Allen Bradley PLC Programming: Multiple Inputs to Step Sequencer

This blog post will guide you through the process of programming multiple inputs to a step sequencer in an Allen Bradley PLC. This is a valuable skill for anyone learning PLC programming, especially those working with Allen Bradley systems.

Understanding Step Sequencers

A step sequencer is a common element in PLC programming used to control a series of sequential events. It’s like a recipe, guiding the PLC through a series of steps in a specific order. Each step represents a different state in the process, and the sequencer moves from one step to the next based on specific conditions, such as the activation of an input.

Programming Multiple Inputs

When you have multiple inputs controlling a step sequencer, you need to establish a clear logic to determine the next step based on the input combinations. This often involves using logic gates like AND, OR, and XOR gates to process the inputs effectively.

Example: A Simple Conveyor System

Let’s consider a simple conveyor system with two inputs: a sensor at the start of the conveyor and a sensor at the end. We want to use a step sequencer to control the conveyor motor based on the sensor states.

Step 1: Define the Inputs

• Input 1: Start Sensor (Activated when an item is detected at the start of the conveyor)
• Input 2: End Sensor (Activated when an item is detected at the end of the conveyor)

Step 2: Define the Steps

• Step 1: Idle – The conveyor motor is off.
• Step 2: Conveyor Running – The conveyor motor is on, moving the item along the conveyor.
• Step 3: Item Complete – The conveyor motor is off, indicating the item has reached the end of the conveyor.

Step 3: Implement the Logic

We can use the following logic to control the conveyor motor:

• **Step 1 to Step 2:** Transition occurs when the Start Sensor is activated (Input 1 = ON).
• **Step 2 to Step 3:** Transition occurs when the End Sensor is activated (Input 2 = ON).
• **Step 3 to Step 1:** Transition occurs when the Start Sensor is deactivated (Input 1 = OFF).

Step 4: Programming in RSLogix 5000

In RSLogix 5000, you would use the ‘SFC’ (Sequential Function Chart) instruction to implement the step sequencer. The SFC instruction allows you to define the steps and transitions, and you can use ladder logic to define the logic for each transition.

Here’s a simplified example of the ladder logic for the conveyor system:

// Step 1: Idle
XIC I:1/0  // Start Sensor ON
OTE O:1/0  // Conveyor Motor ON
// Step 2: Conveyor Running
XIC I:2/0  // End Sensor ON
OTE O:1/0  // Conveyor Motor OFF
// Step 3: Item Complete
XIC I:1/0  // Start Sensor OFF
OTE O:1/0  // Conveyor Motor ON

Key Points to Remember

• Clearly define your inputs and outputs.
• Use logic gates to process multiple inputs effectively.
• Implement the step sequencer using SFC instructions in RSLogix 5000.
• Test your program thoroughly to ensure it operates as expected.

Conclusion

Programming multiple inputs to a step sequencer in an Allen Bradley PLC can be a powerful technique for controlling complex processes. By understanding the fundamentals of step sequencers and using logic gates effectively, you can design and implement robust PLC applications.