Abstract:
Apparatus for detecting excessive slippage of a conveyor belt, as used, for example, in the coal mining industry includes electronic switching devices. A ring having a plurality of magnets thereon is mounted on a belt roller, and a detecting coil produces voltage pulses in response to the passage of the magnets. The voltage pulses are amplified, squared, and replaced by fixed width pulses at a frequency proportional to belt speed. The fixed width pulses are integrated to provide an average voltage having a value proportional to the conveyor belt speed. The average voltage is compared with an adjustable reference, and is used to turn off the belt driving motor when belt speed drops below a predetermined threshold.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to slip-responsive devices, and more specifically to devices for shutting off driving motors when the element driven thereby experiences excessive slip. 
     2. Description of the Prior Art 
     Slipping switches are used extensively in the coal mining industry and are generally required on mine conveyor belts. Centrifugally controlled mechanical switches have been used extensively but such switches have proven unreliable and are subject to wear and damage due to improper lubrication. A switch jammed in a closed position because of a mechanical failure will not disconnect a motor when belt speed drops. An electronically activated switch avoids such problems. 
     A prior art attempt to solve the problems associated with using mechanical switches for conveyor belt slippage detectors is illustrated by Stiebel U.S. Pat. No. 3,845,375 which teaches the generation of pulses, used to trigger a monostable multivibrator thereby producing a sequence of pulses whose frequency is proportional to the belt speed. In operation, a capacitor is charged linearly, the voltage thereon being used to cause a unijunction transistor to fire, thereby deactivating a relay and de-energizing the motor&#39;s circuit. Stiebel, however, utilizes the interruption of a light beam to generate the pulse, and relies upon many discrete electric components which results in a rather complex circuit design. 
     Other prior art references of interest include Kada et al. U.S. Pat. No. 3,877,004 and Hinz U.S. Pat. No. 3,130,393. Neither of these teachings provide for automatic cut-off of the drive motor upon detection of slippage. 
     SUMMARY OF THE INVENTION 
     The instant invention provides a slip-detection apparatus, overcoming the deficiencies of the prior art, by using a simple transducer-- pick-up combination, and comprising high-gain operational amplifiers to disconnect a driving motor in a material conveying system responsive to conditions of excessive slippage. 
     It is thus a primary object of the invention to provide an accurate slip-detection and control device. 
     An additional object of the invention is to provide a slip-detection apparatus utilizing a novel magnetic transducer, readily manufactured and assembled, without providing any mechanical imbalance to the system being monitored. It is a further object of the invention to provide a slip-detection apparatus utilizing highly reliable operational amplifiers in a circuit requiring few components. Yet another object of the invention is to provide a slip-detection apparatus having a variable threshold slip for use in different environments for variable permissible slip levels. 
     The device of the invention overcomes the disadvantages of the prior art by providing a ring, having a pluraity of magnets thereon, for operation in conjunction with a belt roller and a magnetic pick-up coil, to generate pulses responsive to belt speed. The pulses are shaped and converted to provide a DC voltage level proportional to conveyor speed. When the voltage representing conveyor speed drops below an adjustable reference, a relay is de-energized, thereby disconnecting the conveyor driving motor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and features of the invention will become clear from the detailed description thereof, which is intended merely to represent one embodiment of the invention and not in any way to limit the scope of the invention thereto, wherein; 
     FIG. 1 is an illustration of the speed-detection apparatus used in conjunction with a belt roller; 
     FIG. 2 is a block diagram of the electronic portion of the apparatus shown in FIG. 1; 
     FIG. 3 is a detailed schematic diagram of the apparatus of FIG. 2; and 
     FIG. 4 is an illustration of the transfer function of a possible averaging circuit as used in FIG. 2. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a conveyor belt 10 is shown driving a belt roller 12 having thereon a ring 13 with a plurality of magnets 14 thereon. It is within the scope of this invention that the roller be an idling roller, or an idling pulley, or an end roller being rotated by belt 10. Ring 13 may be affixd to roller 12 by bonding, drilling a keyed pilot hole, or any other means for attaching such a ring to an edge of a rotating member. It is within the scope of the invention to have magnets 14 bonded directly to the edge of roller 12. 
     Coil 16 is used to detect the passage of magnets 14 thereby, producing voltage pulses on conductors 17 as a result thereof. Since it is only relative motion between the magnets and the coil which is of interest, it is conceivable that a coil may be mounted on the roller and a magnet be fixed adjacent thereto. It is further contemplated that any magnetically sensitive detecting means may be used in place of coil 16, or that other, non-magnetic speed sensing means might be employed. 
     The pulses generated by magnetic detecting means 16 are conducted to slip detection unit 18, which produces an output signal on conductor 19 to relay control circuits 20, the latter controlling the flow of current to a driving motor 21 for the conveyor belt 10. 
     FIG. 2 illustrates in block diagram form the make-up of slip detection unit 18. The pulses produced by magnetic detecting means 16 are illustrated by a waveform 22 which is amplified by gain stage 24 to provide a waveform of sufficient voltage to be shaped by a wave-shaping stage 28. 
     The wave-shaping stage 28, comprising a squaring means such as a clipping circuit or a Schmidt trigger, produces a square pulse 30 responsive to the occurrence of pulse 26. A pulse-generating means 32 is triggered by pulse 30. Pulse 30 may have widely varying times of duration, corresponding to the speed of rotation of roller 12. Generator 32 produces a sequence of fixed-duration pulses 34, each in response to a leading edge of a pulse 30. Pulse generator 32 may comprise, for example, a monostable multivibrator, and more specifically, a non-retriggerable monostable multivibrator. Thus, taking the area under pulses 34 in a given period of time, and providing an output voltage proportional to that area, effectively yields an output voltage which is proportional to the frequency of the pulses. The process of responding to the area in a period of time is, in effect, averaging the pulse train over a period of time. 
     Averaging circuit 36 may have a voltage to frequency transfer function as shown in FIG. 4 where the output voltage, shown as the ordinate therein, increases linearly with the frequency of pulse train 34. The output of circuit 36 is not a ramp voltage, but a varying voltage dependent on the frequency of pulses 34. 
     With the assumption tht driving motor 21 operates at a constant speed, it is apparent that if the output of averaging circuit 36 drops, then belt 10 is moving at a slower speed, and thus is slipping with respect to the driving motor 21. An adjustable reference voltage source 38 produces a reference voltage which may be selected to correspond to a particular driving motor operating speed, such that the average voltage will drop below that reference voltage only when the belt slip exceeds a particular allowable value. This determination is made by comparing circuit 40, which compares the output of averaging circuit 36 with the adjustable reference voltage. 
     Comprising means 40 may be, for example, a comparator whose output changes from a low to a high value when the average voltage drops below the reference voltage. It is, of course, perfectly possible to reverse the connections of the reference voltage and the average voltage to the inputs of comparator 40, thereby achieving a low output voltage of the comparator when the average voltage drops below the reference. 
     The output of comparator 40 is used to drive relay control circuit 20. Control circuit 20 may be any device which causes energization or de-energization of a relay in the current supply circuit of motor 21. One such apparatus may be transistor, acting as a switch turned on and off by the output of comparator 40. Since the transistor is in series with a relay, it thereby causes the relay to be energized or de-energized in accordance with the output of comparator 40. The control circuit may, of course, be any other device which may be switched on or off, such as a silicon controlled rectifier, a unijunction transistor, a programmable unijunction transistor, a flip-flop, a field effect transistor or any other electronic equivalent of a switch. 
     FIG. 3 shows a detailed schematic diagram of the block diagram of FIG. 2. Specifically, a transformer 52 is used to couple the signal generated by magnetic detector means 16 to gain stage 24. The gain is provided by operational amplifier 54, having a feedback resistor 56 used for gain adjustment. The output of gain stage 24 is provided as input to wave shaping stage 28, shown as a Schmidt trigger comprising a high-gain amplifier 56 which drives the input to clipping, thereby squaring any input waveform. The output of shaping stage 28 is coupled by capacitor 58 to triggered pulse generator 32, comprising operational amplifier 60 used in a non-retriggerable monostable multivibrator configuration. 
     The output of generator 32 is coupled by capacitor 62 to the input of averaging circuit 36, utilizing operational amplifier 64 in an integrator circuit as shown. The output of averaging circuit 36 is provided as one input to comparator 66 in the comparing circuit 40. The other input to comparator 66 is provided by adjustable reference source 38, shown as potentiometer 68. 
     The output of comparator 66 is used to drive base 70 of transistor 72. For illustrative purposes only, transistor 72 is shown as a PNP transistor, having its emitter 74 connected through a manually operable pushbutton switch 76 to operating coil 78, the latter energizing a relay 79 when current passes therethrough. The relay 79 is connected to turn on or off driving motor 21. The other terminal of operating coil 78 is connected to power supply 80 which also supplies power to all the other circuits used in the detection unit. 
     It is completely within the scope of the invention to use an NPN transistor instead of the PNP transistor as shown. The only required modifications would be to connect the output of averaging circuit 36 to the positive input terminal of comparator 66, and the reference voltage to the negative input thereof, as previously described. Additionally, the emitter would be connected to ground terminal 81, while the collector of the NPN transistor would be connected to switch 76. 
     In operation, the input pulses on conductors 17 are coupled to gain stage 24 by transformer 52. Stage 24 provides sufficient gain to the pulses for activation of Schmidt trigger 28, producing square pulses as inputs to triggered pulse generator 32. The output of generator 32, a sequence of fixed width pulses, is averaged by averaging circuit 36, producing an output voltage linearly dependent on the frequency of rotation of roller 12. When the frequency drops below a level established by reference source 38, the output of comparator 66 rises to a voltage which turns transistor 72 off. The emitter terminal 74 then rises in voltage, effectively removing ground from operating coil 78, thereby de-energizing the relay associated therewith. De-energization of the relay interrupts the flow of current to motor 21, thereby turning off the driving motor for conveyor belt 10 in the situation of excessive slippage. 
     It is noted that switch 76 is provided for manual shut-off of the motor in case of circuit malfunction. 
     Diode 82 is provided to permit the current in operating coil 78 to decay without burning out transistor 72 due to the high voltage that would be generated by an abrupt turnoff of the current in the coil. 
     Having thus described the objects, operation and advantages of the embodiment of the invention, it will be obvious to those skilled in the art that various modifications and changes may be made thereto. The scope of the invention as defined by the appended claims is not limited to the above described embodiment and includes the modifications thereof.