches the electromagnet, this touch will therefore not be in the nature of a destructive stroke, but is merely registered as a weak snarl.

When the amplitude of the armature is regulated in the above- mentioned manner, a previously unexploited amplitude range is utilized, thereby making it possible to increase the perform¬ ance of a given vibrator noticeably.

Claims

PATENT CLAIMS

1. A method of regulating the amplitude in a vibration system driven by a vibrator which is driven by an electric current and comprises an electromagnet and an armature, which is arranged oscillatorily with respect to the electromagnet and whose os¬ cillation amplitude is regulated by means of a vibrator regula¬ tion circuit, c h a r a c t e r i z e d in that bottom strokes in the vibration system are registered, and that the oscillation amplitude is regulated such that the distance between the electromagnet and the armature is increased in response to a bottom stroke between these, but is immediately thereafter reduced gradually to zero, where the armature then hits the electromagnet with a new bottom stroke.

2. A vibration system for use in the method of claim 1, c h a r a c t e r i z e d in that the vibrator regulation circuit is adapted for successive upward regulation of the amplitude, and that the system moreover includes a sensor for detecting bottom strokes in the vibration system and to apply a signal, representative of this size, via an auxiliary circuit to the vibrator regulation circuit to regulate the voltage and thereby the amplitude of the armature downwards in response to detected bottom strokes.

3. A vibration system according to claim 1, c h a r a c ¬ t e r i z e d in that the sensor is adapted to detect the instantaneous acceleration of the system.

4. A vibration system according to claim 2, c h a r a c ¬ t e r i z e d in that the vibrator regulation circuit is adapted to successively regulate the oscillation amplitude of the armature upwards by application of a positive control voltage of e.g. 1-3.5 volts between two set points.

5. A vibration system according to claim 2 or 3, c h a r - a c t e r i z e d in that the sensor is adapted to apply a overshoot signal upon bottom stroke of the armature, and that filters just allowing this overshoot signal to pass are inserted in the auxiliary circuit after the sensor, and that the overshoot signal, via a Schmitt-trigger, serves to charge a capacitor whose voltage thereby reduces the control voltage across the two set points via a transistor and a constant current generator.

6. A vibration system according to claim 4, c h a r a c - t e r i z e d in that the control voltage across the two set points changes its sign so that the oscillation amplitude of the armature is regulated downwards.

7. A vibration system according to any of claims 2-5, c h a r a c t e r i z e d in that the auxiliary circuit contains a resistor, connected in parallel with the capacitor for discharging the capacitor after a predetermined period of time.

8. A vibration system according to any of claims 2-6, c h a r a c t e r i z e d in that the sensor is an accelerometer which is arranged in the vibrator and serves to apply a current signal of e.g. 4-20 milliamperes, and that this signal is converted into a voltage signal in a I/U converter, and that an e.g. 50/60 Hz notchfilter and an e.g. 400 Hz HF filter are arranged between this converter and the Schmitt- trigger.