11. FLOWCHART BASED DESIGN
11.1 INTRODUCTION
A flowchart is ideal for a process that has sequential process steps. The steps will be executed in a simple order that may change as the result of some simple decisions. The symbols used for flowcharts are shown in Figure 126 Flowchart Symbols. These blocks are connected using arrows to indicate the sequence of the steps. The different blocks imply different types of program actions. Programs always need a start block, but PLC programs rarely stop so the stop block is rarely used. Other important blocks include operations and decisions. The other functions may be used but are not necessary for most PLC applications.
A flowchart is shown in Figure 127 A Flowchart for a Tank Filler for a control system for a large water tank. When a start button is pushed the tank will start to fill, and the flow out will be stopped. When full, or the stop button is pushed the outlet will open up, and the flow in will be stopped. In the flowchart the general flow of execution starts at the top. The first operation is to open the outlet valve and close the inlet valve. Next, a single decision block is used to wait for a button to be pushed. when the button is pushed the yes branch is followed and the inlet valve is opened, and the outlet valve is closed. Then the flow chart goes into a loop that uses two decision blocks to wait until the tank is full, or the stop button is pushed. If either case occurs the inlet valve is closed and the outlet valve is opened. The system then goes back to wait for the start button to be pushed again. When the controller is on the program should always be running, so only a start block is needed. Many beginners will neglect to put in checks for stop buttons.
The general method for constructing flowcharts is:
2. Determine the major actions, these are drawn as blocks.
3. Determine the sequences of operations, these are drawn with arrows.
4. When the sequence may change use decision blocks for branching.
Once a flowchart has been created ladder logic can be written. There are two basic techniques that can be used, the first presented uses blocks of ladder logic code. The second uses normal ladder logic.
11.2 BLOCK LOGIC
The first step is to name each block in the flowchart, as shown in Figure 128 Labeling Blocks in the Flowchart. Each of the numbered steps will then be converted to ladder logic
Each block in the flowchart will be converted to a block of ladder logic. To do this we will use the MCR (Master Control Relay) instruction (it will be discussed in more detail later.) The instruction is shown in Figure 129 The MCR Function, and will appear as a matched pair of outputs labelled MCR. If the first MCR line is true then the ladder logic on the following lines will be scanned as normal to the second MCR. If the first line is false the lines to the next MCR block will all be forced off. If a normal output is used inside an MCR block, it may be forced off. Therefore latches will be used in this method.
The first part of the ladder logic required will reset the logic to an initial condition, as shown in Figure 130 Initial Reset of States. The line will only be true for the first scan of the PLC, and at that time it will turn on the flowchart block F1 which is the reset all values off operation. All other operations will be turned off.
The ladder logic for the first state is shown in Figure 131 Ladder Logic for the Operation F1. When F1 is true the logic between the MCR lines will be scanned, if F1 is false the logic will be ignored. This logic turns on the outlet valve and turns off the inlet valve. It then turns off operation F1, and turns on the next operation F2.
The ladder logic for operation F2 is simple, and when the start button is pushed, it will turn off F2 and turn on F3. The ladder logic for operation F3 opens the inlet valve and moves to operation F4.
The ladder logic for operation F4 turns off F4, and if the tank is full it turns on F6, otherwise F5 is turned on. The ladder logic for operation F5 is very similar.
The ladder logic for operation F6 turns the outlet valve on and turns off the inlet valve. It then ends operation F6 and returns to operation F2.
11.3 SEQUENCE BITS
In general there is a preference for methods that do not use MCR statements or latches. The flowchart used in the previous example can be implemented without these instructions using the following method. The first step to this process is shown in Figure 135 Label the Flowchart Blocks and Arrows. As before each of the blocks in the flowchart are labelled, but now the connecting arrows (transitions) in the diagram must also be labelled. These transitions indicate when another function block will be activated.
The first section of ladder logic is shown in Figure 136 The Transition Logic. This indicates when the transitions between functions should occur. All of the logic for the transitions should be kept together, and appear before the state logic that follows in Figure 137 The Function Logic and Outputs.
The logic shown in Figure 137 The Function Logic and Outputs will keep a function on, or switch to the next function. Consider the first ladder rung for F1, it will be turned on by transition T1 and once function F1 is on it will keep itself on, unless T2 occurs shutting it off. If T2 has occurred the next line of ladder logic will turn on F2. The function logic is followed by output logic that relates output values to the active functions.
11.4 SUMMARY
· Flowcharts are suited to processes with a single flow of execution.
· Flowcharts are suited to processes with clear sequences of operation.
11.5 PRACTICE PROBLEMS
(Note: Problem solutions are available at http://sites.google.com/site/automatedmanufacturingsystems/)
1. Convert the following flow chart to ladder logic.
2. Draw a flow chart for cutting the grass, then develop ladder logic for three of the actions/decisions.
3. Design a garage door controller using a flowchart. The behavior of the garage door controller is as follows,
- there is a single button in the garage, and a single button remote control.
- when the button is pushed the door will move up or down.
- if the button is pushed once while moving, the door will stop, a second push will start motion again in the opposite direction.
- there are top/bottom limit switches to stop the motion of the door.
- there is a light beam across the bottom of the door. If the beam is cut while the door is closing the door will stop and reverse.
- there is a garage light that will be on for 5 minutes after the door opens or closes.
11.6 ASSIGNMENT PROBLEMS
1. Develop ladder logic for the flowchart below.
2. Use a flow chart to design a parking gate controller.
3. A welding station is controlled by a PLC. On the outside is a safety cage that must be closed while the cell is active. A belt moves the parts into the welding station and back out. An inductive proximity sensor detects when a part is in place for welding, and the belt is stopped. To weld, an actuator is turned on for 3 seconds. As normal the cell has start and stop push buttons.
b) Implement the chart in ladder logic
4. Convert the following flowchart to ladder logic.
5. A machine is being designed to wrap boxes of chocolate. The boxes arrive at the machine on a conveyor belt. The list below shows the process steps in sequence.
1. The box arrives and is detected by an optical sensor (P), after this the conveyor is stopped (C) and the box is clamped in place (H).
2. A wrapping mechanism (W) is turned on for 2 seconds.
3. A sticker cylinder (S) is turned on for 1 second to put consumer labelling on the box.
4. The clamp (H) is turned off and the conveyor (C) is turned on.
5. After the box leaves the system returns to an idle state.
Develop ladder logic for the system using a flowchart. Don't forget to include regular start and stop inputs.