Control Oscillations by Using the duration Operator

Control Oscillations by Using the `duration` Operator

The following example focuses on the gear logic of a car as it shifts from first gear to fourth gear.

When modeling the gear changes of this system, it is important to control the oscillation that occur. The model sf_car uses parallel state debouncer logic that controls which gear state is active. For more information about how debouncers work in Stateflow^®, see Reduce Transient Signals by Using Debouncing Logic.

You can simplify the debouncer logic by using the duration operator. You can see this simplification in the model sf_car_using_duration. The duration operator evaluates a condition expression and outputs the length of time that the expression has been true. When that length of time crosses a known time threshold, the state transitions to a higher or lower gear.

By removing the parallel state logic and using the duration operator, you can control oscillations with simpler Stateflow logic. The duration operator is supported only in Stateflow charts in a Simulink^® model.

Control Oscillation with Parallel State Logic

Open the model sf_car.

openExample("sf_car")

Select the chart shift_logic and, in the State Chart tab, click Look Under Mask.

The Stateflow chart shift_logic controls which gear the car is in, given the speed of the car and how much throttle is being applied. Withinshift_logic there are two parallel states: gear_state and selection_state. gear_state contains four exclusive states for each gear. selection_state determines whether the car is downshifting, upshifting, or remaining in its current gear.

Stateflow chart that uses the parallel substate selection_state to determine timing for downshifting and upshifting.

In this Stateflow chart, for the car to move from first gear to second gear, the event UP must be sent from selection_state to gear_state. The event is sent when the speed crosses the threshold and remains higher than the threshold for the length of time determined by TWAIT. When the event UP is sent, gear_state transitions from first to second.

Control Oscillation with the `duration` Operator

Open the model sf_car_using_duration.

openExample("sf_car_using_duration")

Select the chart Gear_Logic and, in the State Chart tab, click Look Under Mask.

Within Gear_Logic there are four exclusive states for each gear. The local variables up and down guard the transitions between each state.

Stateflow chart that uses the duration operator to determine timing for downshifting and upshifting.

In this Stateflow chart, for the car to move from first gear to second gear, the condition up must be true. The condition up is defined as true if the length of time that the speed is greater than or equal to the threshold is greater than the length of time that is specified by TWAIT. The condition down is defined as true if the length of time that the speed is less than or equal to the threshold is greater than the length of time that is specified by TWAIT. The operator duration keeps track of the length of time that the speed has been above or below the threshold. When the up condition is met, the active state transitions from first to second.

By replacing the parallel state debouncer logic with the duration operator, you can create a simpler Stateflow chart to model the gear shifting.