We will now look at shifting instructions in the BRX Do-More controller. PLC shifting instructions will move bits in memory areas a fixed amount when instructed. Bits are on/off, 1 or 0, and are usually associated together to form a memory location. The memory location can be used for numbers or positions. PLC BITS NUMBERS AND POSITION is a post that will review the different methods that the PLC will interpret the information in memory
We will be looking at the shifting (moving) of the bits within the memory location in several different ways. ROTL rotate left, ROTR rotate right, Math shift left operator, Math shift right operator, Math unsigned shift right operator and SR shift register are some of the instructions in our BRX Do-More PLC that will shift bits.
Let’s look at some samples of each of the above-mentioned instructions. Continue Reading!
Applying the five steps of PLC development to a plc shift register example. This PLC programming example will use a shift register to reject a product on a conveyor.
A lot of times when programming a PLC you need to track what has previously happened. Shift registers allow you to do just that. We will look at a PLC basic tutorial of a conveyor belt and reject station. Following the 5 steps to program development this PLC programming example should fully explain the function of shift registers. Ladder logic will be our PLC programming language.
We will be using the Do-more Designer software which comes with a simulator. This fully functional program is offered free of charge at automation direct.
Define the task:
Shift Register – Conveyor Reject
What has to happen?
A start pushbutton (NO) is used to start the conveyor and a stop pushbutton (NC) is used to stop. Sensor B detects a product on the conveyor belt and sensor A will detect if it is too large and needs to be rejected. The product is tracked along the conveyor belt and when under the reject station the Reject Blow Off will expel the bad product. The product is randomly placed on the conveyor belt, so an incremental encoder is used to track the conveyor movement. The reset pushbutton (NO) will signal that all of the product on the conveyor has been removed between the sensors and reject blow off.
Define the Inputs and Outputs:
PLC Connections for the Shift Register Conveyor Example
Inputs: Start Switch – On/Off (Normally Open) – NO Stop Switch – On/Off (Normally Closed) – NC Reset Switch – On/Off – NO Motor Encoder – On/Off – This will give a discrete signal when the conveyor is moving. It picks up the movement of the freewheel. Sensor A (Part Reject) – On/Off – NO Sensor B (Part Present) – On/Off – NO
Outputs: Motor – On/Off (Conveyor Run) Air Blow Off – On/Off (Reject)
Develop a logical sequence of operation:
PLC Logic for Shift Register Conveyor Reject
Fully understanding the logic before starting to program can save you time and frustration.
Sequence Table: The following is a sequence table for our conveyor reject application.
It is a simple sequence table but clarifies the following: When the power goes off and comes on the sequence will continue. This means that the shift sequencer must be memory retentive. Sensor A and B must be on to get tracked with a shift register.
Shift Registers: The Shift Register (SR) instruction shifts data through a predefined number of BIT locations. These BIT locations can be a range of BITs, a single Word or DWord, or a range of Words or DWords. The instruction has three inputs. Data, Clock and Reset. The data input will load the beginning bit with a ‘1’ if it is on or ‘0’ if it is not. The clock input is used to shift the data through the shift register. In our example, we will be using the encoder on the conveyor to track the reject container. So each pulse of the clock represents a distance on the conveyor. The last input is the reset. It will place ‘0’ in all of the bits within the shift register.
Develop the PLC program:
Start and stop of the conveyor motor.
Shift register to track the rejected parts. This will move the bits with each pulse of the encoder. Note that the ‘V’ memory is used because it is memory retentive.
This will look at the bit in front of the reject station. We can measure and count off the length (conveyor) and then find out what the bit location will be at the reject location.
Test the PLC program:
Shift Register Conveyor Reject
Test the program with a simulator or actual machine. Make modifications as necessary. Remember to follow up after a time frame to see if any problems arise that need to be addressed with the program.
Notes: Sometimes you can use multiple shift registers in your program. This can be helpful if you want to actually track the container as well as the rejects. You could also use a bit shift right (BSR) and bit shift left instructions (BSR) to do the same thing as we did with the shift register instruction. In the Do-more PLC, it is rotate left (ROTL) and rotates right (ROTR) instructions. Always check your instruction set of the controller that you are working with before starting to program.
If you have any questions or need further information please contact me.
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