Abstract:
A clearing method for use with a conveyor belt driven by a reversible motor for essentially automatically eliminating swarf or other obstructions along the belt including an clearing process including sensing motor load and comparing sensed load to a maximum load, when the sensed load exceeds the maximum, performing a clearing process for a predetermined time period calculated to, given the clearly process, clear the obstruction.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to swarf collecting apparatus and methods and more specifically to a method of controlling a swarf collecting conveyor to clear swarf obstructions which cause excessive conveyor motor and drive component loading. 
     Many industries routinely employ lathes, drills, mills and other machinery having specially configured cutting bits to shape metal work pieces by removing metal “chips” therefrom. The chips which are removed come in many different shapes and sizes which are collectively referred to as swarf. 
     Many work piece shaping processes require a plurality of machines arranged at sequential work stations along a machine line. In these instances, after shaping at one station, a work piece is conveyed to a subsequent station for further shaping, each station generating swarf during the process. 
     To remove swarf from machine stations, often a conveyor belt is positioned below or adjacent a machine line to automatically catch flushed swarf and convey the swarf to a collection bin at the end of the belt. When the bin is full it is emptied or replaced with another bin. To facilitate use of large collection bins and thereby increase the time between emptying or replacement, most conveyors include a section which conveys upwardly at an inclined angle (e.g. 45 degrees) so that a belt end can be located above an elevated bin wall. To maintain swarf on the belt during inclined conveying (i.e. impede swarf from falling off lateral edges of the conveyor), a conveyor housing including a roof section is typically provided along the inclined section. Conveyor belts are particularly useful where the number of work stations and associated metal removing machines is large. 
     To cool work pieces and machine cutting tools and to flush swarf away from cutting bits during machining, a liquid coolant is typically dispersed at or near the cutting bits. In addition, swarf inside the bin or on the belt may be cooled by direct coolant dispersion thereon. 
     Swarf conveyor belts are typically driven by a motor capable of driving the belt in at least a forward direction. During machining, the belt is continuously driven to convey swarf from work stations. 
     Unfortunately swarf removing systems of the above kind can become obstructed by swarf during operation in at least two different ways. First, swarf can cause conveyor clogging. Only a certain swarf volume can pass though a conveyor housing at any time. Where swarf accumulates adjacent or within a housing, eventually, the accumulation can clog between the belt and housing impeding belt movement. 
     Second, swarf can become entangled between a belt and a stationary conveyor component (e.g. the housing) acting as a harness impeding belt movement. In this case, an elongated piece of swarf, typically a long corkscrew shaped shaving, can become ensnared at opposite ends between the belt and another component restricting belt movement. 
     In addition to damaging belt and other conveyor components, clogging and other forms of belt restriction caused by swarf (and or parts, bar ends, tools, etc.) increase motor load and, at some point, can damage motor components if the load becomes excessive. 
     One solution to belt obstructions has been to equip conveyors with manually operable motors capable of both forward and reverse operation. In this case, when swarf conditions cause motor overloading, an operator can stop the belt, reverse the belt, clear the obstruction and again restart the belt. Removing an obstruction is referred to herein as “clearing”. 
     Unfortunately, this solution to the problem has a number of shortcomings. First, this solution requires an operator to assist what is otherwise an automatic system for removing swarf from work stations. While the operator only needs to act after a clog or entanglement is detected, practically the operator must always be present to identify clogs and entanglements. 
     Second, where the time required to clear a belt is appreciable, an entire machine line may have to be shut down during the clearing process, further increasing costs associated with the system. 
     Third, if the obstruction is not noticed immediately, clogged swarf may cause belt, housing and/or motor damage prior to an operator stopping the belt. 
     Fourth, if the obstruction is not noticed immediately, swarf may accumulate upstream of the clog and fall from the belt. In addition, excessive cooling agent may be flushed into the belt system generally causing a mess or overflowing onto the floor. 
     Fifth, where the obstruction occurs inside the housing, it may be difficult for an operator to identify the obstruction until swarf backs up to the mouth of the housing. 
     Another solution for removing swarf obstructions is to provide an automatic clutch on the motor which allows the shaft which drives the belt to slip when motor load becomes excessive. In this case, instead of damaging motor and conveyor components, a clutch allows the motor to operate with a safe load and the belt stops until an operator can perform a clearing process to remove the obstruction. 
     While this solution reduces the possibility of motor and conveyor component damage, it to is encumbered with shortcomings. For example, this solution still requires an operator to be present to clear every obstruction that occurs. In addition, when the belt is stopped due to overloading, either the entire machine line must be shut down or swarf will continue to accumulate on the belt. Shutting down the entire line is costly. However, swarf accumulation can eventually exceed belt receiving capacity with excess swarf falling off the belt onto a floor surface. This is especially dangerous when swarf is extremely hot as is often the case with metal shavings or the like. 
     Moreover, as swarf accumulates on a stationary belt during clearing, the accumulated swarf causes conditions which will likely lead to further obstruction once the belt is again running in the forward direction. 
     One solution to the swarf jamming problem is described in U.S. patent application Ser. No. 09/081,538 entitled “Method and Apparatus for Controlling Conveyor” filed on May 19, 1998. That application teaches a system wherein conveyor motor load is sensed and, when the load exceeds a predetermined load likely to correspond to a jam, the conveyor is stepped through a jam clearing process a specific number of times, the process and number of times calculated to likely clear the jam. For example, the clearing process may be to reverse the conveyor motor a given number of turns and then, once again, drive the motor in the forward direction. In the alternative the clearing process may be to reverse the conveyor until the conveyor has traveled in the reverse direction a specific distance and then, once again, drive the conveyor in the forward direction. 
     While this solution including counting the number of clearing processes is much better than prior solutions, under certain circumstances even this solution can be insufficient to protect the motor and conveyor components. For example, where each clearing process includes reversing the conveyor motor until a clearing process milestone is achieved prior to driving the motor in the forward direction, the milestone may never be reached if the jam also prohibits reverse conveyor travel. For example, where a clearing process requires 10 motor rotations prior to again driving the motor in the forward direction, if a jam impedes reverse conveyor travel, the 10 rotations are never achieved and the motor may either be damaged or destroyed. Similarly, if the milestone is a specific conveyor reverse travel distance, the reverse distance will never be achieved if reverse motion is impeded. 
     Moreover, even where a jam does not prohibit reverse motion, the jam may impede reverse motion such that reverse motion is slowed to the point where excessive load is placed on the motor. 
     In addition, even with a single machining process swarf characteristics may vary appreciably in ways that affect the optimum clearing protocol. For example, where swarf consists of relatively light weight pieces of metal, the torque required to drive the motor and conveyor in the reverse direction may be much smaller than the torque required to drive in reverse when swarf consists of relatively heavy metal pieces. Given a threshold total amount of motor work acceptable during a clearing process, the threshold is achieved with less clearing processes when the swarf includes heavy pieces and the load is large than with light weight pieces when the load is small. Thus, the optimum number of clearing processes where swarf pieces are light weight will often be greater than the optimum number when the pieces are large. 
     Furthermore, the likelihood of eliminating a jam via a clearing process is also related, in some respects, to swarf piece size. For example, On one hand where swarf pieces are relatively small jams that do occur will likely be relatively easy to eliminate as any jam will likely constitute a small swarf piece that can be dislodged relatively easily. On the other hand, where swarf pieces are relatively large jams that do occur will likely constitute swarf piece that are much larger and hence more difficult to move. With that said, it is likely that, given the same clearing process, it can be predicted that a small swarf jam would be easier to clear than a large swarf jam. For this reason the optimum number of clearing processes to be performed would also depend upon swarf size. A simple clearing process counting mechanism does not account for these differences. 
     Thus, a need exists for a system that will facilitate a clearing process when a conveyor jam occurs but that will protect the conveyor motor in the event that clearing process milestones cannot be achieved and that will cause an optimum clearing process independent of swarf characteristics. 
     BRIEF SUMMARY OF THE INVENTION 
     The present inventor has recognized that all of the problems with the prior art systems described above can be address by providing a clearing procedure that is time based instead of being based on a specific number of completed clearing processes. To this end, when a jam is detected due to excessive motor load, the present invention requires that a clearing protocol including a series of clearing processes commence and that a timer begin timing the duration of the clearing protocol. When the timer reaches a specific threshold value calculated to likely clear any jam, the system aborts the protocol independent of the number of separate processes that were completed. 
     Thus, even if a jam prohibits reverse conveyor motion the present invention protects the conveyor motor from damage. Similarly, even where a jam impedes reverse motion the inventive system will stop driving the motor in reverse prior to motor damage. 
     After a specific time is set for the timer, the system automatically adjusts the number of clearing processes as a function of swarf characteristics so that the number of processes varies and is at least closer to the optimum number corresponding to the specific swarf characteristics. This is because heavy swarf typically results in a greater load on the motor and hence slows the reverse and forward conveyor motion. In this case any given reverse and forward clearing process takes longer when swarf is heavy than when swarf is relatively light. Therefore, given a specific clearing protocol period, the number of clearing processes corresponding to light swarf is greater than the number corresponding to light swarf. 
     Similarly, where swarf size is small the overall weight corresponding to a jam will likely be much greater than where swarf size is large as swarf density on the conveyor would likely be greater. Thus, the great weight of small swarf pieces would slow the clearing processes and hence a smaller number of processes would occur in a given clearing protocol time period when compared to large swarf pieces. This is the desired effect. For example, as indicated above, where swarf pieces are small it is relatively more likely that a clearing process will eliminate a jam than where swarf pieces are large. Thus, where swarf pieces are small, the number of clearing processes expected to clear a jam also small. The number of clearing processes with the present invention is related to swarf size such that small swarf naturally results in a reduced number of clearing processes and large swarf results in a greater relative number of clearing processes. 
     These and other objects, advantages and aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a side elevational view of a swarf conveyor belt assembly and a control system according to the present invention; 
     FIG. 2 is an end elevational view of the conveyor of FIG. 1; 
     FIG. 3 is a cross-sectional view taken along the line  3 — 3  of FIG. 1; 
     FIG. 4 is a cross-sectional view taken along the line  4 — 4  of FIG. 1; 
     FIG. 5 is a schematic of the inverter of FIG. 1; 
     FIG. 6 is a flow chart illustrating an inventive method used by a controller to control the conveyor of FIG. 1; 
     FIG. 7 is a flow chart illustrating another inventive method according to the present invention; and 
     FIG. 8 is a block diagram illustrating the components of the controller of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     I. Hardware Configuration 
     Referring now to the drawings, where like reference characters represent corresponding elements throughout the several views, and more specifically referring to FIGS. 1 and 2, the present invention will be described in a context of an exemplary swarf conveyor system  10 . System  10  generally comprises five components or assemblies including a conveyor  12 , a motor  14  for driving the conveyor  12 , an inverter  16  for driving motor  14 , a parameter programming unit  18  and a computerized numerical controller  20 . 
     Referring to FIG. 4, conveyor  12  includes a belt-guiding track  22 , a track cover  24  and a conveyor belt  26 . Referring specifically to FIG. 1, track  22  defines a course of movement for belt  26 . To this end, track  22  includes a first horizontal section L 1 , a second horizontal section L 2  disposed generally above section L 1 , and an inclined section L 3  between horizontal sections L 1  and L 2 . 
     Referring to FIGS. 1,  3  and  4 , track  22  includes several different walls including a bottom horizontal wall  19 , a central horizontal wall  21  and two lateral vertical walls  23  and  25 . Walls  19 ,  21 ,  23  and  25  together define an upper channel  30  and a lower channel  32  below channel  30  along the entire length of track  22 . 
     Two low friction runners  36  and  38  are positioned on an upwardly facing surface  40  of wall  19 . Runners  36 ,  38  are parallel, separated and extend along the entire length of track  22 . Similarly, a pair of low friction runners  42 ,  44  are secured to an upwardly facing surface  46  of wall  34  in channel  30 . Runners  42  and  44  are also parallel, separated and extend along the entire length of track  22 . 
     Swarf guidance extensions  48  and  50  extend inwardly from facing surfaces of walls  23  and  25  above runners  42  and  44 . An upper surface  52 ,  54  of each extension  48 ,  50  respectively, slopes downwardly as it extends inwardly. Surfaces  52  and  54  help guide swarf onto belt  26  within channel  30 . 
     Referring to FIGS. 1 and 2, track  22  is supported by a leg  58  connected to section L 3 . Leg  58  includes two wheels collectively referred to by the numeral  60  that facilitate conveyor  12  movement. A rotating pulley-type hub  28  is located at a first end  22   a  and a laterally extending motor housing (see FIG. 2)  56  is located at a second end  22   b  of track  22 . 
     Track cover  24  (see FIGS. 1 and 4) is provided above channel  30  along sections L 2  and L 3  and along a section of L 1  adjacent L 3 . A door  62  is hinged to cover  24  at track end  22   b  which covers end  22   b  when closed but is openable by swarf exiting the track. A swarf collection bin  69  is illustrated in phantom. 
     Belt  26  can be of any design known in the art and therefore will not be explained here in detail. Suffice it to say that belt  26  is continuous and passes from channel  30  into channel  32  around a motor shaft at end  22   b  and passes back from channel  32  into channel  30  around hub  28  at end  22   a.  Belt  26  is sized so that it rests on, and is supported by, runners  36 ,  38 ,  42  and  44 . 
     Motor  14  is a typical three-phase squirrel cage induction motor, the characteristics of which should be understood by those of ordinary skill in the art and therefore will not be explained here in detail. However, it should be understood that motor  14  receives three-phase alternating voltage from inverter  16  which causes the motor to rotate in either a forward or reverse directions at either a high, a medium or low speed, depending on the frequency of the received alternating voltage. Motor  14  is located inside housing  56  and includes a shaft which extends from housing  56  into track  22  at end  22   b  and is suitably linked to belt  26  to provide a rotating motivating force thereto. Therefore, when motor  14  operates in either the forward or reverse directions, the motor shaft causes belt  26  to move accordingly, conveying belt  26  in either forward or reverse directions. 
     Referring now to FIG. 5, inverter  16  receives three-phase AC line voltage from a utility on lines  64 ,  66  and  68  at a utility frequency (e.g. 60 Hz) and modifies that frequency to provide three-phase alternating voltage at a controlled frequency on output lines  70 ,  72  and  74 . Lines  70 ,  72  and  74  supply motor  14 . Inverter also includes a brake output line  78  connected to motor  14  for stopping the motor  14  when required. A preferred inverter is the Mitsubishi Freqrol-A024 or the Freqrol-A044. Inverter  16  includes several input leads including forward start STF, reverse start STR, high speed HS, medium speed MS, stop select SS, and alarm reset RES leads. When a command signal to any of the leads (STF, STR, HS, MS, SS or RESP) is high, inverter  16  operates accordingly. For example, when a command signal to forward start lead STF is high, inverter  16  provides AC voltages on lines  70 ,  72  and  74  driving motor  14  in the forward direction. Similarly when a command signal to reverse start lead STR is high, inverter  16  drives motor  14  in the reverse direction. When the signal at STF is high and neither the signal at high-speed lead HS nor at medium speed lead MS is high, inverter  16  drives motor  14  at a low speed. However, when either of the signals at HS or MS is high, inverter  16  drives motor  14  at the high or medium speeds, respectively. When the signal at stop select lead SS is high, inverter  16  uses line  78  to immediately stop motor  14 . 
     In addition to the three-phase voltages on line  70 ,  72  and  74  and the brake output  78 , inverter  16  also includes at least one other output, a high load output on line  76 . As inverter  16  provides voltages on lines  70 ,  72  and  74 , inverter  16  monitors the current drawn by motor  14  on one of the three lines  70 ,  72  or  74 . For the purposes of this explanation it will be assumed that inverter  16  at least monitors a drawn current I f  on line  70 . When the monitored current I f  exceeds a threshold current level I th , a signal is provided on line  76  indicating that a high load has occurred. As well known in the motor controls art, current drawn by an induction motor increases as load on the motor increases. Therefore, when the load on motor  14  reaches a level which draws a current equal to the threshold current level I th , a signal is provided on line  76 . 
     In addition, inverter  16  can be provided with an alarm output  77  to indicate when monitored current I f  exceeds threshold current I th . 
     Referring to FIGS. 1 and 5, parameter-programming unit  18  is connected to inverter  16  via a first bus  80 . Unit  18  includes a digital readout  82  and a keypad  84  which allow a user to program various inverter parameters via bus  80 . To this end, unit  18  can be used to set the threshold current level I th  which is required prior to inverter  16  generating a signal on line  76 . In addition, unit  18  can be used to set a number of other parameters including high speed, medium speed and low speed frequencies, and can be used to manually run motor  14  in reverse, forward and at various speeds via inverter  16 . 
     Referring to FIG. 1 controller  20  includes a touch screen  88  and an abbreviated keypad  90  which allow an operator to control inverter  16  and monitor motor  14  operation. In addition, referring also to FIG. 8, controller  20  includes a programmable microprocessor  200  that controls inverter  16  during motor operation as a function of the output on line  76 . To this end, line  76  is received by controller  20  and a second bus  86  provides control signals from controller  20  to inverter leads STF, STR, HS, MS, SS and RESP. Thus, controller  20  can drive motor  14  via inverter  16  in the forward direction or the reverse direction at various speeds, can stop motor  14  and can reset an inverter alarm via the inputs. Processor  200  may include a timer  202  for timing the duration of a series of clearing processes as explained in more detail below. 
     Controller  20  can also be used to alter operating parameters such as the duration T r0  of the reverse rotation periods during a clearing process and the maximum number X m  of clearing processes in a clearing method. 
     II. Control Method 
     Generally speaking, according to the inventive control method, with motor  14  operating in a forward direction so that conveyor  12  is moving forward, load on motor  14  will remain relatively constant and within an acceptable range during normal operation. However, when swarf obstructs belt  26  movement, motor load increases substantially. When load increases, the current drawn by motor  14  from inverter  16  also increases substantially. At some point, if the obstruction causes excessive loading, the drawn current exceeds the threshold current. Inverter  16  detects excessive load by comparing the monitored current I f  drawing by motor  14  to the threshold current I th . When current I th  is exceeded the maximum load is exceeded. At that point, inverter  16  generates a signal on line  76  which is provided to controller  20 . Then, to clear the obstruction, controller  20  sends a series of command signals via bus  86  to inverter  16  to stop motor  14 , reverse motor  14  for the predetermined reversal time period T r0 , stop motor  14  and restart motor  14  in the forward direction. This sequence of stopping, reversing, stopping and restarting in the forward direction will typically be sufficient to jostle an obstruction free. 
     If the obstruction persists, monitored motor drawn current I f  will again quickly exceed the threshold current I th  and the excessive load will again be identified. Once again, inverter  16  provides a signal via line  76  to controller  20  which in turn cycles through the clearing process. After a predetermined number X m  of times through the clearing process, if the obstruction persists, controller  20  causes inverter  16  to stop motor  14  and sound an alarm via output  77 , either audio or visual or both, altering an operator that belt  26  has been halted. 
     Referring now to FIGS. 1,  5  and  6 , prior to conveyor  12  operation, controller  20  and unit  18  are used to set various operating parameters. Specifically, unit  18  is used to set the threshold current I th  parameter while controller  20  is used to set the predetermined number X m  of clearing processes which should be performed prior to stopping conveyor  12  and is used to set reverse time period T r0 . These parameters are set at process block  100 . Next, at block  102  controller  20  initializes a process number variable X and sets variable X equal to 0. Continuing, at process block  104  a counter T r  is set equal to period T r0  and controller  20  provides a high command signal to the forward start lead STF of inverter  16 . When the STF command is received, inverter  16  provides output voltages on line  70 ,  72  and  74  driving motor  14  and belt  26  in the forward direction. 
     At decision block  106  inverter  16  determines whether or not monitored current I f  is equal to or exceeds the threshold current I th . To this end, inverter  16  monitors current I f  through line  70  and compares that current I f  to the threshold current I th . If monitored current I f  is less than threshold current l th , the motor load is less than the maximum load and controller  20  control passes back up to process block  102 . However, when monitored current I f  is greater than or equal to threshold current I th , control passes to process block  108  where variable X (i.e. number of clearing processes performed) is incremented by 1. Control then passes to decision block  110  where controller  20  determines whether or not variable X is equal to maximum number of clearing processes X m . Where variable X is equal to maximum number X m , the clearing process including stopping, reversing, stopping and restarting the belt in the forward direction has been completed X m  times without successfully clearing the obstruction which is causing the excessive load. In this case, the clearing process will not likely be able to clear the obstruction and therefore, control passes to process block  112  where controller  20  sends a control signal via bus  86  to the stop select lead SS of inverter  16 . When inverter  16  receives the SS signal, inverter  16  stops motor  14  via brake output  78  thus causing conveyor belt  26  to stop. In addition, at block  112 , inverter  16  generates an alarm signal via output line  77  indicating that belt  26  has been halted. 
     Referring still to FIGS. 1,  5  and  6 , at decision block  110 , when variable X is less than maximum number X m , control passes to block  114  where the clearing process begins. To this end, at block  114 , controller  20  first provides a high control signal at lead SS causing inverter  16  to stop motor  14  and belt  26 . Then, controller  20  provides a high signal at reverse start lead STR which in turn causes motor  14  and belt  26  to move in the reverse direction. In addition, at block  114  controller  20  starts a timer which tracks the amount of time motor  14  is operating in the reverse direction. The timer counts down counter T r  to zero. At decision block  116  controller  20  determines whether or not counter T r  is equal to zero. Where counter T r  is not equal to zero, control loops back to decision block  116 . When counter T r  is equal to zero, control passes to block  118  where controller  20  again sends a high signal to inverter lead SS causing inverter  16  to stop motor  14  and belt  26 . Next, control again passes up to process block  104  where controller  20  resets counter T r  to period T r0  and again provides a forward rotation start input signal STF to converter  16 . Again, when signal STF is received, inverter  16  drives motor  14  in the forward direction-causing belt  26  to move forward. At block  106 , if the motor load is less than the maximum load, the monitored current I f  will be less than the threshold current level I th  and control will again pass up block  102 . 
     Thus, a simple, inexpensive and reliable method and apparatus for implementing the method for automatically clearing swarf obstructions on a conveyor belt have been described. 
     Referring now to FIGS. 1,  8 , controller  20  can also be used to manage a swarf clearing procedure or process as a function of time as opposed to a function of the number of clearing attempts or cycles performed. To this end, referring also to FIG. 7, an exemplary inventive time based clearing method is illustrated as a flow chart beginning at process block  210 . At block  210  a clearing cycle is programmed selected or defined to be performed by processor  200 . The defining step can be performed via any type of interface (e.g., keypad  90  or touch screen  88 ). In addition, the predetermined duration for an ensuing clearing procedure or series of clearing cycles is defined at step  210  via the interface. To this end it is assumed that a system user that understands the process in which the swarf conveyor is used is available to provide the predetermined time. The skilled user bases the predetermined time on the likely duration of each swarf clearing cycle (i.e., the separate clearing efforts), the type of swarf (e.g., large or small, etc.) expected, the nature of a likely obstruction (i.e., easy or difficult to clear), etc. 
     After the process and time have been set a timed period counter Tt that is tracked by timer  202  (see FIG. 8) is set equal to zero at block  212 . At block  214  the inverter is powered to drive the conveyor belt in the forward direction, 
     At decision block  216  processor  200  determines whether or not monitored current I f  is equal to or exceeds the threshold current I th . To this end, processor  200  monitors current I f  through line  70  via the inverter  16  and compares that current I f  to the threshold current I th . If monitored current I f  is less than threshold current I th , the motor load is less than the maximum load and controller  20  control passes back up to process block  212 . However, when monitored current I f  is greater than or equal to threshold current I th , control passes to decision block  218 . 
     At block  218  processor  200  enables timer  202  and timer  202  begins to time the duration of the clearing procedure that follows. After block  218 , at decision block  222  processor  200  compares the timed period Tt to the predetermined period T 1 . Where the timed period Tt is equal to or greater than the predetermined period T 1  the clearing procedure has already been performed for a period equal to the predetermined period and processor  200  turns the conveyor off. In addition, at this time processor  200  sounds an alarm indicating that a conveyor operator should manually check the conveyor to determine the cause of the obstruction and how to clear the obstruction. To this end, where the timed period is equal to the predetermined period control passes to block  224 . 
     Where the timed period Tt is less than the predetermined period T 1 , control passes to block  200  and the clearing procedure is enabled. When an obstruction has just occurred, enabling means commencing a first clearing cycle. For example, the defined clearing cycle may include stopping the motor, reversing the motor for a time or distance or for a number of rotations or until a specific reverse speed is obtained, etc., stopping the reverse action and then restarting the motor in the forward direction. 
     Next, control passes back up to decision block  216  where the measured current If is again compared to the threshold current and control continues to loop through steps  216 ,  218 ,  222  and  220 , possibly stopping at block  224  if an obstruction does not clear. 
     An example of how the time limited clearing procedure might operate is instructive. To this end, assume that an average clear cycle (e.g., stopping, reversing, stopping and again driving the conveyor forward) takes approximately 4 seconds and that a system user has programmed processor  200  to attempt to clear any obstructions for a predetermined 20 second period. In operation, referring still to FIGS. 7 and 8, with period Tt set equal to zero, the predetermined period T 1  set equal to 20 seconds and the motor operating in a forward direction, if the measured current If is less than the threshold current Ith, control continues to loop through blocks  212 ,  214  and  216 . However, when the measured current If exceeds or is equal to the threshold current Ith, control passes to block  222 . Because period Tt is initially zero, at block  222  control passed to block  218  where clock  202  is enabled and begins to time period Tt. At block  220  the clearing process is commenced and control passes back up to block  216 . 
     The second time through blocks  216 ,  222  and  218 , if the measured current If is less than the threshold current Ith control passes back to block  212  where time Tt is re-zeroed. However, assuming measured current If is equal to or greater than the threshold current Ith, at block  222  time Tt will be approximately 4 seconds after the first clearing cycle is completed and time period Tt will be less than the 20 second period T 1 . Thus, control will again pass to block  218  where clock  202  will remain enabled. 
     Assuming the obstruction remains uncleared by the clearing cycles, during the next four 4 second cycles as control passes through blocks  216 ,  222 ,  218  and  220  time period Tt will be less than predetermined period T 1  such that control does not pass back up to block  212 . During the clearing cycle following the next four (i.e., during the sixth overall sequential 4 second clearing cycle), time period Tt will exceed predetermined period T 1  and control will pass to block  224  where the conveyor is turned off and the alarm is sounded. 
     It should be understood that the methods and apparatuses described above are only exemplary and do not limit the scope of the invention, and that various modifications could be made by those skilled in the art that would fall under the scope of the invention. For example, while the method is described in the context of an inverter and an inverter controlled motor, clearly the inventive method could be implemented using some other motor drive type wherein motor load could be determined by some other means. For example, a clutch type motor could be used wherein, when the clutch disconnects the motor from a driving shaft due to excessive motor load, a sensor could detect the disconnection and start the clearing process above. In addition, while the conveyor described above includes a single motor, clearly the inventive method applies to other systems that require two or more conveyors. Moreover, while the invention is described above as one for use with a swarf conveyor, the invention is meant to cover all types of conveyors such as parts or material conveyors, conveyors including a belt and other types of non-belt conveyors which may become jammed. Furthermore, the invention is also meant to include control wherein various operating parameters could be modified. For example, when the belt is reversed, different speeds and durations might be specified and/or the system might be equipped to identify obstruction during a cleaning process (i.e. during belt/conveyor reversal). 
     To apprise the public of the scope of this invention, we make the following claims: