Method For Transporting Loads Safely By Utilizing A Permanent Magnet Type Lifting Magnet

Abstract

A method for transporting loads safely utilizing a permanent magnet type lifting magnet having a permanent magnet provided in its magnetic circuit and an exciting winding for controlling the magnetic flux developed in said magnetic circuit and adaptable to be operated in an attraction mode in which a load is to be attracted, a lifting mode in which the attracted load is lifted and a transportation mode in which the load is transported, said method comprises the steps of applying an exciting current of a predetermined value to the exciting winding for producing an attractive magnetic force during the attraction mode, reducing the exciting current to a value of zero or less thereby making the magnetic flux less than the residual magnetic flux due to its remanence during the lifting mode and increasing the exciting current to larger than zero during at least the initial period of the operations in the transportation mode, thereby providing the lifting magnet with magnetic flux larger than the residual magnetic flux corresponding to the remanence under the condition in the lifting mode.

Description

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

This invention is relating to a method for transporting loads safely by utilizing a permanent magnet type lifting magnet.

2. DESCRIPTION OF THE PRIOR ART

A permanent magnet type lifting magnet, which utilizes a permanent magnet provided in its magnetic circuit and an exciting winding for controlling the intensity of magnetic field through the magnetic circuit to provide the lifting magnet with a necessary attractive force or reduce the force, is employed to transport loads of magnetizable material such as steel and iron in such a way that an exciting current is first supplied to the exciting winding for a few seconds thereby providing the lifting magnet with a magnetic force sufficient to attract a load to be transported, the attractive force of the lifting magnet is held during the transportion of the subject by its high residual magnetic flux and high coercive force after removal of the exciting current and then a negative exciting current, i.e. an exciting current flowing in a reverse direction to the original exciting current, is applied to the exciting winding thereby erasing the residual magnetic flux to release the load from the lifting magnet. This type of lifting magnet is advantageous in that the exciting current is applied to the exciting winding only for a short time when the load is attracted to or released from the lifting magnet so that the electric power consumption is very low and that the load is held by the lifting magnet even at failure of the power supply.

This type of a permanent magnet type lifting magnet is usually operated in such a way that: First, in order to attract a load, an exciting current of a predetermined value is applied to the exciting winding for a short time of 2 to 3 seconds, as shown in FIG. 1B, thereby to change the magnetic force or the magnetic flux density to point a on a hysteresis curve, as shown in FIG. 1A in which the ordinate and abscissa represent the magnetic flux density and the intensity of excitation, respectively, corresponding to the intensity H.sub.1 of the excitation. Then the excitation is removed and thereby the magnet flux density of the magnet changes to point b corresponding to its remanence B.sub.1. Under this condition the lifting magnet is capable of lifting and transporting the load by its residual magnetic force due to the remanence B.sub.1. Finally the exciting winding is supplied with a negative exciting current for providing a negative excitation H.sub.2 thereby to change the magnetic flux density to point c to release the load.

The permanent magnet type lifting magnet may be operated in a different manner such that, as shown in FIGS. 2A and 2B first an exciting current of a predetermined value is applied to the exciting winding for providing a magnetic force, or a magnetic flux density corresponding to point a in FIG. 2A for attracting a load and then the exciting current is reduced to a smaller value, as shown in FIG. 2B. The load is lifted and transported, as holding the small exciting current, by a magnetic force due to the magnetic flux density B.sub.2 corresponding to point d in FIG. 2A. The exciting current is finally changed to a negative value for releasing the load.

However, in the case of transporting loads in the manner as described referring to FIGS. 1A and 1B, if the weight of the load is very near to or slightly exceeds the allowable maximum weight for the lifting magnet or if the lifting magnet is subject to abnormal vibrations or sudden change of its moving speed during transportation of even a light load, it is not assured that there is no dangerous fall of the load from the magnet.

On the other hand, in the case of transporting loads in the manner as described referring to FIGS. 2A and 2B, the load is held by the sum of the magnetic flux due to an applied small exciting current and that due to its remanence during the lift and transportation of the load. Therefore, failure of the power supply may cause the lifting magnet to fail to hold the load especially when the load is too heavy to be held by only the residual magnetic flux due to its remanence.

Thus the conventional methods have problems of dangerous fall of loads during the transportation.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method for transporting loads reliably and safely utilizing a permanent magnet type lifting magnet.

Another object of the present invention is to provide a method for transporting loads free from such dangerous fall of loads utilizing a permanent magnet type lifting magnet even at failure of the power supply.

The present invention is based on such principal concept that the attractive force of the lifting magnet, which is proportional to the square of its magnetic flux density, is always held larger during the operations in a transportation mode in which loads are transported than that during the operations in a lifting mode in which the loads to be transported are lifted and the atractive force produced by the lifting magnet is checked as to whether it is sufficiently strong to hold the loads securely during the transportation of loads by changing its attractive force to a predetermined value prior to the transportation.

According to the present invention, for realizing the above concept, first a predetermined value of exciting current is applied to the exciting current during the operations in an attraction mode in which a load to be transported is attracted thereto and then thereby providing necessary magnetic flux for attraction of the load and then the exciting current is reduced to zero or less, thereby reducing the magnetic flux of the magnet to less than the residual magnetic flux due to its remanance and then the operation is made for lifting the load. The exciting current is again increased to a value adaptable to provide magnetic flux larger than the residual magnetic flux and thereby the safety is assured during the transportation of loads.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 2A and 2B are graphs showing variations of the magnetic flux density and the exciting current during the operation of a permanent magnet type lifting magnet according to typical conventional methods.

FIGS. 3A, 3B, 4A, 4B, 5A and 5B are similar graphs for illustrating the operation according to the present invention.

FIG. 6 is a schematic diagram showing a control circuit for use with the method according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Explanation will be made of the operations of a permanent magnet type lifting magnet according to the present invention. As shown in FIG. 3B, first, during the operations in an attraction mode in which a load to be transported is attracted, an exciting current of a predetermined value is applied to the exciting winding of the lifting magnet. After the load has been attracted by the magnet the exciting current is reduced to zero and then the operation is changed into a lifting mode to lift the load from the floor or ground. Thereafter, during the operations in a transportation mode in which the load is transported, the exciting current is once increased to substantially the same value as that applied during the operations in the attraction mode and then reduced to a smaller, but larger than zero, value. Finally the exciting current is changed to a negative value for releasing the load. During the above operations, the magnetic flux density of the lifting magnet, the square of which is proportional to its magnetic force, changes, as shown in FIG. 3A, according to variation of the exciting current. That is, in the attraction mode, the exciting current provides excitation H.sub.1, whereby the magnetic flux density is changed to B.sub.4 at point a lying within the saturation area of the hysteresis curve of the magnet and the load is attracted to the lifting magnet by its magnetic force. In the lifting mode the exciting current is reduced to zero and thereby the magnetic flux density is changed to a smaller value B.sub.1 at point b corresponding to its remanence. With this magnetic flux density, the operation is made to lift the load. If the magnetic force is in sufficient to hold the load during the transportation, the lifting magnet is unable to lift the load. Thus it is detectable by operations in the lifting mode whether the magnetic force due to the residual magnetic flux is sufficiently strong to hold the load during the transportation. Thereafter, the exciting current is increased to exhibit excitation H.sub.1 so that the magnetic flux density changes through point e, where its value is B.sub.3, to point a where its value is substantially equal to B.sub.4 as developed during the initial attraction and subsequently, the excitation is reduced to H.sub.3 so that the magnetic flux density changes to point d where its value is B.sub.2. The magnetic flux density is held B.sub.2 during the operations in the transportation mode. The values B.sub.2, B.sub.3 and B.sub.4 of the magnetic flux density, which are held during the transportation, are larger than B.sub.1 which is held during the lift mode operations. Thus, the magnetic flux density is increased during the transportation as if the air gap exciting in the magnetic circuit during the lift of loads is eliminated thereby being capable of assuring safety transportation. Finally the magnetic flux density is reduced by a negative excitation H.sub.2 for releasing the loads.

In the case of operations as shown in FIGS. 4A and 4B, the lifting matnet is subject to negative excitation during the lift mode operations as shown in FIG. 4B, and thereafter to positive excitation in the same manner as discribed referring to FIGS. 3A and 3B during the transportation mode operations. However, in this case, the magnetic flux density changes, due to the negative excitation H.sub.4, to B.sub.5 at point f which is smaller than B.sub.1 corresponding to the remanence of the magnet and the difference between the magnetic flux density during the lift of loads and that during the transportation becomes larger than the case of FIGS. 3A and 3B, resulting in more safety transportation.

FIGS. 5A and 5B show another embodiment in which the lifting magnet is controlled to exert a smaller attractive force than its allowable capacity for limiting the weight of loads to a predetermined value, for example, the number of sheets of magnetizable material such as steel to be transported to a predetermined value, thereby assuring attraction of a predetermined number of such sheets and safety transportation thereof. In this case, during the attraction mode operations, the excitation is set at such a value H.sub.5 that the magnetic flux density reaches point i lying below the saturation area on its hysteresis curve and, during the lifting mode operation, reduced to zero thereby changing the magnetic flux density to B7 at point j with which the lifting magnet lifts loads. During the transportation mode operations, the excitation is once increased to H.sub.1 thereby changing the magnetic flux density through point i to H.sub.1 at point a lying within the saturation area and then again reduced to zero thereby returning the magnetic flux density to a value B.sub.1 corresponding to its remanence, which value B.sub.1 is larger than the value B.sub.7 presented during the lift mode operations, whereby the lifting magnet holds strongly loads by a larger magnetic force during the transportation mode operations than during the lift mode operations.

Now referring to FIG. 6 showing a circuit diagram of a system for use with the method according to the present invention, a power circuit 1 is connected at its input side to a power source (not shown) and its output side to a contactor circuit 2, which is adaptable to supply an exciting current to a lifting magnet 5 by means of a power cable which is supported by a cable reel 3 and connected to the lifting magnet through a cable connector 4. If a plurality of such lifting magnets are utilized, a branch box 6 is employed for connecting the power cable to them. The power circuit 1 may include a rectifier means for converting AC power to DC power if an AC power source is utilized. The contactor circuit includes a plurality of contactors for changing the connection of a resistor circuit 9 thereby controlling the exciting current supplied to the lifting magnet, a controller 7 adaptable to control the operation of the contactors for selectively changing the exciting current depending on the operating modes of attraction, lift, transportation and release of loads, and an alarm 8 for warning failure of the power supply. The exciting current may be changed automatically in a predetermined time sequence by a timer means which additionally provided in the controller or, alternately, manually by the controller.

According to the present invention, once attracted loads are lifted by a lifting magnet with a magnetic flux density not larger than its remanence under that situation and transported with a larger magnetic flux density than the above remanence. Hence it is capable of carrying out transportation of the loads more safely and reliably than the conventional method as described previously and also capable of strongly holding the loads even at failure of the power supply during the transportation. Further by applying a smaller exciting current than that applied during the attraction mode during the transportation, the transportation becomes more safe and reliable, and by applying an exciting current substantially equal to that applied during the attraction mode for a short time in the transportation mode and then applying a smaller exciting current as above mentioned during the remaining period, the lifting magnet exhibits a magnetic flux density corresponding to a given point on its saturation hysteresis curve whereby more safe transportation is assured.

Claims

1. A method of transporting loads by utilizing a permanent magnet type lifting magnet having a permanent magnet provided in its magnetic circuit and an exciting winding for controlling the magnetic flux produced in said magnetic circuit, said method comprising the steps of:

a. applying an exciting current of a first value to said exciting winding during the operations in an attraction mode in which a load to be transported is attracted to the lifting magnet,

b. reducing the exciting current to a second value of zero or less thereby making the magnetic flux produced by the circuit less than the residual magnetic flux due to its remanence during the operations in a lifting mode in which the load is lifted, and

c. increasing the exciting current to a third value larger than zero thereby making the magnetic flux produced by the circuit larger than the residual magnetic flux corresponding to the remanence under the condition in the lifting mode during the operations in a transportation mode in

2. A method according to claim 1, wherein the third value of the exciting current is held larger than zero thereby providing the lifting magnet with magnetic flux larger than the value corresponding to the remanence under the condition in the lifting mode during the operations in the

3. A method according to claim 2, wherein the exciting current during the operations in the transportation mode is not larger than said first value.

4. A method according to claim 1, wherein said exciting current during the operations in the transportation mode is once increased to substantially the same value as said first value and then reduced to and held at a

5. A method according to claim 1, wherein said third value of the exciting current is larger than said first value.