Conveyors: power-driven – Conveyor section – Reciprocating conveying surface
Patent
1997-07-25
1999-08-03
Bidwell, James R.
Conveyors: power-driven
Conveyor section
Reciprocating conveying surface
198761, B65G 2732
Patent
active
059312858
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a driving system for vibration conveyors of the resonance type comprising an electromagnet with a yoke.
2. Description of Related Art
In order to convey fairly small items, vibration conveyors are frequently used which are either linear or bowl-shaped. A typical example of a vibration conveyor contains a bowl with a spiral track on the inside for the items to be conveyed. The bowl is suspended on slightly slanted leaf springs which in their turn are fastened to a heavy bottom element. Together these parts form a resonant mechanical system. The bottom element is commonly supported by vibration-damped elastomer feet. On the top of the bottom element one or several electromagnets are fastened. If these are connected. to an alternating current, they will induce the bowl to vibration by the varying magnetic attraction of a yoke on the bowl, in step with the frequency of the alternating current. Normally the alternating current supplied will have a fixed frequency, as the source is the public mains. In order to obtain sufficient vibration at this frequency it is necessary to tune the resonant mechanical system to (approximately) the same frequency as that of the supplied force. In the case of a soft iron core and yoke the frequency of the force will be twice that of the mains because of electromagnetic attraction at both positive and negative going currents. The mechanical resonance frequency will, however, vary in function of a number of factors, such as temperature, ageing of the springs, and the mass of the items to be conveyed. Furthermore, the magnetic force supplied will be dependent on variations in the mains voltage. The operator must hence continuously adjust the current supply to the vibration conveyor in order to obtain stable item conveyance. In case a high item velocity is required, it is furthermore necessary to halt operations from time to time in order to retune the resonance frequency.
Furthermore, it turns out in practice that the mechanical resonance frequency depends on the oscillation amplitude, in a such a manner that the the resonance frequency decreases with increasing oscillation amplitude. If the mechanical system is tuned to a higher frequency than the driving frequency, the above relationship will cause a positive feedback of the oscillation with increasing oscillation amplitude, and a negative feedback in the case of decreasing amplitude. The outcome of this is that the vibration conveyor suddenly runs wild when more energy is fed into it, and suddenly halts when the energy supply is reduced. Furthermore, an increase of the mass of the oscillating system by loading it with heavy items will result in a lower resonance frequency, so that this will approach the drive frequency, whereby the oscillation amplitude increases, and the above mentioned phenomenon of positive feedback manifests itself. In order to obtain a stable conveying system it is hence necessary to tune the vibration conveyor resonance frequency such that it is 5-10% lower than the drive frequency which calls fore more energy input to obtain sufficient oscillation or vibration amplitude.
Even though the energy requirement is reduced because of the resonance, there is still, in large conveying plants, a requirement for an overdimensioned electrical installation due to the large reactive current caused by the large airgaps in the electromagnets. This results in either a higher payment to the electricity company or the need for investment in phase compensators. In traditional systems the amplitude of the vibrator oscillation is adjusted by means of a variable mains transformer which has to be individually adjusted for each individually tuned conveyor unit. Alternatively, a power regulation of the phase control type may be used. All these factors are considered as particular disadvantages of the known systems.
In order to partially remedy these disadvantages it has been tried to use a variable frequency to drive vibration conveyors, i.e., to tune
REFERENCES:
patent: 4331263 (1982-05-01), Brown
patent: 4350243 (1982-09-01), Weyandt
patent: 4811835 (1989-03-01), Bullivant et al.
patent: 4921090 (1990-05-01), Gregor
patent: 5472079 (1995-12-01), Yagi et al.
Christensen Kurt Hesseldahl
Madsen Niels Bent Heegaard
Bidwell James R.
Poul Johansen Development A/S
Safran David S.
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