FirstOrderStockAdjustment

Maintain a stock at a desired level

Diagram

Information

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The output y indicates the rate of inflow required to keep a stock at a desired level. The (perceived) outflow from the stock will be replaced immediately to keep the stock at its current level, while the gap between the current level and the desired level will be closed using a →CloseGap component.

Limitations

The output y can become negative to adjust the stock to a lower level; one needs to be careful in that case with using this value as we cannot always drain a stock in a production process in the same way that we fill it.

Notes

While often modelers simply adjust the outflow from a stock that is controlled by this component to u_outflow one should be aware of the fact, that in reality there will likely be a perception lag which may be modeled by a →Smooth or a more elaborate structure.

Filling the stock under control may not be possible without delay, so the modeler may with to explicitly model a supply chain with explicit delays instead of connecting the output to an immediate inflow.

See [6, pp. 38-39] or Jim Hines' Molecules of Structure website (→First Order Stock Adjustment) for more details on the use of this structure in system dynamics.

Parameters (6)

adjTime	Value: 1 Type: Time (s) Description: Time constant for the adjustment to the desired level (optional)
maxRate	Value: inf Type: OutputType Description: Maximum rate feasible (clippedRate.maxValue)
hasConstantAdjTime	Value: true Type: Boolean Description: = true, if adjustment time is to be given by a constant parameter
clipOutput	Value: false Type: Boolean Description: = true, if the indicated rate is to clipped to not exceed limitations
strict	Value: true Type: Boolean Description: = true, if strict limits with noEvent(..) (constantLimiter.strict) (clip1.strict)
homotopyType	Value: LimiterHomotopy.Linear Type: LimiterHomotopy Description: Simplified model for homotopy-based initialization (clip1.homotopyType)

Connectors (5)

	y	Type: RealOutput Description: Policy output signal
	u_reference	Type: RealInput Description: Desired or reference amount in the stock under control
	u_current	Type: RealInput Description: Current amount in the stock under control
	u_outflow	Type: RealInput Description: Current outflow from the stock under control
	u_adjTime	Type: RealInput Description: Variable time for adjustment (optional)

Components (6)

	modelSettings	Type: ModelSettings Description: Setting important global variables within a model
	clippedRate	Type: Clip Description: Limit rate to feasible values
	unchanged	Type: PassThrough Description: Leave the indicated rate unchanged
	adjustment	Type: CloseGap Description: Inflow needed to reach desired level of stock
	totalAdjustment	Type: Add_2 Description: Closing gap and compensate for outflow from stock
	parAdjTime	Type: ConstantConverterTime Description: Constant time of adjustment (optional)