Patent Abstract:
A magnet controller supplied by a DC generator controls a lifting magnet. Four transistors, forming an H bridge, allow DC current to flow in both directions in the lifting magnet. During “Lift”, full voltage is applied to the lifting magnet. During “Drop”, reverse voltage is applied briefly to demagnetize the lifting magnet. At the end of the “Lift” and the “Drop”, most of the lifting magnet energy is returned to the DC generator. A transient voltage suppressor protects against voltage spike generated when current reverses in the generator.

Full Description:
RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 11/757,304, filed Jun. 1, 2007, which is hereby incorporated herein by referenced in its entirety. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for controlling a lifting magnet of a materials handling machine for which the source of DC electrical power is a DC generator. It finds particular application in conjunction with lifting magnets used on crawlers in the scrap metal industries. 
     2. Prior Art 
     Lifting magnets are commonly attached to crawler booms to load, unload, and otherwise move scrap steel and other ferrous metals. 
     While lifting magnets have been in common use for many years, the systems used to control these lifting magnets remain relatively primitive. During the “Lift”, a DC current energizes the lifting magnet in order to attract and retain the magnetic materials to be displaced. At the end of the “Lift”, when the materials need to be separated from the lifting magnet, most of the controllers automatically apply a reversed voltage across the lifting magnet for a short period of time to allow the consequently reversed current to reach a fraction of the “Lift” current. This phase is known as the “Drop” phase, during which a magnetic field in the lifting magnet of the same magnitude but in an opposite direction of the residual magnetic field is produced that the two fields cancel each other. When the lifting magnet is free of residual magnetic field, all scrap metal detaches freely from the lifting magnet. This is known as a “Clean Drop”. 
     Some known control systems operate to selectively open and close contacts that, when closed, complete a “Lift” or “Drop” circuit between the DC generator and the lifting magnet. At the end of the “Lift”, which is called the “discharge” and at the end of the “Drop”, which is called the “secondary discharge”, these systems generally use either a resistor or a varistor to discharge the lifting magnet&#39;s energy. The higher the resistor&#39;s resistance value or varistor breakdown voltage, the faster the lifting magnet discharges, but also the higher the voltage spike across the lifting magnet. High voltage spikes cause arcing between the contacts. In addition, fast rising voltage spikes also eventually wear out the DC generator collector and its winding insulation, the lifting magnet insulation, and the insulation of the cables connected to the lifting magnet and the generator. To withstand these voltage spikes, generally in the magnitude of 750 V DC with systems using DC generators rated 240 V DC, the lifting magnet, cables, and the control system contacts and other components must be constructed of more expensive materials, and must also be made larger in size. These systems waste lifting magnet&#39;s energy. Lifting magnet&#39;s energy is transformed into heat, dissipated through a voltage suppressor or resistor bank. This results in poor system efficiency and oversized components to dissipate the heat. 
     To avoid these issues, some other known control systems connect directly to DC generator excitation shunt field. They eliminate arcing across contacts and minimize voltage spikes in the lifting magnet circuit but at the expense of a slower response time, caused by the induced DC generator time constant. 
     SUMMARY 
     A new and improved method and apparatus for controlling a lifting magnet is provided. 
     In one embodiment, the lifting magnet energy produced during the “Lift” phase is returned to the DC generator which in turn converts it back into mechanical energy. 
     In one embodiment, a Transient Voltage Suppressor (TVS) is provided to control DC generator maximum voltage when current is reversed in the DC generator. 
     In one embodiment, a circuit is provided to protect the TVS against overload. TVS overload can occur, for example, by accidental disconnection between the controller and the DC generator such that energy stored in the lifting magnet cannot be returned to the DC generator. 
     In one embodiment, at least a portion of the energy stored in the lifting magnet is returned to the source rather than being dissipated in resistor, varistor, or other lossy elements. 
     In one embodiment, switching of current for the magnet is provided by solid-state devices. 
     In one embodiment, the control system is configured to reduce voltage spikes in the lifting magnet circuit. 
     In one embodiment, the control system is configured to increase the useful life of the lifting magnet, the generator supplying power to the lifting magnet, and/or the associated circuitry. 
     In one embodiment, the control system is configured to reduce the “Drop” time. Shorter “Drops” helps to increase production by reducing lifting magnet cycle times. Some existing systems are using a resistor, which causes voltage to decay with the current leading to a longer discharge time. This invention uses a constant voltage source provided by the DC generator to discharge the lifting magnet energy, allowing a faster discharge. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates a lifting magnet controller circuit. 
         FIG. 2  graphically shows a voltage and current signals as the lifting magnet is operated through “Lift” and “Drop” cycle. 
         FIG. 3  shows the circuit of  FIG. 1  during the “Lift” mode. 
         FIG. 4  shows the circuit of  FIG. 1  during the “Lift” off mode. 
         FIG. 5  shows the circuit of  FIG. 1  during the Discharge mode. 
         FIG. 6  shows the circuit of  FIG. 1  during the “Drop” mode. 
         FIG. 7  shows the circuit of  FIG. 1  during the “Drop” off mode. 
         FIG. 8  shows the circuit of  FIG. 1  during the secondary discharge mode. 
         FIG. 9  shows the circuit of  FIG. 1  during an open circuit in the “Lift” mode. 
         FIG. 10  shows the circuit of  FIG. 1  during the Freewheel TVS protection mode after the “Lift” mode. 
         FIG. 11  shows the circuit of  FIG. 1  during an Open circuit in the “Drop” mode. 
         FIG. 12  shows the circuit of  FIG. 1  during the Freewheel TVS protection mode after the “Drop” mode. 
         FIG. 13 , consisting of  FIGS. 13A-13K , is a schematic diagram of one embodiment of the logic controller. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically illustrates a lifting magnet controller circuit that includes a logic controller  108 . Outputs from the logic controller  108  are provided to respective switches  101 ,  102 ,  103  and  104 . One of ordinary skill in the art will recognize that logic controller  108  can be a Printed Circuit Board, Programmable Logic Controller, etc. The switches  101 - 104  are configured in an “H” bridge arrangement to provide current to a magnet  150 . The switches  101 - 104  can be any type of mechanical or solid-state switch device so long as the devices are capable of switching at a desired speed and can withstand the desired current and voltage. For convenience, and not by way of limitation,  FIG. 1  shows the switches  101 - 104  as insulated gate bipolar transistors. One of ordinary skill in the art will recognize that the switches  101 - 104  can be bipolar transistors, insulated gate bipolar transistors, field-effect transistors, MOSFETs, etc. 
     In  FIG. 1 , a first output from the logic controller  108  is provided to a gate of the switch  101 , a second output from the logic controller  108  is provided to a gate of the switch  102 , a third output from the logic controller  108  is provided to a gate of the switch  103 , a fourth output from the logic controller  108  is provided to a gate of the switch  104 . An emitter from the switch  101  is provided to a first terminal of the magnet  150  and to a collector of the switch  102 . An emitter from the switch  103  is provided to a second terminal of the magnet  150  and to a collector of the switch  104 . Flyback diodes  111 - 114  are provided to respective collectors and emitters of the switches  101 - 104 . 
     A positive output from a DC generator  101  is provided through a fuse  130  to a first terminal of a current sensor  121 . A second terminal of the current sensor  121  is provided to a first terminal of a transient voltage suppressor (TVS)  123 , and to the collectors of the switches  101  and  103 . A negative output from the DC generator  101  is provided through a current sensor  122  to a first terminal of a resistor  124  and to the emitters of the switches  102  and  104 . A second terminal of the resistor  124  is provided to a second terminal of the TVS  123 . 
     The transistors,  103  and  102  form the “Lift” circuit, and transistors  101  and  104  form the “Drop” circuit. One of ordinary skill in the art will recognize that when any of the diodes  111 - 114  are forward biased, the switch  101 - 104  can be closed to provide a current path in parallel with the diode (e.g., to protect the diode, to provide a lower impedance path for current, etc.) Thus, for example, during discharge and/or drop, the switches  104  and  101  can be closed to provide current through the switches, or open to allow current to flow through the respective diodes. The current sensors  121 ,  122  can be configured as Hall Effects sensors, current shunts, resistors, current transformers, etc. The current sensors  121 ,  122  monitor current and detect “Drop current threshold” current, short-circuits, and ground faults. The system  100  (shown in FIGS.  1  and  3 - 12  as the system  100  with the addition of the generator  101 , the fuse  130  and the magnet  150 ). controls the maximum voltage when current reverses direction in the generator. The resistor  124  is provided to monitor energy dissipated in the TVS  123 . 
       FIG. 2  shows voltage and current during the lift mode. When the operator activates “Lift” at time “L”, the logic controller  108  closes the switches  103  and  102 . Current flows from the generator  101  to the magnet  150 . Current from the DC generator  101  is applied to the lifting magnet through the switches  103  and  102  as shown in  FIG. 3 , and the current ramps to the lifting magnet rated current value. The operator ends “Lift” at time “D 1 ”, whereupon the circuit is configured shown in  FIG. 4 , the voltage rises to the TVS breakdown value, and the current in the lifting magnet decays. When the current direction reverses in the DC generator (at time D 2 ), the circuit is as shown in  FIG. 5  where the lifting magnet energy discharges into the DC generator. When the lifting magnet energy is released (at time D 3 ), current in the lifting magnet reaches zero and then starts to ramp in the reverse direction as shown in  FIG. 6 . When the current value becomes equal to the “Drop current threshold” (at time D 4 ), the circuit is in the configuration shown in  FIG. 7 , the voltage steps to TVS breakdown value, and the current in the lifting magnet decays. When the current direction reverses in the DC generator (at time D 5 ), the circuit is as shown in  FIG. 8 , the lifting magnet energy discharges into the DC generator, and the current decays until substantially all lifting magnet energy is released (at time D 6 ). 
       FIG. 3  shows current in the system  100  during the “Lift” mode. During lift, the logic controller  108  keeps the switches  101  and  104  open (e.g., off), and closes (e.g., turns on) the switches  103  and  102 . Current flows from the positive terminal of the DC generator  101  through the switch  103 , through the lifting magnet  150 , through the switch  102  and back to the generator  101 . Rated current establishes in the lifting magnet  150  after a few seconds, based on the time constant of the circuit, which is primarily due to the inductance to resistance ratio (L/R) of the lifting magnet  150 . 
       FIG. 4  shows current in the system  100  during the “Lift” off mode. When operator needs to release the material being lifted by the magnet, the operator instructs the logic controller  108  to start the drop process. The drop process includes lift off ( FIG. 4 ), discharge ( FIG. 5 ), drop ( FIG. 6 ), drop off ( FIG. 7 ) and secondary discharge ( FIG. 8 ). During lift off, switches  103  and  102  are turned off and a few milliseconds later switches  101  and  104  are turned on. Due to the inductance of the generator, the generator current is still flowing in the same direction as it was flowing during “Lift”. Because the switches  103  and  102  are off, the generator current flows through the TVS  123 . Due to the inductance of the lifting magnet, the lifting magnet current is still flowing in the same direction as it was flowing during “Lift”. So, if for example, during “Lift”, a current of 100 Amps was flowing through the DC generator  101  and the lifting magnet  150 , at the time  103  and  102  turn off, a current of 200 amperes flows through the TVS  123 , with the DC generator  101  contributing for 100 amperes, and the lifting magnet  150  contributing for  100  amperes. 
       FIG. 5  shows current in the system  100  during the discharge mode. The lifting magnet  150  has a longer time constant than the DC generator  101 , so the direction of current will reverse in the DC generator  101  before it can reverse in the lifting magnet  150 . When the DC generator  101  allows current to reverse its direction, the lifting magnet current flows back into the DC generator  101 . The difference of potential V M2 -V M1  across the lifting magnet is positive. Therefore, the lifting magnet  150  acts as a source of energy, and energy from the lifting magnet is transferred from the lifting magnet  150  to the DC generator  101 . 
       FIG. 6  shows current in the system  100  during the “Drop” mode. During drop mode, switches  101  and  104  are closed. When there is insufficient energy left in the lifting magnet  150  to maintain the reverse current flow into the DC generator  101 , the DC generator  101  generates a “reverse” current in the lifting magnet  150 . Based on the time constant of the circuit, the reverse current gradually increases. 
     In one embodiment, the switches  101  and  104  are closed during the lift-off phase. Since the flyback diodes  114  and  111  are forward biased during the lift-off phase, the switches  101 ,  104  need not to be forward biased (in other words, the switches  101 ,  104  can be closed by the logic controller  108  but nevertheless not conducting current because they are reversed biased). Once the magnet  150  is discharged, the current through the magnet will reverse during the drop phase and thus the switches  101 ,  104  will become forward biased. 
       FIG. 7  shows current in the system  100  during the “Drop” off mode. When the current measured by the current sensor  121  (and/or the current sensor  122 ) matches the “Drop current threshold”, the logic controller turns the switches  101  and  104  off. Due to the inductance of the generator  101 , the generator current is still flowing in the same direction as it was flowing during “Drop”. Because all of the switches  101 - 104  are off, generator current flows through the TVS  123 . Due to the inductance of the lifting magnet  150 , the lifting magnet current is still flowing in the same direction as it was flowing during “Drop”. If for example, during the “Drop” a “reverse” current of 20 Amps was flowing through the DC generator and the lifting magnet, at the time the switches  101  and  104  turn off, 40 amperes would flow in the TVS  123 , with the DC generator  101  contributing for 20 amperes, and the lifting magnet  150  contributing for 20 amperes. 
       FIG. 8  shows current in the system  100  during secondary discharge. The lifting magnet  150  has a longer time constant than the DC generator  101 , so the direction of current will reverse in the DC generator  101  before it can reverse in the lifting magnet  150 . When the DC generator  101  allows current to reverse its direction, the lifting magnet current flows back into the DC generator  101 . The difference of potential V M1 -V M2  across the lifting magnet is positive. Therefore the lifting magnet  150  acts as a source of energy, and energy is transferred from the lifting magnet  150  to the DC generator  101 . Then the “reverse” current into the generator  101  gradually decays to zero when all the energy left in the lifting magnet  150  is dissipated. 
       FIG. 9  shows current in the system  100  during an open circuit in the “Lift” mode. If during “Lift”, the DC generator  101  is accidentally disconnected, such as in the case of a loose connection or if the fuse  130  opens, the path for the lifting magnet current is through the circuit formed by the diodes  111 ,  114  and the TVS  123 . In one embodiment, the TVS is not sized to absorb all the lifting magnet energy. The logic controller  108  measures the current in the TVS  123  by sensing a voltage across the resistor  124 . If excess current in the TVS  123  is detected, then the circuit switches into “Freewheel TVS protection” mode to protect the TVS  123  against overload. 
       FIG. 10  shows current in the system  100  during the “Freewheel TVS protection” mode after an open circuit in the “Lift” mode. In the “Freewheel TVS protection” mode, the switch  103  is closed and the diode  111  is forward biased, thus providing a loop for the current circulating in the lifting magnet  150  to maintain the same direction that it had during “Lift”. 
       FIG. 11  shows current in the system  100  during an open circuit in the “Drop” mode. If during “Drop”, the generator  101  is accidentally disconnected such as in the case of a loose connection or if the fuse  130  opens, the path for the lifting magnet current is through the circuit formed by the diodes  113 ,  112  and the TVS  123 . In one embodiment, the TVS  123  is not sized to absorb all the lifting magnet energy. The logic controller  108  measures the current in the TVS  123  by sensing a voltage across the resistor  124 . If excessive current in the TVS  123  is detected, then the circuit switches into “Freewheel TVS protection” mode to protect the TVS  123  against overload. 
       FIG. 12  shows current in the system  100  during the Freewheel TVS protection mode after an open circuit in the “Drop” mode. In “Freewheel TVS protection” mode, the switch  101  is closed and the diode  113  is forward biased, thus providing a loop for the current circulating in the lifting magnet  150  to maintain the same direction that it had during “Drop”. 
     reewheel TVS protection mode is not polarity sensitive. When a TVS overload is detected, Freewheel TVS protection mode is activated by closing switches  101  and  103  to divert the current from the TVS. As described above, the switch  101  can be closed to form a loop with diode  113 , and the switch  103  can be closed to form a loop with diode  111 . 
     Logic controller  108  monitors currents passing through sensors  121  and  122 . If an unbalance occurs, then the logic controller  108  signals a ground fault alarm. In one embodiment, the logic controller  108  will turn off the switches  101 - 104  if an overload condition is detected. 
       FIG. 13 , consisting of  FIGS. 13A-13E , is a schematic diagram of one example circuit embodiment for the logic controller. In  FIG. 13 , a LIFT INPUT is received from a “Lift” user control (e.g., a such as, for example, a lift push button provided to the circuit of  FIG. 13  via an opto-isolator). The “Lift” control initiates the “Lift” operation. After the “Lift” push button is released, circuit stays in “Lift”. A thermostat that senses the temperature of the one or more of the switches  101 - 104  (or a heat-sink for the switches  101 - 104 ) can be provided to a THERMOSTAT input shown in  FIG. 13 . If the switches get too hot, the thermostat sends a signal to the THERMOSTAT input that prevents initiation of the next Lift operation, however, a lift currently in progress is not terminated (for safety reasons). A “cycle” control (e.g., push button and associated electronics) can be provided to a CYCLE INPUT. The “Cycle” control can be used to replace (or supplement) the lift and drop controls. Activating the cycle control (e.g., pressing the cycle button) causes the status of the Magnet Controller to cycle through “Lift”, then “Drop” and automatically to “OFF”, and then again to “Lift” etc. Basically U301A with its complemented output fed in its data input acts as a divider by 2. A POWER UP RESET line is temporary held ON when control power is applied (or after power has been cycled to reset a fault) to set the status of D Type Flip-Flop (latches) in the circuit. A DROP INPUT receives signals from a “Drop” control (e.g., a “Drop” push button and associated opto-isolator and electronics). The “Drop” push button terminates the “Lift” and initiates the “Drop”. After the “Drop” push button is released, the circuit finishes “Lift” and then automatically goes to “Off”. A NO CONTROL POWER input is configured to receive a signal indicating that the 24V DC power supply has fallen below 18V. A typical 24V to 15V voltage regulator needs at least 18V on its input to guarantee 15V output. So if control power supply is too low, to protect against unexpected behavior, the switches  101 - 104  are turned off when the NO CONTROL POWER signal is received. The “Drop” current can be adjusted by an optional potentiometer P 201 . An HE POS input receives current sensor signals from the current sensor  121 . An HE NEG input receives current sensor signals from the current sensor  122 . A SHORT CIRCUIT input is provided to receive a signal if an overload or short condition is detected. A connector CN 521  provides inputs from the TVS current sensor  124 . The circuit of  FIG. 13  is configured to use a 0.1 ohm resistor as the TVS current sensor. If a TVS overload signal is received at the TVS input, the switches  101  and  103  are then turned on to protect  123 . 
       FIG. 13B  shows “LIFT” and “DROP” outputs. The “LIFT” output is provided to drivers that control the switches  102  and  103 . The “DROP” output is provided to drivers that control the switches  101  and  104 . The “LIFT” output is activated to produce the lift function. The “DROP” output is activated to control the drop function. 
     It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrated embodiments and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributed thereof; furthermore, various omissions, substitutions and changes may be made without departing from the spirit of the inventions. The foregoing description of the embodiments is, therefore, to be considered in all respects as illustrative and not restrictive, with the scope of the invention being delineated by the appended claims and their equivalents.

Technology Classification (CPC): 1