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| Solenoid Provides Positive Position Feedback using Optics |
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Many applications for solenoids in today's products require more out of a solenoid than simply moving a load. They require many added features which gives engineers the flexibility they need to accomplish the many complex tasks which must take place in their end products.
One such recent application involves the use of a tubular solenoid in a banking machine. The solenoid had to be used to lock a motorized transport in various positions while other tasks were being performed. The transport mechanism is moved rapidly from one position to the next and weighed several pounds. This created the need to know when the mechanism was safe to move and when it was locked in place by the solenoid. In addition due to the weight involved, the solenoid had to be strong enough to hold the unit in place and to continually withstand the side loads. This created some interesting challenges for the solenoid. It had to have a special shaped hardened plunger tip; a built in indicator which would alert the system as to its position; a built in spring; the reliability to withstand the load of the transport; and last but not least the long term reliability and integrity associated with the banking industry.
To breakdown the requirements, we looked first at the need for the solenoid to give the system an indication of whether or not it was in the energized or de-energized position. This could be accomplished in a variety of ways, such as external switches operated by the solenoid's plunger which would indicate its position. Since reliability was of major concern, Saia-Burgess engineers chose a previously proven technology of using optical sensors. This technology provides for non-contacting components where an infrared beam of light emitted by a L.E.D. is sensed by a corresponding photo cell. The plunger of the solenoid passes through this beam of invisible light at certain points in the stroke of the solenoid and thus either blocks the beam or allows it to reach the detector.
The plunger of the solenoid actually has an extension rod with an adjustable tip which is used to set the desired trip point. In this manner the output of the detector can be set to indicate the precise position of the plunger needed by the system to insure that the transport is either safe to move (solenoid energized) or is locked in place (solenoid de-energized).
The output from the sensor is TTL compatible and therefore interfaces nicely with the computer controlled system.
As was indicated earlier the unit also had to have a built in return spring. This was accomplished by locating the spring inside the unit around the extension rod which was used to activate the sensor. The spring was designed to provide a life compatible with the number of cycles the solenoid would see in the application.
Another feature of the solenoid is a special molded front bearing system. This was needed to help keep the plunger aligned accurately and to support the plunger against the heavy side loads due to the weight of the transport mechanism. This holds the mechanism in place with a minimum of movement.
In the application the solenoid is mounted in tight quarters, very close to the moving transport. This required that all wiring be contained within a single harness and that special routing of the wires needed to be considered. The wires for the solenoid and the sensor circuit are contained in a single five wire bundle. The two solenoid wires exit the back of the solenoid and enter directly into the bottom of the sensor housing and up to the internal PC board. An arc suppression diode for the solenoid is located on this board also. From the PC board a total of five wires exit the opposite side of the housing, two for the solenoid and three for the sensor. The wires are then bundled down the side of the solenoid in a particular position so they do not interfere with the moving transport mechanism. They are then terminated in a customer specified connector.
The solenoid evolved from a 3/4" standard STA® tubular solenoid. The length of the solenoid with the sensor package is increased only about 0.75 inch. The overall diameter is not affected as the diameter of the sensor housing is 0.75 inch which is the same as the solenoid.
The solenoid also features other standard items such as a built in air gap spacer which eliminates the need for external retaining rings and washers. These parts are normally used to stop the plunger and control the air gap. Eliminating these parts also eliminates the inherent problems of early failure. On this design, any external parts on the plunger such as these could have caused some additional problems.
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