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,
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
The output signal named
Motor will be
1 when the motor is ON and
0 when the motor id OFF.
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.
Start input is toggled to
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.
Stop input is toggled to
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.
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:
Stoptoggle switches are used to change the value of the respective inputs. When the toggle switch is in
Onposition the value of the corresponding input will be 1.
Motor: This indicates the status of the motor. Green colored
Motor ONindicator 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
Motoragainst time as the simulation progresses.