Relay - Switch output between two constants

Library

Discontinuities

Description

The Relay block allows its output to switch between two specified values. When the relay is on, it remains on until the input drops below the value of the Switch off point parameter. When the relay is off, it remains off until the input exceeds the value of the Switch on point parameter. The block accepts one input and generates one output.

The Switch on point value must be greater than or equal to the Switch off point. Specifying a Switch on point value greater than the Switch off point models hysteresis, whereas specifying equal values models a switch with a threshold at that value.

Note   When the initial input falls between the Switch off point and Switch on point values, the initial output is the value when the relay is off.

Data Type Support

The Relay block accepts signals of the following data types:

• Floating point

• Built-in integer

• Fixed point

• Boolean

• Enumerated (output only)

If Output when on or Output when off is an enumerated value, both must be of the same enumerated type as the output.

For more information, see Data Types Supported by Simulink in the Simulink documentation.

Parameters and Dialog Box

The Main pane of the Relay block dialog box appears as follows:

Switch on point

The "on" threshold for the relay. The Switch on point parameter is converted to the input data type offline using round-to-nearest and saturation.

Switch off point

The "off" threshold for the relay. The Switch off point parameter is converted to the input data type offline using round-to-nearest and saturation.

Output when on

The output when the relay is on.

Output when off

The output when the relay is off.

Input processing

Specify whether the block performs sample- or frame-based processing. You can select one of the following options:

• Elements as channels (sample based) — Treat each element of the input as a separate channel (sample-based processing).

• Columns as channels (frame based) — Treat each column of the input as a separate channel (frame-based processing).

  Note   Frame-based processing requires a DSP System Toolbox license. For more information, see Sample- and Frame-Based Concepts in the DSP System Toolbox documentation.

• Inherited — Inherit the processing mode from the input signal and delay the input accordingly. You can identify whether the input signal is sample or frame based by looking at the signal line. Simulink represents sample-based signals with a single line and frame-based signals with a double line.

  Note   When you choose the Inherited option for the Input processing parameter, and the input signal is frame-based, Simulink® will generate a warning or error in future releases.

Use Input processing to specify whether the block performs sample- or frame-based processing. The block accepts frame-based signals for the input u. All other input signals must be sample based.

Input Signal u	Input Processing Mode	Block Works?
Sample based	Sample based	Yes
Frame based		No, produces an error
Sample based	Frame based	Yes
Frame based		Yes
Sample based	Inherited	Yes
Frame based		Yes

For more information about these two processing modes, see Sample- and Frame-Based Concepts in the DSP System Toolbox documentation.

Enable zero-crossing detection

Select to enable zero-crossing detection. For more information, see Zero-Crossing Detection.

Sample time (-1 for inherited)

Specify the time interval between samples. To inherit the sample time, set this parameter to -1. See How to Specify the Sample Time in the Simulink documentation for more information.

The Signal Attributes pane of the Relay block dialog box appears as follows:

Output minimum

Specify the minimum value that the block should output. The default value is [] (unspecified). Simulink software uses this value to perform:

• Parameter range checking (see Checking Parameter Values)

• Simulation range checking (see Signal Ranges)

• Automatic scaling of fixed-point data types

Output maximum

Specify the maximum value that the block should output. The default value is [] (unspecified). Simulink software uses this value to perform:

• Parameter range checking (see Checking Parameter Values)

• Simulation range checking (see Signal Ranges)

• Automatic scaling of fixed-point data types

Output data type

Specify the output data type. You can set it to:

• A rule that inherits a data type, for example, Inherit: Inherit via back propagation

• The name of a built-in data type, for example, single

• The name of a data type object, for example, a Simulink.NumericType object

• An expression that evaluates to a data type, for example, fixdt(1,16,0)

• An enumerated data type, for example, Enum:BasicColors

  In this case, Output when on and Output when off must be of the same enumerated type.

Click the Show data type assistant button to display the Data Type Assistant, which helps you set the Output data type parameter.

See Specifying Block Output Data Types for more information.

Lock output data type setting against changes by the fixed-point tools

Select to lock the output data type setting of this block against changes by the Fixed-Point Tool and the Fixed-Point Advisor. For more information, see Fixed-Point Tool and Fixed-Point Advisor in the Simulink Fixed Point documentation.

Examples

The sldemo_househeat model shows how you can use the Relay block.

The Relay block appears in the Thermostat subsystem.

The thermostat allows fluctuations of 5 degrees Fahrenheit above or below the desired room temperature. If air temperature drops below 65 degrees Fahrenheit, the thermostat turns on the heater. The Relay block outputs a value of 1 to turn on the heater and a value of 0 to turn off the heater.

Characteristics

Direct Feedthrough	Yes
Sample Time	Specified in the Sample time parameter
Scalar Expansion	Yes
Dimensionalized	Yes
Multidimensionalized	No
Zero-Crossing Detection	Yes, if enabled

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