Abstract:
A current monitoring system for an electric motor, whereby the current is halted to the motor if the electrical load exceeds calculated electrical load limits. The system includes a current measuring device, sensors for detecting operational functions, a system for setting an electrical current limit setpoint, and a switch for halting application of electrical current to the motor. A method of halting an electrical motor if the electrical load exceeds a calculated electrical current limit is also disclosed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an overcurrent monitoring system for use with an electrical motor and, in particular, a system for determining press operating conditions based on the electrical load on an electrical press drive motor used in a machine press. 
     2. Description of the Related Art 
     As a machine press operates, there are variations in the electrical load drawn or created by the press motor. If the current load on the motor exceeds the motor&#39;s capacity, damage and loss of use may ensue. For example, current overload may cause burnout to the motor and associated portions such as the bearings, bushings, or commutators. 
     Load variations may be attributed to normal press operational functions. Alternatively, load variations may not be the result of normal press functions. For effective protection, it is essential to detect normal press functions which effect electrical load. 
     Normal press operations that effect the electrical load include initial starting of the press drive motor, engaging the press clutch, and varying the press operating speed. Other changes which effect the electrical load include lubricating oil temperature changes and die changes. 
     If the electrical load increase is not attributed to normal press operations, the current increase could be an indication of a potential problem in press operation. For example, when bearings or bushings overheat, the press speed tends to slow down and there is an increase in press motor drive load. Therefore, an increase in the electrical load may indicate that press bushings or bearings are overheating. In order to safeguard the press, it is advantageous to cut power to the motor when an increase in current load is not attributed to a normal press function to permit service personnel to determine the actual cause of the increased motor load. 
     In the art, monitoring systems have been limited to monitoring the effective power consumption of the press motor. In addition, such motor monitoring is not constant; rather, the monitoring is limited to only a portion of the press stroke. 
     It is advantageous to account for variations in motor electrical current draw attributed to normal press operations. Such detection would make it possible to stop the drive motor and press before potential damage may be caused to the motor, press bearings and bushings. In turn, there is a financial benefit in preventing damage to the machine press. 
     The present invention prevents potential damage to a machine press motor and press when the motor electrical current draw is above predetermined limits. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a system accomplishes detection of steady or transient electrical current overloads on a machine press motor. The invention measures the electrical current draw or load on the press drive motor. If the electrical current draw exceeds an acceptable load limit setpoint, i.e. a predetermined or calculated limit setpoint, the new system halts the drive press motor. 
     The invention determines the limit setpoint by incorporating and compensating for the current load variations associated with known operational functions. Such operational functions include, but not limited to, drive motor startup, clutch engagement, and speed change commands. When the inventional system detects one or more of these known operational functions, it may accordingly adjust the limit setpoint. 
     The procedure for adjusting the limit setpoint after the system detects an operational function is to add a predetermined offset to the present percent of full load current. The predetermined offset is a predetermined value which is added to the present percent of full load current to account for the electric load attributable to the detected operational function. 
     The invention utilizes countdown or delay timers when calculating the limit setpoints. These timers allow there to be a temporary increase in electrical load without halting current to the drive motor. When an electrical load is attributable to a detected operational function, a timer is set for a finite period of time. Consequently, the electrical current will not be halted while the timer is counting down. 
     The invention, in one form thereof, calculates a limit setpoint after it detects the press motor startup and activates the drive motor startup delay timer. The system then sets the limit setpoint by adding a predetermined offset to the present percent of full load current. The system continues to update the limit setpoint until the drive motor startup delay timer has expired. 
     The invention calculates a limit setpoint after it detects the press clutch has engaged and starts the clutch engaged startup delay timer. The system then sets the limit setpoint by adding a predetermined offset to the previous limit setpoint. The system continues to update the limit setpoint until the clutch engaged startup delay timer has expired. 
     The invention calculates a limit setpoint after it detects a requested large press speed change and starts the range dropout delay timer. The system creates a speed range which is calculated by taking a predetermined percentage of the requested speed and adding or subtracting the calculated percentage of speed from the requested speed. 
     For example, if the predetermined percentage was 15 percent and the speed request was 100 spm, the speed range would be 100 spm plus or minus 15 percent, in other words, 85 spm to 115 spm. 
     After the timer has expired, if the press speed is not within the requested press speed range, the system sets the limit setpoint by adding a predetermined offset to the present percent of full load current. It continues to update the limit setpoint until the press speed is within the requested press speed range. 
     The invention calculates a limit setpoint after it detects a small press speed change request and starts the new speed holdout delay timer. The system continuously updates the limit setpoint until the new speed holdout delay timer has expired. The system sets the limit setpoint by adding a predetermined offset to the present percent of full load current. 
     The invention also includes a nuisance check system that prevents halting current to the drive motor when the measured electrical load increase is short lived. The invention with this system sets a fault delay timer. While this timer is running, the invention will not halt current supplied to the drive motor. This prevents halting current supplied to the motor when the electrical load exceeds the limit setpoint for a duration shorter than the time period measured by the fault delay timer. 
     The invention further includes an averaging function which calculates the average electrical load over a predetermined period of time, e.g. an eight second period. If the average electrical load has dropped 2.5 percent of full load current below the previous limit setpoint, the limit setpoint is set by adding a predetermined offset to the calculated average electrical load. The purpose of the averaging function is to lower the limit setpoint as the average of the measured electrical load decreases. 
     For example, if the average measured electrical load is 47 and the previous limit setpoint is 50, the invention will compare 47 to 2.5 percent less than the previous set point, i.e., 47.5. Since 47 is less than 47.5, the invention will update the limit setpoint by adding a predetermined offset to the calculated average electrical load. 
     While the invention measures short term variations of electrical load while a machine press is operating, it can easily be adopted to measuring long term variations in electrical load. The invention could store limit setpoints for various different tool sets, die combinations, machine press conditions and tolerances. 
     One embodiment of the present invention is an apparatus for monitoring current draw of an electric motor. It measures the electric current draw of an electric motor and checks for operational functions. The system sets a limit setpoint and compares the limit setpoint to the measured electrical draw. The apparatus may halt current supplied to the motor on various conditions. 
     Another embodiment of the present invention is a process for monitoring a machine press motor supplied with electrical current comprising the following steps. Operational functions which contribute to a load on the motor are detected. The limit setpoint is set based on the detected operational functions. The electrical load on the motor or current draw is determined. The determined electrical load is then compared to the limit setpoint. Current supplied to the motor is halted if the electrical load exceeds the limit setpoint. 
     Yet another embodiment of the present invention is a method for monitoring a machine press motor load. A limit setpoint is calculated based on the detected operational functions effecting electrical load on the motor. The electrical load is compared to the limit setpoint. The electrical current supplied to the motor is halted if the determined electrical load exceeds the calculated limit setpoint. 
     A further embodiment of the present invention is an apparatus for monitoring current overload on a press drive motor. The apparatus comprises a detector for detecting electrical current draw of the motor. In addition, it includes memory means for storing the limit setpoint. It also contains a means for halting electrical current to the motor if the detected electrical current draw exceeds the limit setpoint stored in memory. 
     An advantage of the present invention is that the apparatus removes power from the drive motor when the identified electrical current overload is not attributed to a normal operational cause. 
     Another advantage of the present invention is when a current overload is detected, the apparatus halts the motor and press before potential damage is done to the motor, bearings, bushings, or the rest of the machine press. 
     A further advantage of the present invention is the ability to detect drive motor electrical overcurrent when the press motor is operating at less than one hundred percent capacity of its rated maximum capacity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a flowchart of the program for executing the operation of an embodiment of the invention; 
     FIG. 2 is the continuation of the flowchart of the program in FIG. 1; 
     FIG. 3 is continuation of the flowchart of the program in FIG. 2; and 
     FIG. 4 is a diagram showing an example of one embodiment of the current monitoring apparatus for a machine press drive motor. 
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring generally to FIG. 1-3, there is shown a flow chart illustrating a procedure for monitoring press motor load on a machine press according to one embodiment of the present invention. The procedure broadly involves monitoring electrical draw on a motor while accounting for operational causes of electrical current load, such as, but not limited to, motor startup, press clutch activation, and speed changes. The system sets a limit setpoint based on detected operational functions. The system then halts current to the press motor if the electrical load exceeds the limit set point. 
     Referring specifically to FIG. 1, the monitoring procedure begins at “start.” Branches of step  10 ,  22 , and “A” run concurrently. Steps  10 - 20  corresponds to a procedural branch for motor startup. Steps  22 - 32  refer to a procedural branch for press clutch activation. First, the system determines that the motor has been started (step  10 ). If the motor has been started, the system sets a drive motor startup delay timer for a period of time, preferably, twenty-five seconds (step  12 ). If the system has not detected drive motor startup, the procedure rechecks for motor startup (step  10 ). 
     After the drive motor delay timer has been set, a limit set point is set by adding a predetermined offset to the present percent of full load current (step  14 ). The system checks to see if the drive motor delay timer has expired. If the drive motor startup delay timer has not expired, the system updates the limit setpoint by adding a predetermined offset to the present percent of full load current (step  16 ). The predetermined offset is a predetermined value which is added to the limit setpoint to account for the electric load attributable to the detected operational function. Once the drive motor delay timer has expired, the system stops updating the limit setpoint (step  18 ). 
     The system then checks to see if the press motor has been stopped. After the drive motor delay timer has stopped, the system continuously re-checks to see if the motor has stopped (step  20 ). If the motor has stopped, the procedure starts from the beginning (step  10 ). 
     Steps  22 - 32  refer to the procedure for accounting for electrical load attributed to the press clutch engaging. The procedure determines if the clutch has recently been engaged (step  22 ). If the clutch has not been engaged, the system continues to recheck for clutch engagement (step  22 ). 
     If the system has determined that the clutch has recently been engaged, the system sets a clutch engagement startup delayed timer (step  24 ). A limit setpoint is set by adding a predetermined offset to the present percent of full load current (step  26 ). The system checks to see if clutch engagement start timer has expired (step  28 ). 
     If the timer has not expired, steps  26  and  28  are repeated, whereby the limit setpoint is updated by adding a predetermined offset to the present percent of full load current (step  28 ). When the clutch engagement startup timer has expired, the system stops recalculating the limit set point (step  30 ). 
     Finally, this procedure branch checks to see if clutch was just disengaged. If the clutch was just disengaged, the procedure repeats, starting at step  22 . Alternatively, if the clutch is engaged, the system continuously rechecks to see if it has just disengaged (step  32 ). 
     Referring specifically to FIG. 2, steps  34 - 44  deal with large speed change requests to press speed. The system continuously checks to see if the press speed is within the speed range requested (step  34 ). If the press speed is not within the requested speed range, a speed range dropout timer is set (step  36 ). The system then checks to see if the press speed is within the speed range requested. If the press speed is within the speed range requested, the procedure starts again at step  34  to determine if present speed is within requested range. 
     Alternatively, if the press speed is not within requested speed range, the system checks to see whether the speed range dropout delay timer has expired (step  40 ). If the speed request range timer has not expired, it repeats step  38  (step  40 ). 
     If the speed range drop-out delay timer has expired, a limit setpoint is set by adding a predetermined offset to the present percent of full load current (step  42 ). The system rechecks to see if the press speed is within the speed range requested (step  44 ). 
     If it is not within the speed range requested, step  42  is repeated. Alternatively, if the present speed is within the speed range requested, the procedure goes back to  34  and checks to see if the speed is within the requested range (step  44 ). 
     Steps  46 - 54  refer to small changes to the speed of a press. First, the system determines whether a new speed has been entered (step  46 ). If there has not been a new speed entered, the procedure repeats step  46  to determine whether a new speed has been entered (step  46 ). 
     If a new speed has been requested, a speed holdout delay timer is set (step  48 ). Once again, the system checks to see if a new speed has been entered (step  50 ). If a new speed has been entered, step  48  is repeated. If a new request speed has not been entered, the system checks to see if the speed holdout delay timer has expired (step  52 ). 
     If the speed holdout delay timer has expired, step  50  is repeated. Alternatively, if the speed hold out delay timer has not expired, a limit setpoint is set by adding a predetermined offset to present percent of full load current (step  54 ). Then step  50  is repeated. 
     Steps  56 - 64  refer to a procedural branch stopping the press drive motor if the electrical draw exceeds the limit setpoint. The system first measures the electrical draw on the press motor and then compares the electrical draw to the limit setpoint (step  56 ). If the electrical draw does not exceed the limit setpoint, the system remeasures the electrical load and again compares the electrical load to the limit setpoint (step  56 ). Alternatively, if the electrical current load exceeds the setpoint, the system starts a fault delay timer (step  58 ). The system then measures the electrical load and compares it to the limit setpoint (step  60 ). If the electrical load does not exceed the setpoint, the procedure repeats step  56  (step  60 ). Alternatively, if the electrical load exceeds the setpoint, the system checks to determine whether the fault delay timer has expired (step  62 ). If the fault delay timer has not expired, step  60  is repeated. Alternatively if the fault delay timer has expired, the system halts electrical current to the press drive motor (step  64 ). 
     FIG. 3 refers to setting the limit setpoint when there is a decrease in electrical load. The procedure averages the electrical load over eight seconds (step  66 ) although other time averages may be equivalently used. Next, the procedure compares the average electrical load to the previous limit setpoint. If the average electrical current load has dropped 2.5 percent of full load current below the previous setpoint, the procedure repeats step  66  (step  68 ). Alternatively, if the average electric load has dropped 2.5 percent of full load current below the previous setpoint, the procedure sets a new limit setpoint by making the limit setpoint the calculated average from step  66  (step  70 ). Of course other percentage drops in average electrical load may also be utilized to cause updating of the limit setpoint. 
     The above procedures may be implemented in a conventional PLC or microcomputer. An example apparatus is shown in FIG.  4 . Referring to FIG. 4, there is shown one embodiment of a current monitoring system for a machine press drive motor. The system includes an applied current source  72  which supplies current to press drive motor  84  through a member of different devices. 
     Specifically, applied current  72  is connected to current switch  76  by line  74 . Applied current may continue to current sensor  80  by line  78  is current switch  76  is closed. Applied current from current sensor  80  is connected to press drive motor  84  by a line  82 . Current sensor  80  measures the electrical load on press motor  84 . 
     Current switch  76  is connected to the control unit  106  by line  104 . Control unit  106  sends a signal via line  104  to current switch  76  to either open or close, allowing or halting applied current  72  to press motor  84 . 
     Press start sensor  86  detects a press motor  84  startup. The press start sensor  86  is connected to control unit  106  by line  98 , whereby detection of press motor  84  startup is recorded by control unit  106 . 
     Clutch engagement sensor  88  detects machine press clutch engagement and is connected to the control unit by line  96 , whereby detection of clutch engagement is sent by a signal to control unit  106 . 
     Press speed control  90  detects operator requested changes of press speed. Press speed control  90  is connected to the control unit  106  by line  96  whereby detection of requests changes in press speed are sent by a signal to the control unit. 
     Press speed sensor  92  measures the press speed and transfers the measured speed to the control unit  106  by a line  94 . 
     Other operational sensors  112  may be connected to control unit by line  114  whereby detection signal of other operational functions are sent to the control unit  106 . 
     Control unit  106  contains at least three separate functional units of interest here, including, comparator  107 , timers  108 , and Memory  110 . Memory  110  in combination with the control unit microprocessor calculates and sets a limit setpoint. Comparator  107  compares the limit setpoint to the current draw measured by current sensor  80 . Timers  108  are set as previously when operational sensors such as clutch engagement  88 , press speed control  90 , press speed sensor  92 , press start sensor  86 , or other operational sensors  112  detect operational functions. Timers  108  countdown from a predetermined period of time as previously discussed. 
     While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.