Stateflow

Simulating the "fuel rate controller" Model

On starting the simulation, and assuming no sensors have failed, the Stateflow diagram initializes in the Warmup mode in which the oxygen sensor is deemed to be in a warmup phase. If Stateflow is placed into animation mode, the current state of the system can clearly be seen highlighted on the Stateflow diagram, as shown.

After a given time period, defined by o2_t_thresh, the sensor is deemed to have reached operating temperature and the system settles into the normal mode of operation, shown above, in which the fueling mode is set to NORMAL.

As the simulation progresses, the chart is woken up synchronously every 0.01 second. The events and conditions that guard the transitions are evaluated and if a transition is valid, it is taken and animated on the Stateflow diagram.

To illustrate this, you can provoke a transition by switching one of the sensors to a failure value on the top-level Simulink model. The system detects throttle and pressure sensor failures when their measured values fall outside their nominal ranges. A manifold vacuum in the absence of a speed signal indicates a speed sensor failure. The oxygen sensor also has a nominal range for failure conditions but, because zero is both the minimum signal level and the bottom of the range, failure can be detected only when it exceeds the upper limit.

Switch the Simulink switch for the manifold air pressure sensor to the off position to witness the following sequence of transitions (note the diagram that follows).

1. Switching the Simulink manifold air pressure sensor switch causes a value of zero to be read by the fuel rate controller.
2. When the chart is next woken up, the transition from the press_norm state becomes valid as the reading is now out of bounds and the transition is taken to the press_fail state, as shown.
3. Regardless of which sensor fails, the model always generates the directed event broadcast Sens_Failure_Counter.INC, which makes the triggering of the universal sensor failure logic independent of the sensor.

1. This event causes a second transition from FL0 to FL1 in the Sens_Failure_Counter superstate. Both transitions are animated on the Stateflow diagram.

4. With the Sens_Failure_Counter state showing one failure, the condition that guards the transition from the Low_Emissions.Normal state to the Rich_Mixture.Single_Failure state is now valid and is therefore taken.
5. As the Fuel_Disabled state is entered, the data fuel_mode is set to RICH, as shown.

The transitions taken in the preceding steps are depicted in the following simulation diagram. Step numbers appear next to the dashed indicator line.

A second sensor failure causes the Sens_Failure_Counter to enter the Multifail state, broadcasting an implicit event that immediately triggers the transition from the Running state to the Shutdown state. On entering the Fuel_Disabled superstate the Stateflow data fuel_mode is set to DISABLED.

Implicit Event Broadcasts

The preceding example shows how the control logic can be represented in a clear and intuitive manner. The Stateflow diagram (or chart) has been developed in a way that allows the user, or a reviewer, to easily understand how the logic is structured. Implicit event broadcasts (such as enter(multifail)) and implicit conditions (in(FL0)) make the diagram easy to read and the generated code more efficient.

Modifying the Model

To illustrate how easy it is to modify the model, consider the Warmup fueling state in the fuel control logic. At the moment the fueling is set to the low emissions mode (note the highlighted default transition at the bottom).

You might decide that when the oxygen sensor is warming up, changing the warmup fueling mode to a rich mixture would be beneficial. In the Stateflow chart you can easily achieve this by changing the parent of the Warmup state to the Rich_Mixture state. This is accomplished by enlarging the Rich_Mixture state and moving the Warmup state into it from the Low_Emissions state. This alteration is obvious to all who need to inspect or maintain the code as shown in the following result (note the highlighted default transition at the bottom):

The results of changing the algorithm can be seen in the following graphs of air/fuel mixture ratio for the first few seconds of engine operation after startup. The left graph shows the air/fuel ratio for the unaltered system. The right graph for the altered system shows how the air/fuel ratio stays low in the warming up phase indicating a rich mixture.

Control Logic of the "fuel rate controller" Model

Using Stateflow