Download PDF version (217.5k). The full text of this article is available as a PDF document.
Every plant seems to have one production bottleneck. One plant might have a cement auger that occasionally gets plugged when a chunk of silo buildup falls into the discharge chute. Another plant might require the watchful eye of a plant laborer to keep a bucket elevator operating. And at still another plant, drivers might have a difficult time positioning their hoppers under the material discharge chute. Ultrasonic technology can help solve these production difficulties at concrete plants. Inexpensive ultrasonic systems are easy to install, program and maintain. Although the technology is basic, producers can solve complex job-specific problems not normally addressed by a plant automation system. Ultrasonic systems use distance and time to monitor cyclical events. In practical terms, ultrasonic systems can determine target movement, how far the target has moved or how fast the target is moving. An ultrasonic system has only two components. One component is a fixed-location sensor that both emits a high-frequency sound wave and receives the echo after the sound wave hits the target. The second component is a programmable controller that measures and analyzes the sensor-signal activity. When the controller detects an unplanned change in sensor-signal activity, it sends an output signal to the producer's existing plant warning devices. Sensors can monitor targets at distances of an inch to 200 feet. Sensor sizes depend on the separation of the sensor from the target. Other application factors might influence the selection of a sensor, including target orientation and size, presence of steam and extreme temperatures. The controller times how long it takes for the echo to travel from the sensor to the target and back. Producers can program the controller for different situations. For instance, if the sound wave never returns, the target is out of position. Or if the sound waves return too fast, the target may be too close to the sending sensor, or there may be an obstruction. Ultrasonic systems can monitor material levels in a storage bin, vehicle loading positions, a conveyor belt's edge, whether buckets on a bucket elevator are missing and spillage off the end of a conveyor belt or feeder. A single sensor and controller can cost less than $600. Controller selection also needs some consideration. While ultrasonic sensors can be used with sophisticated control systems, there are many inexpensive controllers that can be programmed in two minutes. The set-point controller is a basic style. It compares a current sensor reading with a predetermined reading. These controllers usually have two set points that allow for minimum/maximum monitoring or for caution/warning levels of alarm. Another controller type, a PID, is often used in "closed-loop" systems such as process temperature or batching. The controller often provides a third signal option for controlling a variable-speed motor that can either increase or decrease the speed of a preceding activity. Even when plants already use large programmable logic controllers or personal computer control systems, producers can use an ultrasonic sensor system. The controller produces a single and discrete alarm output signal that easily fits into any ladder-logic control sequence and requires little, if any, reprogramming.