by Michael Robins
In the past I have read a great deal of literature regarding the control of models through the home personal computer (PC), fitted either with a game port or a specifically designed Input/Output (I/O) card, but I believe the model I have recently built to be the first employing a PLC for this purpose.
The PLC is in essence an industrial version of a PC, containing no keyboard or screen, only a micro-processor and other electronic gadgetry to perform its task. The screen and keyboard are obviously necessary for the programming of such a unit, but once this is done, the program is downloaded to the PLC's memory cartridge for use. Unlike a PC, this unit is specifically designed for an industrial application, ie, it is robust, has no moving parts and is fairly compact.
Programming the PLC is done in a graphical language, composed of steps and transitions, these defining the structure of the program. Where most PC based controllers and game ports have limited input and output options, various PLCs can have anything from 20 to a couple of thousand input/output ports (at a price).
The particular model that I built was a mock-up of a production line, where standard Meccano ball bearings were loaded into match boxes. The entire production line comprised 5 stages, each with a specific task. Thirteen different sensors were placed throughout the model and nine motors and 2 solenoids were required to operate the model (although the PLC that was being used was capable of 24 inputs and 16 outputs). The model was 37" long, 12½" high and 5½" wide. A 65 core cable joined the control board to the model.
Many of the sensors such as the photo detector (602) and contact strips (532) acting as micro-switches were made up from standard Meccano parts. In places where space was limited, commercially available micro-switches were used. The disadvantage of the Meccano photo-detector is that it is fairly slow acting and sensitive to ambient light. In cases where high response was necessary, infra-red LED transmitters and receivers were used which have the advantage of being insensitive to ambient light sources and being extremely compact.
The first stage of the model consisted of a storage space where 6 match boxes were stored on top of each other. On starting the production line, two photo sensors first confirmed the presence of a match box in the storage area, and that stage two was clear of any match boxes. If these two conditions were true, a solenoid was energised which forced the bottom box between two rotating 1" pulleys. These pulleys shot the box forward into stage 2 of the model.
Once in stage 2, the photo sensor confirmed the presence of the box. Had this not been the case, it would be assumed that a malfunction had occurred and the plant would be tripped. In the case of the box arriving safely, a motor operating a stamp would start up, stamping the top of the box with the name MECCANO. Once the stamp had returned to its original position, confirmed by a micro-switch, the motor stopped.
Stage 3 of the model was an inclined conveyer, allowing time for the wet ink to dry. Again numerous checks were made to ensure the correct position of the conveyer belt, and that no other boxes were present in this stage. If all these conditions checked out, the box was tilted upside down onto the conveyer and was slowly moved upwards.
The 4th stage was relatively simple in that all that was required was that the box be tipped upside down again so as to be in the correct orientation for filling. Again, prior to this action various checks were made to ensure that stage 5 was clear and ready to accept the box.
From a mechanical and programming point of view, stage 5 was by far the most complex to develop. After being tipped onto a second conveyer from stage 4, the box was moved along until it interrupted an infra-red light beam, so stopping the conveyer immediately and in so doing accurately positioning the box in place. Once this had been done and confirmed by a micro-switch, a second solenoid was energised, dropping the box opening mechanism into position. This mechanism was so arranged as to open the box, allowing it to be filled. A rotatory mechanism now started counting the precise number of balls to be dropped into the box. When all the balls were in the box, the opening mechanism reversed, so closing the box. The solenoid now de-activated, allowing the mechanism to move out of the way so permitting the full box to exit the end of the production line.
Once in operation, the model was absolutely fascinating to watch, with matchboxes flipping and balls rolling, creating the impression of a frenzy of movement. Due to the precise nature of the operation of such a model, it could be left running unattended for considerable periods of time. When gremlins did creep in, careful thought in programming ensured that the production line would stop prior to any damage being caused. The model generated much interest, and a further model has been commissioned by an automation company using their sensors and equipment.
Further details can be obtained from the author:
11 Latona Street
Kensington 2094, Johannesburg, South Africa.