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Model a Redundant Sensor Pair by Using Atomic Subcharts

This example uses:

Open Model

This model shows how to model a redundant pair of sensors. By using atomic subcharts, you can compose a large Stateflow® chart from other charts that reside in a library file.

Main Chart

In this model, the chart RedundantSensors contains two linked atomic subcharts (Sensor1 and Sensor2) and a state (Alarm).

At the start of the simulation, the subchart Sensor1 is active. This atomic subchart reads the input u1. If the input value remains between 75 and -75, Sensor1 passes this value as the output of y. Otherwise, the sensor fails and subchart Sensor2 becomes active.

In a similar way, Sensor2 reads the input u2 and checks that its value remains between 100 and -100. Sensor2 passes this value as the output of y. Otherwise, the sensor fails and the chart transitions to the Alarm state.

Library Chart

The logic for both Sensor1 and Sensor2 is defined in a library model. In this model, the chart SingleSensor accepts an input u and provides a filtered sensor output y.

The chart detects out-of-range errors in the sensor input u. Initially, the sensor is in the state OK. If u goes out of range, the chart takes the transition from OK to the state FailOnce. If u stays out of range for longer than one second, then the chart transitions to the state Fail. In this case, the sensor outputs a constant value of zero. This pattern allows the sensor to ignore transient spikes in the sensor reading.

Map Inputs, Outputs, and Parameters

The chart RedundantSensors has two inputs (u1 and u2), while the library chart that defines the atomic subcharts has only one input (u). To enable the atomic subcharts to access a different chart input, right-click each subchart and select Subchart Mappings. In the Mappings tab of the properties dialog box, you can:

  • Specify which symbol in the main chart corresponds to each symbol in the subchart.

  • Assign values to parameters defined in the subchart.

For example, in the case of subchart Sensor1:

  • The subchart input u is mapped to the main chart input u1.

  • The subchart output u is mapped to the main chart output y.

  • The subchart parameters HIGH and LOW are assigned the values 75 and -75.

In the case of subchart Sensor2:

  • The subchart input u is mapped to the main chart input u2.

  • The subchart output u is mapped to the main chart output y.

  • The subchart parameters HIGH and LOW are assigned the values 100 and -100.

Simulation Behavior

Initially, both inputs and both outputs to the chart are zero. To change the value of the chart inputs, double-click the Gain blocks and drag the slider.

As long as the value of u1 is between -75 and 75, the output value y tracks the input value u1. If the value of u1 exceeds these bounds, the value of y begins to track the input value u2. If the value of u2 falls outside the range from -100 to 100, y returns a value of zero and Alarm returns a value of one.

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