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Using Timers in Ladder Logic

This example shows how to model and simulate a motor controller in Simulink® by using Ladder Logic. The example uses the ladder Timer instruction to implement the logic for delayed switching of a motor. The timer T1 is used to control the starting delay and the timer T2 is used to control the stopping delay.

Inputs and Outputs

The Motor Controller has two inputs, Start and Stop. Changing the Start input value to 1 will start the motor after 5 seconds. Changing the Stop input value to 1 will stop the motor after 2 seconds. Stop input will override the Start input.

The output signal named Motor will be 1 when the motor is ON and 0 when the motor id OFF.

Motor Controller

The Motor Controller block is a PLC Controller block. It contains a Ladder Program block which houses the ladder logic. Open the Motor Controller block and then open the Ladder Diagram Program block to view the ladder logic for the controller.

This ladder logic has a TON timer named as T1 which is responsible for the delay during starting the motor, and a TOF timer named as T2 which is responsible for the delay during the stopping of the motor.

When the Start input is toggled to 1, the MotorStart output in the first rung gets activated which starts the timer T1 counting operation. T1.DN bit is set when the T1 finishes counting. This causes the third rung with timer T2 to become activated. Since T2 is a TOF timer, the T2.DN bit is set but, timer starts counting operation only when this rung becomes false. Hence, both the inputs to the lowermost rung are true and the Motor output gets activated.

When the Stop input is toggled to 1, the MotorStart coil gets deactivated and hence the T1.DN bit is reset and the timer T2 starts counting. Once the T2 finishes counting operation, the T2.DN bit gets reset and the Motor output gets deactivated.

Timer Configuration

The timer configurations are specified in the InitFcn callback inside the Model Properties. To modify the start and stop delays, open the InitFcn callback from the Model Properties dialog from Modeling > Model Settings > Model Properties.

T1_InitialValue.PRE specifies the Preset value of timer T1 and the T2_InitialValue.PRE specifies the Preset value of timer T2. Both these values are specified as milliseconds.

Human Machine Interface (HMI)

Double click the HMI Subsystem if it is not already open to bring up the Human Machine Interface (HMI) for this example. This HMI has the following sections:

  • Inputs: The Start and Stop toggle switches are used to change the value of the respective inputs. When the toggle switch is in On position the value of the corresponding input will be 1.

  • Motor: This indicates the status of the motor. Green colored Motor ON indicator means that the motor is running whereas grey colored indicator means that the motor is stopped.

  • Graphs: This section of the HMI Subsystem displays the status of Start, Stop and Motor against time as the simulation progresses.