Given is the following machine that is sorting boxes of two different heights. High boxes go to the north side of the platform, low boxes have to go to the south side of the platform. Realize the control with ladder logic:
The machine consists of the following components:
– A conveyer belt CB1 that transports boxes from left to right
– A conveyer belt CB2 that transports boxes onto and off a platform (Describe the direction by the following variables: “CB2 onto” and “CB2 off”)
– A platform that can turn left only (switch on “platform” to turn left)
– A sensor element to measure the height of a box. It consists out of two push buttons (PB1 and PB2)
o PB1 and PB2 are pressedà The box that is on the platform is a high box
o Only PB1 is pressedà The box on the platform is a low box
– A light sensor L2 is sitting on the platform that is being deactivated if a box has left the platform. A box on the platform is long enough to activate the light barrier even it has reached the end of CB2.
– A light sensor L1 at the end of the conveyer belt CB1 to activate the conveyer belt CB2. The light sensors (L1 and L2) are activated if a box is in between the light beam.
– Three positioning sensors to detect the position of the platform (PS center, PS north, PS south)
The machine works as follows:
– The conveyer belt CB1 runs until a box is in the right position on the platform
– The conveyer belt CB2 runs (CB2 onto) when a box has reached the end of CB1. This is being detected by the light barrier L1. If the box has reached the position on the platform the conveyer belt CB2 (CB2 onto) is being deactivated.
– Deciding on the status of the push buttons PB1 and PB2 the platform is turning left to the desired position
– If the platform has reached the desired position (PS north or PS south) the conveyer belt CB2 starts to unload the box (CB2 off)
– After unloading (L2 is being deactivated) CB2 stops and the platform is returning to the center position. This will activate CB1.
– CB1 is switched on until there is an active blocking signal “blocking CB1”. PB1 is the trigger for the blocking signal.
– The blocking signal is active until there is an active PS center signal without a PB1 signal. It is possible to realize this with an unblocking signal or Boolean algebra (negation of a specific combination of signals)