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Increase your conveyor uptime with motion detection.

Properly applied, motion detection devices can signal underspeed conditions be[ore conveyor drive motors overload and trip.

As the complexity and sophistication of conveyor systems increase, so does the need to monitor and control conveyor speed. By detecting underspeed conditions before thermal overload relays in motors trip, motion detection improves productivity and increases conveyor uptime.

Because the drive motors on the majority of conveyor systems operate at a fixed speed, the factors that cause an underspeed condition also result in the motor(s) working harder to maintain the correct speed. Once a motor trips, the overload relays must be allowed to cool down before the conveyor can be restarted. In the case of a typical medium-sized motor, this waiting period runs three to five minutes. Adding the ability to detect an underspeed condition (and therefore an overloaded motor) largely eliminates this non* productive waiting period.

Motion detection is based on the principle of detecting an object in a predetermined amount of time. It uses some form of a presence sensor, coupled with a device that generates pulses at set time intervals, to accomplish the following four steps: (1.) detecting the presence of several targets; (2.) counting by measuring a known time interval; (3.) comparing this data with the known value of a previously set speed; and (4.) signaling the results of this comparison.

Three basic methods of motion detection are commonly available. A system can be assembled using available components; a dedicated motion detection device can be used; or a detection device can be coupled with a control device.

1. In the first case, a presence sensor (such as a photoelectric cell) is combined with separate devices (most often a timer) to count, compare, and signal. The timer is coordinated with the speed at which the target should be moving. Because the timer is positioned between the sensor and the output, the output signal is delayed, allowing a target to move out of the sensing zone before the time delay expires. However, if the target remains in the sensing zone for too long, the signal will indicate an underspeed condition.

2. Dedicated, self-contained motion detection systems incorporate all of the necessary components (sensor, timer, and comparitor) into a single housing. While other forms of motion detection systems may function by signaling the presence of a target, a dedicated device provides signals related to target speed from its normally closed output.

3. In some applications such as remote monitoring, the sensor and the accompanying electronics are mounted in separate housings. Target presence is detected by an inductive or capacitive sensor, or by using a photoelectric cell. An electronic pulse analyzer performs the actual speed com* parison, and switches the output circuit to signal underspeed conditions.

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Author:Gould, Les
Publication:Modern Materials Handling
Date:Aug 1, 1992
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