Backlash

Model behavior of system with play

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Libraries:
Simulink / Discontinuities
HDL Coder / Discontinuities

Description

The Backlash block implements a system in which a change in input causes an equal change in output, except when the input changes direction. When the input changes direction, the initial change in input has no effect on the output. The amount of side-to-side play in the system is referred to as the deadband. The deadband is centered about the output. This figure shows an initial state, with the default deadband width of 1 and initial output of 0.

A system with play can be in one of three modes.

Mode	Input	Output
Disengaged	Inside deadband zone.	Remains constant.
Engaged-positive direction	Outside deadband zone and increasing.	Equals input minus half of deadband width.
Engaged-negative direction	Outside deadband zone and decreasing.	Equals input plus half of deadband width.

The Initial output parameter value defines the initial center of the deadband zone.

This table shows output values when initial conditions are: Deadband width = 2 and Initial output = 5.

Output Value	Condition
5	4 <= input <= 6
input + 1	input < 4
input - 1	input > 6

For example, you can use the Backlash block to model the meshing of two gears. The input and output are both shafts with a gear on one end, and the input shaft drives the output shaft. Extra space between the gear teeth introduces play. The width of this spacing is the Deadband width parameter. If the system is disengaged initially, the Initial output parameter defines the output.

These figures illustrate operation when the initial input is within the deadband and the system begins in disengaged mode.

When the input increases and reaches the end of the deadband, it engages the output. The output remains at its previous value.

After the input engages the output, the output changes by the same amount as the input.

If the input reverses direction, it disengages from the output. The output remains constant until the input reaches the end of the deadband and engages again.

Examples

View Backlash Output on Sine Wave

This example shows the effect of the Backlash block on a sine wave using default parameters. The initial Deadband width is 1 and the Initial output is 0.

The initial deadband is centered around 0 and has a width of 1, which extends .5 in each direction. The output from the Backlash block begins at 0 and does not change until the input reaches the edge of the dead zone at .5. Then the output engages in a positive direction and changes an equal amount as the input. After the input reaches a value of 1, it starts moving in a negative direction. At this point the output disengages and stays flat until the input passes through the deadband width of 1. Once the input reaches the end of the deadband zone at 0, then the output engages and starts moving in a negative direction with the input.

Ports

Input

Port_1 — Input signal

scalar | vector

Input signal to the backlash algorithm. The value of this signal is either in the deadband or engaging the output in a positive or negative direction.

Data Types: double | single | int8 | int16 | int32 | uint8 | uint16 | uint32

Output

Port_1 — Output signal

scalar | vector

Output signal after the backlash algorithm is applied to the input signal. When the input is in the deadband, then the output remains unchanged. If the input is engaged with the output, then the output changes an equal amount as the input.

Data Types: double | single | int8 | int16 | int32 | uint8 | uint16 | uint32

Parameters

Deadband width — Specify the width of the deadband

1 (default) | scalar | vector

Specify the size of the deadband zone centered on the output value. When the input signal is inside the deadband, then a change in input does not cause a change in output. When the input signal is outside of the deadband, then the output changes an equal amount as the input.

Programmatic Use

Block Parameter: BacklashWidth

Type: character vector

Values: real scalar or vector

Default: '1'

Initial output — Specify the initial output value

0 (default) | scalar | vector

Specify the initial center of the deadband zone. If the initial input value is in the deadband zone, then the output value is equal to Initial output. If the initial input value is outside of the deadband zone, then the output value is Initial output plus or minus half of the deadzone width.

Programmatic Use

Block Parameter: InitialOutput

Type: character vector

Values: real scalar or vector

Default: '0'

Input processing — Specify sample- or frame-based processing

Elements as channels (sample based) (default) | Columns as channels (frame based)

Specify whether the block performs sample- or frame-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 (DSP System Toolbox).

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

Use Input processing to specify whether the block performs sample- or frame-based processing. For more information about these two processing modes, see Sample- and Frame-Based Concepts (DSP System Toolbox).

Programmatic Use

Block Parameter: InputProcessing

Type: character vector

Values: 'Columns as channels (frame based)' | 'Elements as channels (sample based)'

Default: 'Elements as channels (sample based)'

Enable zero-crossing detection — Enable zero-crossing detection

on (default) | off

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

This block supports zero-crossing detection only in simulations that use a variable-step solver. When you use a fixed-step solver for simulation, the software does not detect or locate zero crossings for this block.

Programmatic Use

Block Parameter: ZeroCross

Type: character vector | string

Values: 'off' | 'on'

Default: 'on'

Block Characteristics

Data Types	double | integer | single
Direct Feedthrough	yes
Multidimensional Signals	no
Variable-Size Signals	no
Zero-Crossing Detection	yes

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using Simulink® Coder™.

Generated code relies on memcpy or memset function (string.h) in certain conditions.

HDL Code Generation

Generate VHDL, Verilog and SystemVerilog code for FPGA and ASIC designs using HDL Coder™.

HDL Coder™ provides additional configuration options that affect HDL implementation and synthesized logic.

HDL Architecture

This block has one default HDL architecture.

HDL Block Properties

ConstrainedOutputPipeline	Number of registers to place at the outputs by moving existing delays within your design. Distributed pipelining does not redistribute these registers. The default is 0. For more details, see ConstrainedOutputPipeline (HDL Coder).
InputPipeline	Number of input pipeline stages to insert in the generated code. Distributed pipelining and constrained output pipelining can move these registers. The default is 0. For more details, see InputPipeline (HDL Coder).
OutputPipeline	Number of output pipeline stages to insert in the generated code. Distributed pipelining and constrained output pipelining can move these registers. The default is 0. For more details, see OutputPipeline (HDL Coder).

Restrictions

The Deadband width and Initial output parameters support only scalar values.

Version History

Introduced before R2006a