AN ULTRASONIC ECHO-RANGING SENSOR FOR BOARD INSPECTION AND SELECTION

Once reserved for the most sophisticated machinery, ultrasonic noncontact position sensors are rapidly being accepted by a variety of industrial machinery manufacturers as low-cost, reliable solutions to demanding or unique applications, especially in areas where no other technology has succeeded (see "An Introduction to Ultrasonic Sensing", Sensors, November 1989). The James L. Taylor Manufacturing Co. of Poughkeepsie, New York, has developed a simple yet ingenious machine called the Opti-Sizer that would not have been possible without ultrasonic technology.

Most people are familiar with butcher block furniture and cabinetry, in which random-width wood stock is placed edge to edge, glued together, and then cut and assembled into finished products. The random-width pattern of butcher block is, by design, the result of optimizing the yield of parallel-ripped boards (cut lengthwise) and thereby eliminating waste. The Opti-Sizer completely automates the process of selecting specific random-width boards so that when they are glued together the total width will be within a user specified tolerance.

Random width boards (3/4-6 in.) are placed edgewise in slots on a specially designed continuous conveyor that feeds the boards into a series of gates, one for each slot in the conveyor (see Figure 1). An ultrasonic sensor is mounted on a traveler that is passed once across all the gates, looking down at each board and gate. As the sensor passes over each board, it transmits bursts of ultrasonic sound that bounce off the top edge and return as an echo. The sensor is connected to an electronic system that measures the amount of time it takes for the echo to return and uses the known speed of sound in air to calculate the distance to the top edge of each board. The distance measurement is converted by the sensor to a 4-20 mA output and is sent to the machine's computer so that board width can be determined.

The gates are separated by a metal divider designed to be taller than the boards. As the sensor is moved from one gate to the next, it passes over the divider, receiving echoes off the top edge. The computer registers this height as a boundary to a gate area and counts each echo event to keep track of which gate the sensor is measuring, thus eliminating any need to monitor the position of the traveler. The fixed height of the dividers also serves as a reference to monitor changes in sound velocity, verifying measurement accuracy.

When all the board widths are measured and the information is stored in the computer, it becomes a simple matter for the computer to select which ones will, when combined, equal a preset desired width within prescribed tolerances. The gates are opened on the selected boards and they are fed through the gate by the conveyer system. The gates then close and the operation is repeated. Up to 6.8 sets of boards can be processed per minute depending on board length, number of conveyor slots, and number of boards in each selected panel. Maximum width error of the assembled panel is advertised at + 0.13/ - 0.00 in., but typically the results are half that, or + 0.06/ - 0.00 in., because of the reliable accuracy of the ultrasonic measuring system. If no combination of boards will create a proy sized panel, the machine can be programmed to make a larger panel, if possible, or to request intervention by an operator.

An experienced worker can process a maximum of 6 sets of boards/min., with a maximum error of up to 1.00 in. wider than desired (boards are later trimmed as required.) Of course, this production rate is not consistently achievable with manual operation, and the trim step (with its resulting scrap) becomes unnecessary if the set of boards is selected to tolerance in the first place. Taylor Manufacturing has also designed automated gluing equipment that takes the selected boards and automatically glues, compresses, and stores the panels in an automated clamp carrier. This is an ideal application for ultrasonic echo-ranging technology. The level of precision required for the noncontact distance-ranging information shortened the list of candidate technologies. Taylor Manufacturing also evaluated ranging photoelectrics and laser devices. Photoelectric sensors were not selected because the lenses might become covered with wood dust (affecting accuracy) and the target wood pieces might be dirty or not always the same color (affecting reflectivity). Lasers were rejected for the same reasons and also because of cost considerations and limited sensing range.

The Model M-4000 Ultrasonic Measurement and Control System, manufactured by Massa Products Corp., was chosen for several reasons: small transducer (sensor) size, high-speed operation (150 measurements/s), and the availability of a high-frequency narrow beam ultrasonic transducer with no side lobes (preventing interference from adjacent gate dividers).

Ultrasonics has been used successfully in many woodworking and saw mill machinery applications. Long sensing ranges, insensitivity to target color and translucency, and the ability to reliably receive echoes from most types of targets qualifies ultrasonic technology to be the ideal method of solving a wide variety of proximity and position sensing applications.

As a final note of interest, development of the Opti-Sizer came about as a result of a letter to the Sensors Wish List. Taylor Manufacturing has received two Challengers awards for advancing the state of the art in edge-gluing machinery. The awards are given by the International Woodworking Machinery and Furniture Supply Fair held in Atlanta every two years.

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