Patent Description:
Power transmission belts rely on proper tension in order to operate properly. Tension can be applied by adjustment of a driver sprocket center with respect to a driven sprocket center. An automatic tensioner can also be used.

In systems where an automatic tensioner is not used the proper operating tension of the belt can gradually decay over time. Loss of tension can cause the belt to slip ultimately resulting in belt failure. Belt failure results in down time for the system.

Belt tension can be determined as a function of oscillation frequency of the installed belt. Acoustic belt tension meters measure the vibrational frequency of a belt in Hz in a stationary condition, that is, the belt system is not operating. Higher frequency indicates a higher tension while a lower frequency indicates a lower tension, not unlike a stringed instrument.

Representative of the art is <CIT> which discloses a lateral sensor positioned proximate to at least one of the edges of a conveyor belt continually monitors the position of the edge of the conveyor belt. If lateral movement is detected by the lateral sensor, an adjustment motor rotates to move an end of a non-drive pulley to adjust for the lateral movement. The lateral sensor can be a non-contacting inductive proximity sensor, a proportional sensor such as a linear variable displacement transducer or a linear potentiometer which determines if the edge of the conveyor belt has moved laterally by monitoring the resistance in the spring, or a Hall effect sensor.

<CIT> discloses a belt monitoring apparatus for monitoring a belt in a pulley system comprising: a first sensing device including a transmitter configured to transmit a wireless signal and a receiver configured to receive the wireless signal reflected, in use, by a span of the belt, and to produce a corresponding output signal; processing means configured to detect in the output signal a signal component caused by transverse vibration of the belt span and to determine the frequency of the transverse vibration from the span vibration signal component; and a second sensing device to monitor teeth on the belt for speed and tooth position.

What is needed is a system having a signal processor operating on a first signal and a second signal to calculate a dynamic belt tension. The present invention meets this need.

An aspect of the invention is to provide a system having a signal processor operating on a first signal and a second signal to calculate a dynamic belt tension.

Other aspects of the invention will be pointed out or made obvious by the following description of the invention and the accompanying drawings.

The present invention comprises a belt sensor system as recited in claim <NUM>. Optional features are recited in the dependent claims.

The accompanying drawings, which are incorporated in and form a part of the specification, illustrate preferred embodiments of the present invention, and together with a description, serve to explain the principles of the invention.

<FIG> is a schematic of the system. A sensor array comprises two high accuracy, fast sampling, non-contact proximity sensors arranged in parallel.

An example system comprises a driver pulley <NUM>, a driven pulley <NUM> with a belt <NUM> trained between them. The sensor array comprises a first infra-red (IR) proximity sensor <NUM> and a second proximity sensor <NUM>. Both proximity sensors are connected to a digital signal processor <NUM>. Processor <NUM> is connected to a control system network.

Infra-red (IR) sensors are available from numerous sources including InfraTec, Mouser Electronics (#<NUM>-GP2Y0D815Z0F) and STMicroelectronics (#VL53L1X). These examples are only offered to illustrate the breadth of the invention and are not intended to limit the system to only these devices.

Signals from each sensor may be transmitted wirelessly <NUM>, <NUM> by Bluetooth™ to a receiver <NUM>, or via hardwire <NUM>, <NUM>. Bluetooth™ is a widely adopted wireless technology standard for exchanging data over short distances. The technology uses UHF frequencies in the ISM band from <NUM> to <NUM>. It is used on both fixed and mobile devices.

The drive length (L) is between the center of pulley <NUM> and the center of pulley <NUM>.

<FIG> is a detail of the sensor array. Proximity sensor <NUM> generates an analog signal and is used to detect the relative distance of the belt back <NUM>. Sensor <NUM> generates a digital signal and is used to detect passage of each tooth land area <NUM> while the belt is in operation. The tooth land area <NUM> is disposed between adjacent teeth <NUM>. A relative distance to each surface <NUM>, <NUM> can be determined based on sensor placement and known datum of the belt and of each sensor <NUM>, <NUM>. Preferably, the sensors are placed at or near the belt centerline equidistant between the driver pulley <NUM> and driven pulley <NUM>.

Sensors <NUM>, <NUM> measure the vibrations of the first or third nodes of fundamental harmonics the belt <NUM>. The raw signals are considered a half-rectified cosine/sine wave within a double amplitude waveform, see <FIG>.

Using the signal from each sensor in conjunction will yield the net, total, or peak-to-peak dimensional displacement of the vibrating belt. After data acquisition, amplitude signal processing techniques are performed by the DSP on the sensor signals. Tooth side proximity sensor <NUM> detects the meshing/excitation frequency. The DSP filters it from the span vibration signal from sensor <NUM>.

The excitation frequency is a function of the linear tooth velocity of the belt while in operation, hence sensor <NUM> detects belt velocity. Each flat surface <NUM> reflects the IR signal to the sensor receiver <NUM>. Since each tooth <NUM> scatters the IR light, the signal periodically drops out, hence, the signal transmitted by sensor <NUM> is periodic. The distance between each surface <NUM> is known and is based on the belt pitch P. The period between each signal <NUM> from surface <NUM> can be used to determine the velocity v of belt <NUM> in direction D.

Similarly, backside proximity sensor <NUM> measures the excited span vibration ±y and the related frequency of oscillation. Direction ±y is normal to direction D. Surface <NUM> reflects the IR signal to sensor receiver <NUM>.

A DSP/microcontroller and off-the-shelf IR sensors were used. Two different Sharp IR sensors are selected. A Sharp GP2Y0A51SK0F analog distance sensor rated <NUM>-<NUM> is used for sensor <NUM>. A Sharp GP2Y0D805Z0F digital distance sensor rated for <NUM> is used for sensor <NUM>.

The DSP microcontroller used to dual sample the data was an Arduino Pro™ branded Atmel™ Atmega™328P SMD running on <NUM>. 3V at a <NUM> clock rate, which can be programmed to sample an analog input channel at <NUM> with <NUM>-bit resolution, and can sample a digital input channel greater than <NUM>. The microcontroller was also programmed using the Arduino Integrated Development Environment (IDE), a Java based program used to create C-code/firmware for the controller.

The system also includes a MatLab™ based GUI used to parse the messages, log data to file, and display the tooth frequency and vibration of the belt. The com port settings, baud rate, and type of flow control are hard coded into both the microcontroller firmware and GUI software.

Three separate pieces of C code are combined into one main loop on the microcontroller, and a MatLab based user display was written for the purposes of testing the theory and application of this technical investigation; the firmware on the microcontroller for timing control, data acquisition and sending serial messages, and the MatLab script for the graphical user interface (GUI) and datalogging.

The firmware written for the microcontroller is a combination of three separate algorithms; analog sampling of the proximity sensors, and calculating a large array of time series based proximity data through Fast Fourier Transforms (FFT), and a microsecond frequency counter. After setting up non-volatile global variables, timers are declared to accurately control the analog sampling and serial output rates. The serial output rate is statically set to update the COM port at <NUM>, and the analog sample rate is based on the array size (<NUM>^n term) used for FFT. After each sample, the analog value is stored in an circular buffer array for later use. In this system, the array is set to <NUM> (<NUM>^<NUM>) terms of which half are real and half are imaginary values; only the real terms are used in the frequency analysis. Since the FFT is a process intensive series of functions, it is only called to operate on the array before the serial output is sent to COM port.

Another algorithm used in the firmware of the DSP/microcontroller is the tooth frequency counter. The logic is identical to RPM sensors in which the time, in microseconds, is measured between the low to high pulse transitions, and placed into a rolling average array. The digital input for the proximity sensor is tied to the pin interrupt function of the microcontroller.

The rolling average of the array is then stored in a global variable where it will be averaged in order to calculate the meshing frequency and drive speed in the serial output to the user display or network.

The serial messages are based on a timer set to call the function every <NUM> milliseconds, or <NUM>/<NUM>th of a second, from the microcontroller, and follow a very simple form: <NUM> bytes used for header, <NUM> bytes used for the FFT message, <NUM> bytes used for tooth meshing frequency, and <NUM> bytes used for endline characters.

The Matlab based GUI script runs user set COM port settings before allowing any messages from the microcontroller through. Once settings are matched, the Matlab pulls each byte from the COM ports' circular buffer, and begins to look for the header bytes sent from the microcontroller. After a correct header comparison, the script will log a timestamp, read the buffer until the endline characters, and write the raw bytes to file. The script also updates plot for FFT, convert the raw the bytes into decimal form, and update values for display.

Sensors <NUM>, <NUM> detect the relative distances of the belt back and tooth/land areas while the belt is in operation. In the example system the analog (span vibration) sensor <NUM> has a range of <NUM>-<NUM>, and can be placed at approximately <NUM> from the backside <NUM> of the belt. Similarly, the digital (tooth counter) sensor <NUM> has a ranged hysteresis of <NUM>-<NUM>, and can be placed approximately <NUM> from the tooth <NUM> and land side <NUM> of the belt.

<FIG> is a system flowchart. System start occurs at <NUM>. Setup is run <NUM>. User input is read <NUM>. The displacement signal from sensor <NUM> is read <NUM>. Data is stored in the circular buffer <NUM>. A circular buffer is a data structure that uses a single, fixed-size buffer as if connected head to tail in a circular fashion. This structure is useful for buffering data streams such as from the instant sensor array <NUM>, <NUM>. Data is read from buffer <NUM>, or directly from user input <NUM>.

A FFT is performed at <NUM>. The FFT samples a signal over a period of time and divides it into its frequency components. These components are single sinusoidal oscillations at distinct frequencies each with their own amplitude and phase. Hence, FFT is used to convert a signal from sensor <NUM> from its original time domain to a representation in the frequency domain and vice versa.

The meshing frequency from sensor <NUM> is filtered at <NUM>. The data used at <NUM>, <NUM> is then deleted from the buffer <NUM>. The dominant frequency is selected <NUM>. Using the selected dominant frequency (f) at <NUM> Mersenne's law is used to calculate the belt tension (T).

Belt speed is calculated from the tooth frequency, and used at <NUM>. Belt speed can be acquired from a system RPM meter (not shown) or calculated from the meshing frequency. User provided belt material constants are read at <NUM>. User provided drive constants are read at <NUM>. Belt material constants <NUM> are input to Mersenne's law calculation at <NUM>.

The difference of the signal from sensor <NUM> and sensor <NUM> yields a cleaner span vibration waveform that is used to calculate the frequency of oscillation (f). The derived frequency (f) is used to approximate the active belt tension (T) using Mersenne's Law for string vibrations: <MAT> Where.

The dynamic belt tension (T) is calculated using the system constants, namely, drive center distance (L) and linear density of the belt (µ) in addition to measured values from sensors <NUM> and <NUM>.

Execution of the calculation at <NUM> gives the active tension measurement <NUM> (T). The active tension measurement is then input to the dynamic tension Tdyn compensation equation <NUM>. The Tdyn term is the sum of W/<NUM>(Static) + half the difference of the tension between T(t) and T(s) which is the applied torque. The W/<NUM>(Static) is equivalent to the static belt tension (W). W is a static load which is applied to the belt through the pulleys at the time of installation.

Tt - Ts = 2Q/Dp is the active portion of the equation, where Q is the transmitted torque and Dp is the pitch diameter of the pulleys <NUM>, <NUM>. The span vibration of Tt and Tg are measureable using the proximity sensors, and the tension of each side is calculated using Mersenne's Law solved for tension. T(t) (tight side tension) and T(s) (slack side tension) are calculated in step <NUM>.

A centrifugal term is due to a running drive = K*m*v<NUM>. K is a system constant for units and is set to equal <NUM> for metric units. Similarly, K = <NUM> × <NUM>^<NUM> for English units. Lastly, m = µ, and is mass/unit length, and v is the belt speed that is calculated using the measured tooth frequency.

Therefore, the Dynamic tension can be calculated by summing all parts of the equation: <MAT>.

The calculation gives the total dynamic tension Tdyn <NUM>. This result signal can be output to a GUI <NUM> or stored <NUM> in a system memory.

The total dynamic tension Tdyn can be used to control system operation based on dynamic belt tension. For example, alarm limits can be included to alert an operator in the event the system deviates from prescribed limits. System history can be used to estimate remaining belt life.

<FIG> is a chart of the combined signals. The raw signal measurements are considered a half-rectified cosine/sine wave within a double amplitude waveform. Signal <NUM> is from sensor <NUM>. Signal <NUM> is from sensor <NUM>. The periodic nature of signal <NUM> is a function of the velocity of passage of each tooth land <NUM> past sensor <NUM>. The sinusoidal nature of signal <NUM> is the result of the belt span vibration along an axis normal to the direction of movement D.

Claim 1:
A belt sensor system (<NUM>) comprising:
a first infra-red sensor (<NUM>) disposed adjacent to a toothed belt (<NUM>) to detect a relative distance of a first back belt surface (<NUM>) and generate a first signal (<NUM>) therefore;
a second infra-red sensor (<NUM>) disposed adjacent to the toothed belt (<NUM>) to detect a periodic signal and a relative distance of a tooth land area : (<NUM>) between adjacent belt teeth (<NUM>) of a second toothed belt surface (<NUM>) to generate a second signal (<NUM>) therefore;
a signal processor (<NUM>) operating on the first signal (<NUM>) and second signal (<NUM>) to calculate a frequency of oscillation (f) of the belt (<NUM>) and a dynamic belt tension (T) using the equation: <MAT>
Where
T = belt tension
f = the frequency of oscillation of the belt
n = harmonic mode
L = drive length
µ = mass per belt unit length.