Apparatuses and methods for monitoring stall of motors

A system and method for monitoring a level of material in a storage bin, including a gear train, a gear train shaft rotatably coupled to the gear train, a drive shaft, a coupling system to couple the gear train shaft to the drive shaft, a paddle coupled to the drive shaft to rotate in unison therewith, the paddle configured to engage a material in the storage bin when the level of material reaches a threshold level, and a stepper motor. A controller may be communicatively coupled to the stepper motor wherein the controller is configured to monitor the running state and the stall state of the stepper motor.

BACKGROUND

Technical Field

The present disclosure relates to monitoring of motors, and more particularly for monitoring stall of motors used in connection with bin level indicators.

Description of the Related Art

Bin level indicators typically include a rotatable paddle structure extending into a bin and an electric motor. The electric motor may be contained within a protective enclosure and is connected to the rotatable paddle. The rotatable paddle is adapted to engage material within a storage bin when the material rises to the bin level at which the rotating paddle is disposed. Material drag on the paddle causes the motor drive torque to rotate the motor rather than the paddle, which rotation is sensed by one or more switches carried within the enclosure. The switches may be connected to deactivate a conveyor feeding material to the bin, to remove power from the indicator motor and/or to perform other control functions related to material level. In some instances, the indicator may be mounted at the bottom of the bin and will have its load released by emptying of the bin and again a signal system is actuated by the change in load on the motor.

Conventional bin level indicators may further require sensors, such as optical or magnetic sensors, to detect the motor's shaft rotation, fine pitch gears, springs, and/or complex clutch mechanisms. Requiring multiple complex components for proper functioning can increase the failure modes, add costs, and increase the complexity of the bin level indicators. Further, the material in the storage bin may have varying densities, detection of which may require adjusting paddle sizes to paddles of certain width and height to accurately detect material levels in bins. Still further, conventional bin level indicators may not differentiate between stalled paddles and gear or motor failures. By way of an example, a low level bin level indicator may remain in the stalled position until the level of material in the bin is fully empty, thus removing material drag on the paddle. However, an operator may not know if the low level bin level indicator has remained in the stalled position due to material level or due to a gear or motor failure.

BRIEF SUMMARY

Embodiments described herein provide apparatuses and methods that are well adapted to monitor the operating conditions of motors and rotatory equipment in a particularly accurate manner and in a compact and robust form factor. According to one embodiment, an apparatus for monitoring a level of material in a storage bin may be summarized as including a gear train; a gear train shaft rotatably coupled to the gear train; a drive shaft; a coupling system to couple the gear train shaft to the drive shaft; a paddle coupled to the drive shaft to rotate in unison therewith, the paddle configured to engage a material in the storage bin when the level of material reaches a threshold level; and a stepper motor. The coupling system may be configured to impart rotation motion between the gear train shaft and the drive shaft. The stepper motor may have a running state and a stall state, where the stepper motor is operatively coupled to the gear train to drive the gear train shaft to impart torque on the drive shaft and the paddle during the running state and the material engaging the paddle to cause the stepper motor to switch to the stall state.

According to another embodiment, an apparatus for monitoring a level of material in a storage bin may be summarized as including a gear train; a gear train shaft rotatably coupled to the gear train; a drive shaft; a coupling system to couple the gear train shaft to the drive shaft; a paddle coupled to the drive shaft to rotate in unison therewith, the paddle configured to engage a material in the storage bin when the level of material reaches a threshold level; a stepper motor; and a controller. The coupling system may be configured to impart rotation motion between the gear train shaft and the drive shaft. The stepper motor may have a running state and a stall state, where the stepper motor is operatively coupled to the gear train to drive the gear train shaft to impart torque on the drive shaft and the paddle during the running state and the material engaging the paddle to cause the stepper motor to switch to the stall state. The controller may be communicatively coupled to the stepper motor, where the controller is configured to monitor the running state and the stall state of the stepper motor.

According to another embodiment, a method for continually monitoring a running state and/or a stall state of a motor may be summarized as including measuring a duration of counter or back electromotive force (BEMF) during operation of a motor; and comparing the said duration with a respective threshold duration of BEMF of a running motor and a stalled motor, the said duration being higher than the threshold duration of the running motor when the motor is in the running state and being lower than the threshold duration of the stalled motor when the motor is in the stall state.

According to another embodiment, a method for monitoring a stall state of a motor for one of a variety of operating conditions may be summarized as including defining a threshold duration of BEMF for a range of stall torque values of a running motor and a stalled motor, where each stall torque value being proportional to a peak current of the motor and applicable to the respective operating condition; setting the stall torque value of the motor; measuring a sample duration of presence of BEMF during operation of the motor; and comparing the sample duration with the selected threshold duration of BEMF to determine the stall state of the motor. The comparison may be indicative of a stall state when the sample duration is lower than the threshold duration of the stalled motor and the comparison may be indicative of a running state when the sample duration is higher than the threshold duration of the running motor.

According to another embodiment, a method for detecting a defect in a gear train may be summarized as including rotating the motor in a direction counter to a direction of motor rotation during operation such that the gear train shaft disengages from the drive shaft through the coupling system and determining the stall state of the motor. Measuring the stall state at this time insures that the gear train is free to move in spite of the drive shaft's mobility. If the motor is in a stalled state during this instance, the gear train is deemed defective. If the motor is in a running state during this instance, the gear train is deemed operational.

According to another embodiment, a method for detecting a defect in a motor may be summarized as including rotating the motor in a direction counter to a direction of motor rotation during operation of the motor; measuring a sample duration of presence of a back electromotive force (BEMF) for both windings during counter rotation of the motor; and comparing the sample durations with a threshold duration of a defective motor. The comparison may be indicative of the defective motor when the sample duration for either winding is lower than the threshold duration and the comparison may be indicative of an operational motor when the sample duration for both windings is higher than the threshold duration.

According to another embodiment, a method to monitor a fault in an apparatus for monitoring a level of material in a storage bin for one of a variety of operating conditions may be summarized as including defining a threshold duration of BEMF for a range of stall torque values of a running motor, with each stall torque value being proportional to a peak current of the motor and applicable to the respective operating condition; setting the stall torque value of the motor; measuring a sample duration of presence of BEMF during operation of the motor; and comparing the sample duration with the selected threshold duration of BEMF to determine if the apparatus has a fault. The comparison may be indicative of a fault in the apparatus when the sample duration is lower than the threshold duration of the running motor and the comparison may be indicative of an operational apparatus when the sample duration is higher than the threshold duration of the running motor.

DETAILED DESCRIPTION

The following detailed description is directed toward apparatuses and methods for use in connection with monitoring the operating condition of motors and rotating equipment. The description and corresponding figures are intended to provide an individual of ordinary skill in the art with enough information to enable that individual to make and use embodiments of the invention. Such an individual, however, having read this entire detailed description and reviewed the figures, will appreciate that modifications can be made to the illustrated and described embodiments, and/or elements removed therefrom, without deviating from the spirit of the invention. It is intended that all such modifications and deviations fall within the scope of the invention, to the extent they are within the scope of the associated claims.

FIG. 1illustrates an apparatus10for detecting material storage in a bin, such as a tank, hopper, silo, or other structures. The apparatus10includes a housing28that is formed by coupling an upper cover12to a lower base14. The apparatus10may further include a sealing structure, such as a gasket, sandwiched between the upper cover12and the lower base14, to provide a substantially sealed apparatus10. Within the housing28, the apparatus10includes a gear train20, a stepper motor24, and an electronic controller22. The stepper motor24may be a permanent magnet, variable reluctance, hybrid reluctance, or other type of a stepper motor. The electronic controller22may include a microcontroller and a stepper motor driver to control and drive the stepper motor24in a known manner. Moreover, the microcontroller may comprise a microprocessor, memory, and a plurality of peripheral devices to form a system on a chip that may be applicable for a wide variety of applications.

More generally, a control system that operates the embodiments of the apparatuses and methods described herein may include, without limitation, one or more computing devices, such as processors, microprocessors, digital signal processors (DSP), application-specific integrated circuits (ASIC), and the like. To store information, the control system may also include one or more storage devices, such as volatile memory, non-volatile memory, read-only memory (ROM), random access memory (RAM), and the like. The storage devices can be coupled to the computing devices by one or more buses. The control system may further include one or more input devices (e.g., displays, keyboards, touchpads, controller modules, or any other peripheral devices for user input) and output devices (e.g., displays screens, light indicators, and the like). The control system can store one or more programs for processing any number of different apparatuses and methods according to various embodiments described herein to detect the operating conditions of a motor. The control system, according to one embodiment, may be provided in the form of a general purpose computer system. The computer system may include components such as a CPU, various I/O components, storage, and memory. The I/O components may include a display, a network connection, a computer-readable media drive, and other I/O devices (a keyboard, a mouse, speakers, etc.). A control system manager program may be executing in memory, such as under control of the CPU, and may include functionality related to detecting the running and stall states of the motors described herein.

It should further be noted that the control system may include one or more sensors for determining various operating parameters of the apparatus such as temperature, humidity, vibration, etc. These sensors are able to collect data and store the information within the apparatus or transmit the data to a data collection system outside of the apparatus. For example, the apparatus may also include a wired or wireless transmission system for enabling the sensor data or operating conditions of the apparatus to be transmitted to a storage or processing device remote from the apparatus. Such remote devices may include on-site data processing systems or cloud computing storage for remote data processing.

With continued reference toFIG. 1, the gear train20may include two or more gears rotatably engaged with each other. Further, the gear train20is rotatably coupled to a gear train shaft31that extends downwardly from the gear train20and is coupled to a drive shaft32through a coupling system26, which is discussed in more detail elsewhere. While the illustrated embodiment includes a gear train20, in alternate embodiments, other means or drive mechanisms may be used instead, such as a belt drive system, pulleys, sprockets, or similar systems, which may connect a motor shaft to a drive shaft and transmit power.

At a bottom end, the drive shaft32is fixedly coupled to a paddle30, such that the paddle rotates in unison with the drive shaft32. In operation, the drive shaft32extends into a storage bin and the stepper motor24drives the drive shaft32, thus enabling the paddle30to rotate freely. However, as the paddle30engages material in the storage bin, the rotational resistance of the material will result in a counteracting torque, causing the stepper motor24to stall. Such material may include, for example, feed, silica sand, rocks, pellets, wood, calcium dust, rubber, metals, regrind materials, coal, peanuts, malt, clay resin, limestone, grain, foundry materials, sand pre-mix, rawhide, sawdust, and other materials.

Use of the stepper motor24in the manner described in further detail elsewhere can advantageously detect this stall condition. In stepper motors, the stall torque can be defined as being directly proportional to the peak current. By way of an example, the following equation can be used to show this relationship:

T=(PN2⁢⁢π)⁢∅⁢⁢I
where T is torque; P is number of poles; N is number of coil turns; Φ is flux; and I is current. In the embodiment illustrated inFIG. 1, the peak current, which is the highest current a motor will produce, can be adjusted to determine the stall torque of the apparatus10. By way of an example, the microcontroller of the electronic controller22may be used to measure the peak current. Thus, changing the peak current to various values, the stall torque of the motor may then be determined. Moreover, the stall torque may then be appropriately manipulated to suit a wide variety of applications. For example, in some embodiments, the electronic controller22may be electrically coupled to a potentiometer, DIP switches, jumpers or other means to create an adjustable voltage divider or the like, which the electronic controller22can read to control the stall torque by adjusting the current thresholds.

FIG. 2illustrates a method200to detect a stepper motor24that has been stalled or is in a running state, according to one embodiment. In stepper motor24, the current supplied to a coil sitting in a magnetic field causes it to turn. As the coil of the motor rotates, it experiences a change in magnetic flux with time and by Faraday's Law an electromotive force (EMF) is induced in the coil. As explained by Lenz's Law this induced EMF must oppose the supplied EMF driving the coil. This induced EMF is known as a counter or back EMF (BEMF) and is represented as a voltage induced across the motor windings when the motor driver is momentarily disabled. Throughout this specification, the terms “counter electromotive force” and “back electromotive force” may be used interchangeably. The BEMF can be defined as being directly proportional to the angular velocity of the stepper motor24. Thus, as a load is applied to the stepper motor24, the angular velocity will decelerate faster and thus reduce the duration of the presence of the BEMF. The method200advantageously allows for using the BEMF duration measurements to determine whether the stepper motor24is stalled or running without using any electro-mechanical means, such as optical or mechanical sensors, clutch or spring mechanisms, or the like.

In step202, the motor is in a run state and turns the stepper motor24on at204. At step206, the motor coil or drive current is at zero-cross and the BEMF is measured. The motor24drive current at zero-cross, i.e., where the motor24drive current is at its lowest value, provides an advantageous point to measure the duration of the BEMF because the duration of the BEMF is the longest at this point in an unloaded condition. If the motor24drive current is at the zero-cross point at step206, the motor driver is disabled and a BEMF counter is initiated at step208. Thereafter, at step210, the presence of BEMF is measured. BEMF may be measured by including a comparator electrically coupled to the electronic controller22, or other similar means to compare voltage inputs. The comparator may compare the BEMF voltage VBEMFto a pre-set Voltage threshold VThresholdand send an output signal to the electronic controller22when the VBEMFexceeds the VThresholdto initiate the BEMF counter. Once initiated, the BEMF counter continues the duration count until BEMF is no longer present, i.e., when VBEMFdrops below the VThreshold, and at step212enables the motor driver and stops the BEMF counter. At step214, whether the motor is in a stall or run state is determined. At step214a, if the BEMF counter is less than a pre-set motor stall threshold value, then the motor is determined to be in a stall state at step216. On the other hand, at step214b, if the BEMF counter is greater than the pre-set motor run threshold, then the motor is determined to be in a run state at step218.

FIGS. 3 to 5illustrate oscilloscope traces of electrical waveforms generated at various states of the motor.FIG. 3illustrates the current waveform of an unloaded motor, and shows a short pulse at the zero cross of the waveform, thus indicating the point at which the BEMF measurement is taken. As discussed previously, this is the point at which the longest duration of BEMF is observed.FIGS. 4 and 5illustrate electrical waveforms when the motor is running and stalled, respectively. As observed from the traces of channel1inFIGS. 4 and 5, when the motor is temporarily disabled, the duration of the BEMF is reduced when the motor is in the stalled state due to the load being applied to the motor. Using the method200can advantageously detect this condition, which may be caused by load on the motor, such as material build-up on the paddle30, gear fatigue, deterioration of the motor performance, and other failure modes, and can be communicated to a user through a user interface by use of indicators, such as an audible alarm, a buzzer, a gauge, a light emitting diode (LED), or other similar indicators. Using the method200described above can advantageously provide for detection of stalled motors at a wide range of stall torque levels.

FIG. 6illustrates a plot of coil current versus BEMF counts for a running motor and a stalled motor. As explained in more detail elsewhere, the peak motor24drive current may be directly proportional to the stall torque; thus, the stall torque may be controlled by adjusting the peak motor24drive current. As illustrated inFIG. 6, the BEMF counts of a running motor is generally higher than the BEMF counts of a stalled motor across a wide range of peak motor24drive current values. Thus, the BEMF thresholds for determining whether the motor is running or stalled can be pre-set for a wide range of peak motor24drive currents and can be advantageously used for a wide variety of applications. By way of an example, using the method200, the rotatable paddle30may detect lower density materials or higher density materials by reducing or increasing the stall torque values, respectively, and monitoring whether the BEMF duration measurements are less than the motor run threshold and/or the motor stall threshold. In other embodiments, the pre-set BEMF threshold may be defined over a set of stall torque values, such as by defining a high, medium, and low range covering for the densities of material likely to be received by the storage bins.

FIGS. 7-9illustrate the coupling system126that transmits rotational energy to drive the rotatable paddle30. The coupling system includes a slotted gear train shaft131, a clutch housing134, a drive shaft132, c-pin136, spring142, and two bearings140. The slotted gear train shaft131fits into one end of the clutch housing134such that the slot mates with a c-pin136, pressed into the clutch housing134allowing for the direct transmission of torque. The opposite end of the clutch housing134includes a slotted opening on one side and a hole on the other. The drive shaft132includes an opening in which a spring142and two bearings140are seated. The drive shaft132is rotatably coupled to the clutch housing134, through the interlocking of the bearings140, held in place by the spring142due to the applied spring force.

As best illustrated inFIG. 8, torque transmission between the clutch housing134and the drive shaft132is achieved through the spring142force acting on both bearings140. When the gear train shaft131is turning in the direction of146, the drive shaft132can achieve a torque proportional to the force acting between both bearings140and the clutch housing134. As the torque applied by the geartrain through the clutch134to a secured drive train shaft132is increased, the bearings140are forced into the clutch housing134, disengaging the clutch housing134and allowing the geartrain shaft131to rotate without load for 180 degrees of rotation.

Moreover, a gap G′ exists in the clutch housing134due to the slotted opening on one side. The gap in this embodiment is 90 degrees, in alternative embodiments this gap may range between 5 to 135 degrees. Providing this gap allows the geartrain to disengage from the clutch housing134at a lower torque when driven in the reverse direction148, as illustrated inFIG. 9, which may assist in detecting if the gear train is defective, if the motor windings are damaged, or if the motor connector has been improperly installed, among other possible faults with the motor and gear train.

An alternate embodiment of the coupling system is illustrated inFIGS. 10-12, wherein the coupling system26transmits rotational energy to drive the rotatable paddle30. The coupling system26includes a gear train coupler34and a drive shaft coupler36. The gear train coupler34includes a first half main body38and a pair of gear train coupler jaws40axially projecting from the first half main body38, each jaw40being positioned opposite to the other. The drive shaft coupler36includes a second half main body42and a pair of drive shaft coupler jaws44axially projecting from the second half main body42. The first half main body36is rotatably coupled to the gear train shaft31and the second half main body42is rotatably coupled to the drive shaft32. As best illustrated inFIG. 11, the gear train coupler jaws40and the drive shaft coupler jaws44are spaced apart such that when the motor is running in the direction46, the drive shaft32rotates when the gear train coupler jaws40interdigitate with the drive shaft coupler jaws44.

Moreover, a gap G exists between the gear train coupler jaws40and the drive shaft coupler jaws44. While the gap G shown in the embodiment is 30 degrees, in alternative embodiments, this gap may range between 5 to 60 degrees. Providing this gap G can advantageously allow for reversing the direction of the motor for a short duration47, as illustrated inFIG. 12, which may assist in detecting if the gear train is defective, if the motor windings are damaged, or if the motor connector has been improperly installed, among other possible faults with the motor and gear train.

FIG. 13illustrates an exemplary method3000to detect defective gear trains and/or proper functioning of the motor. A test timer may be pre-set to run motor tests at appropriate time intervals. At step3002, it is determined whether the test timer has expired. If the timer has not expired, the motor continues its normal operation at step3004. If the timer has expired, the motor is set to rotate in a reverse direction at step3006. The motor remains in the reverse direction for the duration specified by step3008. At step3010, the motor is set to rotate in a forward direction. After the delay specified in step3012, the system performs a stall check in step3020and determines in step3028whether the motor is stalled. If the motor is found to have stalled then the system indicates a fault condition of the gear status in step3030. If a stall condition is not found, then the gear status is determined to be correctly operational in step3026. After gear status has been determined, a delay is initiated in step3032for a predetermined duration, the test timer is reset in step3034and the system returns to normal operation in step3004.

In addition to, or contemporaneously with, the stall check, the duration of BEMF on both motor windings is measured in step3014. In steps3016and3018, coil BEMF duration is compared against a bad motor threshold reference value. If one or both of the duration values is below the threshold value, then motor is determined to be defective in step3022. If both the BEMF counters are greater than the bad motor threshold, the motor is considered to be fully operational at step3024. After motor status has been determined, there is a delay in step3032, the test timer is reset in step3034and the system returns to normal operation in step3004.

The methods described above may also be advantageously used to detect additional faults in the operation of the apparatus10, such as material build-up on the paddle30, wear on the gear train20, decline in electronic performance of the motor24, wear on a motor24bearing, and/or wear on a shaft bearing. Such conditions will add additional torque to the drive shaft, thus reducing the BEMF duration count in the manner described previously.FIG. 14illustrates a plot of motor24coil current versus BEMF counts for a running motor, a motor under additional load, and a stalled motor across a wide range of peak motor24currents. As illustrated inFIG. 14, when additional load is added to the drive shaft, the BEMF duration count shifts downwardly, reflecting a reduction in the count. Thus, the methods described above may include additional checks to compare the BEMF duration counts with a reduced count caused by conditions that may increase the torque of the drive shaft. Upon discovery of such conditions, means to indicate this information to a user may be used through a user interface in a known manner; such means may include an audible alarm, a buzzer, a gauge, a light emitting diode (LED), or other similar indicators.