Patent Application
20100131232
1. Field of the Invention
This invention relates generally to a method and apparatus for measuring relative rotational speed of two rotating machine elements, more particularly to a method and system or apparatus for measuring belt slip in a belt drive system, and specifically to a belt slip meter.
2. Description of the Prior Art
Belts, including V-belts, multi-ribbed belts and flat belts, transmit power between rotating machine elements through friction and are therefore prone to some degree of slippage between belt and pulley or sheave. Excessive slippage, or slip, can rapidly wear out a belt, damage pulleys, create noise, generate heat, waste energy, and the like. These problems can be observed by stopping the drive and inspecting the belt and/or pulleys; listening for or measuring noises; or measuring temperatures or speeds and tracking them over time; or the like. However, these detection methods either require interrupting the operation of the equipment, or severe enough malfunction of the drive to draw attention, or to create inconvenience.
U.S. Pat. No. 3,637,998 discloses a speed ratio measuring system with a ratio counter that provides a measure of the speed ratio of a cooperative pair of work rolls on a reversing roughing mill.
U.S. Pat. No. 4,849,917 discloses a device for measuring the speed difference between the speed of a belt and the peripheral speed of a drum in strip casting.
U.S. Pat. No. 4,823,080 discloses a combination touchless (photo type) and contact type tachometer having a digital display.
The present invention is directed to systems and methods which provide a non-invasive, direct measurement of belt slip which can indicate a belt drive problem before severe malfunction occurs. The present invention provides for automated belt slip measurement in real time on an operational belt drive. The present invention thus provides a monitoring and diagnostic device for belt drives based on belt slip measurement.
The present invention is directed to a meter for measuring belt slip in a belt drive system having at least a belt trained about two rotating elements, comprising: two sensor inputs, to receive a signal related to the speed of a rotating element; an external input, to accept a command or value; an output, to display or transmit a measurement or calculation result; and a controller, to calculate the relative speed of the two rotating elements, compare the relative speed to a set-point, and output a result of the comparison that is indicative of belt slip.
The set-point may be a stored measurement of the relative speed of the two rotating elements or a value provided via an external input and subsequently stored. The result indicative of belt slip may be a ratio of a relative speed measurement to the set-point or a percent difference between them. The result indicative of belt slip may be compared to a stored value indicative of a tolerable amount of slip and an alarm may be generated if the belt slip result exceeds that amount. Alternately, the belt slip result may be compared to additional stored values representing multiple alarm or warning levels indicating various degrees of slip.
Sensors useful in embodiments of the invention include various non-contact sensors, such as optical, infrared, or laser sensors sensitive to one or more targets rotating in conjunction with or on rotating elements of a drive system. The targets may be a reflective surface such as reflective tape, paint, or the like. If more than one target is present on a rotating element, then the number of targets may be input to the meter so that the controller may properly calculate the time of one full revolution of the rotating element. The speed measurement of each rotating element may be accomplished by measuring for each corresponding signal the width of a high voltage pulse and a subsequent low voltage trough to determine the time for one revolution of the element. The drive ratio of two drive elements may then be calculated from two such times, which may also be inverted and scaled to provide rotational speed data.
Embodiments of the present invention are also directed to methods for measuring and displaying belt slip. The inventive embodiments are also directed to a belt drive system having a belt drive with at least a belt and two pulleys, two targets, two sensors, a belt slip meter, and a tensioning device. The sensors and meter may communicate wirelessly.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.
The accompanying drawings, which are incorporated in and form part of the specification in which like numerals designate like parts, illustrate embodiments of the present invention and together with the description, serve to explain the principles of the invention. In the drawings:
The slip meter according to the invention dynamically measures and displays the percent slip or other indication of slip or slip rate of a belt drive. The meter compares measured values of slip to a set, acceptable value of slip, and provides a visual warning indicator when slip becomes excessive. A maintenance person or end user is thus able to continuously monitor the belt drive without shutting it down for a belt inspection. The slip meter may be a portable, handheld meter or a permanently mounted meter. Updates can occur continuously, providing a real-time measure of slip, which may also be viewed as a measure of energy loss, and ultimately a measure of belt performance. It should be understood that generally the term “percent slip” herein can mean any desired indication of belt slip.
The present invention is directed to a belt slip measurement system which integrates two tachometers, a speed ratio and slip calculator, and outputs. An embodiment of such a system is illustrated in
Markers or targets 25 and 26 are shown on an axially facing pulley surfaces in
It is preferred that the sensors generate a digital signal in response to the rotating markers. By digital is meant a signal with primarily two states, for example a high and a low voltage level, or an on and off state. The two voltage levels may conform to any conventional digital signal standard, for example TTL (i.e., Transistor-Transistor Logic, which uses discrete levels of 0 and +5 volts). In a preferred embodiment, the signal is high or on when the target is detected by the sensor and low or off when the target is not detected by the sensor. Thus, a signal such as illustrated in
While a single target on a pulley is preferred, multiple targets may be used provided that the number of targets may be provided to the microcontroller by some input means so that the proper number of pulses may be timed to determine the time of a single revolution of a rotating element. Subsequent calculations carried out by the controller may be the same as in the single marker case.
Alternately, when one or multiple markers are used on a rotating element, in combination with an internal timer, the controller could determine the total number of consecutive signal pulses in a given time interval. Thus, the rotational speed may be determined by the number of times a marker is detected in a predetermined time reference, using knowledge of the number of markers per revolution. An additional stored constant is needed to use multiple targets. Accuracy would be a function of the length of time chosen for counting pulses and the number of targets. A large number of targets and/or a long time may be needed to achieve the accuracy of the preferred method of timing one target for a single revolution. Regardless, it simplifies the meter and reduces the programming space or memory required by using a single target all the time.
In various embodiments, once the rotational speeds of two pulleys have been measured, the microcontroller calculates the ratio of the two speeds. The speed ratio is independent of the units used for rotational speed and therefore may be determined directly from either the measured time for one revolution of each pulley or from the inverted or converted form of the rotational speeds. The percent slip is determined by a comparison of the speed ratio to a value of a speed ratio which represents zero slip. The zero slip value may be known from the design speed ratio of the belt drive system, or may have been measured previously, for example by the same slip meter. The belt slip meter can store such a zero slip value, or similar set-point for the drive, for later use. The meter can perform a drive ratio measurement on command and store it as the set-point speed ratio for later use. The meter can also, or instead, receive a set-point value directly from a user or some electronic input. The percent slip of the belt on the drive system is then easily calculated as the magnitude of the per cent difference between the current value of speed ratio and the set-point value. Alternately, if desired, some other indication of slip or slip rate may be utilized, such as a speed ratio difference, slip ratio, percent of nominal, fractional slip, or fraction of nominal or the like, instead of actual percent slip. Regardless of the form of the slip rate indication chosen in a particular embodiment of the slip meter, the slip indication may then be programmatically compared to a predetermined and programmed-in level or levels by the controller to decide if a warning indication is needed. In the case of actual percent slip with respect to V-belt drives, it may be useful to send a warning when the percent slip is greater than about 3%. Generally, for example in typical V-belt drives, a value of slip above about 8% indicates a severely degraded drive condition needing maintenance. Thus, two predetermined warning levels may be advantageously utilized.
Thus, the inventive slip meter and slip measurement system provide a number of advantages over other means of diagnosing drive problems. The slip measurement may be carried out on an operational belt drive system. Unlike some other diagnosis techniques, such as direct inspection of a belt, the drive does not need to be shut down first. The slip measurement may be fully automated. The slip measurement is objective and can provide early warning of problems before subjective symptoms, such as belt noise, become severe enough to warrant attention. The slip measurement may be carried out remotely from the belt drive itself. The slip meter can be extremely portable for field use. The slip meter can be incorporated into a network or central control system.
Other or alternate output devices useful in embodiments of the present invention could be interface connectors or wireless transmitters, receivers, or transceivers, Ethernet, USB or Bluetooth capability, for use with external devices such as displays, printers, controllers, computers, or networks. Likewise, alternate external input devices could be used, such as keyboards, touch screens, various wired connections, wireless transmitters, receivers, transceivers or other devices, computers, controllers, or networks or the like.
A slip meter according to one embodiment of the invention and appearing similar to the illustrated embodiment of
Two slip meters according to other embodiments of the invention and appearing similar to the illustrated embodiment of
In both 5-button example meters, the user could input a target drive ratio, or set-point or zero-slip value, based on two measured RPMs by pressing a “ratio” button, using the increase and/or decrease buttons to generate the desired value, and finally pressing an “enter” button. Alternately, the meters could “zero” on any two measured RPMs by calculating the zero-slip ratio at the time a button is pushed. Once a zero-slip value is stored, the meters can provide a measure of relative slip. The LCD screen displayed the two shaft or pulley speeds and the relative slip simultaneously. Three colored indicator lights were included as a visual display of the amount of slip in the drive: a green light indicating normal operation with less than about 3% slip, a yellow light indicating slip greater than 3% but less than about 8%, and a red light to indicate slip greater than 8%. The red light could also indicate that no value had registered on the meter, for example, indicating belt breakage or drive power failure.
It should be noted that the above examples are not meant to limit the invention. For example, details such as the choice of power supply or output device, the range of sensors or transceivers, the speed of the microcontroller and useful speed range of the meter, and the like, may be readily altered by suitable choice of components.
In other embodiments of the invention, a menu structure may be programmed into the device. The menu structure may include simple introductory statements and then prompt the user to enter a drive ratio or alternately a speed for each sheave as the set-point value. The speed ratio may be checked or limited to a range, for example from 1.00 to 10.00, with prompts to inform the user of these limitations. Alternately, the user may select a current measurement to be stored as the set-point. The menu may also prompt for the entry of the number of targets per sheave. There may be a warning about low RPMs and/or the need for multiple targets per sheave.
Either a known or a calibrated speed ratio is used to calculate a dynamic percent slip of a V-belt. The above examples allow the ratio to be set either in the menu structure of the program by user input or for the ratio to be calibrated using the “Set Ratio” command which will then measure the RPMs of each respective pulley and calculate a speed ratio. When the “Set Ratio” button is pressed, the value of ratio is immediately calculated (and stored) based on a measured relative rotation rate. Accordingly, the slip is set to zero and ongoing calculations of percent slip are performed based on that set-point.
Slip meter embodiments do not necessarily require, but can be equipped with, an interface to an existing control system in a building, or industrial, manufacturing or other environment. A slip meter can also be equipped with a multiplexer to make it a central monitoring station for one or more belt drives each transmitting RPM data for that drive. The wireless version of the meter can be equipped with a wireless interface for monitoring one or more drives or communicating with an existing control system.
According to various embodiments of the invention, a number of optional features could be incorporated. “Light towers” can be implemented on or near the meter to display an indication of slip. One or more optional sensors to measure a belt's running temperature, using a heat spy or other thermal sensor, can be included to provide an additional indication of belt performance. Likewise, ambient temperature and/or humidity measurements can be acquired and displayed with appropriate circuitry. Vibration monitoring of the equipment using accelerometers can be designed into the meter. Noise sensors may likewise be included. Intelligent, feedback control of an electronic/pneumatic/hydraulic actuator system can be implemented into embodiments of the slip meter to allow for continuous adjustment of tension in a belt drive system to optimize belt life and performance based on slip measurements.
The slip meter is used to diagnose belt performance while the drive is in operation for predictive maintenance or repair or overhaul purposes. By implementing a slip-meter into a V-belt drive system, the user can easily determine the required time to re-tension a belt drive, optimize belt life, prevent downtime, and reduce energy loss. Energy savings are possible because percent slip is a good indication of energy loss, which can be reduced or substantially eliminated by proper belt system maintenance. Energy loss can be converted to a monetary value or cost and/or displayed for example in currency units. Applicable belt drive systems are found for example in air handling equipment (air conditioning and heating), conveying systems, fluid pumping systems, and the like. A portable meter could be taken from drive to drive to measure the slip of each quickly and easily. A permanently mounted unit would continuously monitor process critical applications.
Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. The invention disclosed herein may suitably be practiced in the absence of any element that is not specifically disclosed herein.
