Page 77 - Instrumentation and Measurement

In order to generate the displacement-event diagram of Figure 5.15 the sequence of operations with
Figure 5.16 is:
Event 1
1. Start push button pressed.
2. Cylinder A extends, releasing limit switch a-.
Event 2
3. Cylinder A fully extended, limit switch a+ operated to start B extending.
4. Cylinder B extends, limit switch b- released.
Event 3
5. Cylinder B fully extended, limit switch b+ operated to start cylinder A retracting.
6. Cylinder A retracts, limit switch a+ released.
Event 4
7. Cylinder A fully retracted, limit switch a- operated to start cylinder B retracting.
8. Cylinder B retracts, limit switch b+ released.
Event 5
9. Cylinder B fully retracted, limit switch b- operated to complete the cycle.
The cycle can be started again by pushing the start button. If we wanted the system to run
continuously then the last movement in the sequence would have to trigger the first movement.
As an illustration of the types of problems that sequential control can be used for, consider the control
required with an automatic machine to perform a number of sequential actions such as positioning
objects, operating clamps and then operating some machine tool (Figure 5.17). This requires the
switching in sequence of a number of cylinders, the movements of the cylinder pistons being the
mechanisms by which the actions are initiated.

Page | 77