# Fixed-Displacement Motor

Hydraulic-to-mechanical power conversion device

**Library:**Simscape / Fluids / Hydraulics (Isothermal) / Pumps and Motors

## Description

The Fixed-Displacement Motor block represents a device that extracts
power from a hydraulic (isothermal liquid) network and delivers it to a mechanical
rotational network. The motor displacement is fixed at a constant value that you specify
through the **Displacement** parameter.

Ports **A** and **B** represent the motor inlet and
outlet, respectively. Port **S** represents the motor drive shaft.
During normal operation, the angular velocity at port **S** is positive
if the pressure drop from port **A** to port **B** is
positive also. This operation mode is referred to here as forward motor.

**Operation Modes**

A total of four operation modes are possible. The working mode depends on the pressure
drop from port **A** to port **B** (Δ*p*) and on the angular velocity at port **S**
(*ω*). The Operation Modes figure maps the modes
to the octants of a Δ*p*-*ω*-*D*
chart. The modes are labeled 1–4:

Mode

**1**: forward motor — A positive pressure drop generates a positive shaft angular velocity.Mode

**2**: reverse pump — A negative shaft angular velocity generates a negative pressure gain (shown in the figure as a positive pressure drop).Mode

**3**: reverse motor — A negative pressure drop generates a negative shaft angular velocity.Mode

**4**: forward pump — A positive shaft angular velocity generates a positive pressure gain (shown in the figure as a negative pressure drop).

The response time of the motor is considered negligible in comparison with the system response time. The motor is assumed to reach steady state nearly instantaneously and is treated as a quasi-steady component.

### Block Variants and Loss Parameterizations

The motor model accounts for power losses due to leakage and friction. Leakage is
internal and occurs between the motor inlet and outlet only. The block computes the
leakage flow rate and friction torque using your choice of five loss
parameterizations. You select a parameterization using block variants and, in the
`Analytical or tabulated data`

case, the **
Leakage and friction parameterization** parameter.

**Loss Parameterizations**

The block provides three Simscape™ variants to select from. To change the active block variant, use the
**Modeling option** parameter. The available variants
are:

`Analytical or tabulated data`

— Obtain the mechanical and volumetric efficiencies or losses from analytical models based on nominal parameters or from tabulated data. Use the**Leakage and friction parameterization**parameter to select the exact input type.`Input efficiencies`

— Provide the mechanical and volumetric efficiencies directly through physical signal input ports.`Input losses`

— Provide the mechanical and volumetric losses directly through physical signal input ports. The mechanical loss is defined as the internal friction torque. The volumetric loss is defined as the internal leakage flow rate.

### Flow Rate and Driving Torque

The volumetric flow rate required to power the motor is

$$q={q}_{\text{Ideal}}+{q}_{\text{Leak}},$$

where:

*q*is the net volumetric flow rate.*q*_{Ideal}is the ideal volumetric flow rate.*q*_{Leak}is the internal leakage volumetric flow rate.

The torque generated at the motor is

$$\tau ={\tau}_{\text{Ideal}}-{\tau}_{\text{Friction}},$$

where:

*τ*is the net torque.*τ*_{Ideal}is the ideal torque.*τ*_{Friction}is the friction torque.

**Ideal Flow Rate and Ideal Torque**

The ideal volumetric flow rate is

$${q}_{\text{Ideal}}=D\omega ,$$

and the ideal generated torque is

$${\tau}_{\text{Ideal}}=D\Delta p,$$

where:

*D*is the specified value of the**Displacement**block parameter.*ω*is the instantaneous angular velocity of the rotary shaft.*Δp*is the instantaneous pressure drop from inlet to outlet.

**Leakage Flow Rate and Friction Torque**

The internal leakage flow rate and friction torque calculations depend on the
block variant selected. If the block variant is ```
Analytical or
tabulated data
```

, the calculations depend also on the
**Leakage and friction parameterization** parameter
setting. There are five possible permutations of block variant and
parameterization settings.

**Case 1: Analytical Efficiency Calculation**

If the active block variant is ```
Analytical or tabulated
data
```

and the **Leakage and friction
parameterization** parameter is set to
`Analytical`

, the leakage flow rate is

$${q}_{\text{Leak}}={K}_{\text{HP}}\Delta p,$$

and the friction torque is

$${\tau}_{\text{Friction}}=\left({\tau}_{0}+K{\text{}}_{\text{TP}}\left|\Delta p\right|\right)\mathrm{tanh}\left(\frac{4\omega}{{\omega}_{\text{Threshold}}}\right),$$

where:

*K*_{HP}is the Hagen-Poiseuille coefficient for laminar pipe flows. This coefficient is computed from the specified nominal parameters.*K*_{TP}is the specified value of the**Friction torque vs pressure drop coefficient**block parameter.*τ*_{0}is the specified value of the**No-load torque**block parameter.*ω*_{Threshold}is the threshold angular velocity for the motor-pump transition. The threshold angular velocity is an internally set fraction of the specified value of the**Nominal shaft angular velocity**block parameter.

The Hagen-Poiseuille coefficient is determined from nominal fluid and component parameters through the equation

$${K}_{\text{HP}}=\frac{{\nu}_{\text{Nom}}}{\rho v}\frac{\text{\hspace{0.17em}}{\rho}_{\text{Nom}}{\omega}_{\text{Nom}}{D}_{\text{Max}}}{\Delta {p}_{\text{Nom}}}\left(\frac{1}{{\eta}_{\text{v,Nom}}}-1\right),$$

where:

*ν*_{Nom}is the specified value of the**Nominal kinematic viscosity**block parameter. This is the kinematic viscosity at which the nominal volumetric efficiency is specified.*ρ*_{Nom}is the specified value of the**Nominal fluid density**block parameter. This is the density at which the nominal volumetric efficiency is specified.*ω*_{Nom}is the specified value of the**Nominal shaft angular velocity**block parameter. This is the angular velocity at which the nominal volumetric efficiency is specified.*ρ*is the actual fluid density in the attached hydraulic (isothermal liquid) network. This density can differ from the specified value of the**Nominal fluid density**block parameter.*v*is the kinematic viscosity of the fluid associated with the fluid network.*Δp*_{Nom}is the specified value of the**Nominal pressure drop**block parameter. This is the pressure drop at which the nominal volumetric efficiency is specified.*η*_{v,Nom}is the specified value of the**Volumetric efficiency at nominal conditions**block parameter. This is the volumetric efficiency corresponding to the specified nominal conditions.

**Case 2: Efficiency Tabulated Data**

If the active block variant is ```
Analytical or tabulated
data
```

and the **Leakage and friction
parameterization** parameter is set to ```
Tabulated data
— volumetric and mechanical efficiencies
```

, the leakage
flow rate is

$${q}_{\text{Leak}}={q}_{\text{Leak,Motor}}\frac{\left(1+\alpha \right)}{2}+{q}_{\text{Leak,Pump}}\frac{\left(1-\alpha \right)}{2},$$

and the friction torque is

$${\tau}_{\text{Friction}}={\tau}_{\text{Friction,Motor}}\frac{1+\alpha}{2}+{\tau}_{\text{Friction,Pump}}\frac{1-\alpha}{2},$$

where:

*α*is a numerical smoothing parameter for the motor-pump transition.*q*_{Leak,Motor}is the leakage flow rate in motor mode.*q*_{Leak,Pump}is the leakage flow rate in pump mode.*τ*_{Friction,Motor}is the friction torque in motor mode.*τ*_{Friction,Pump}is the friction torque in pump mode.

The smoothing parameter *α* is given by the hyperbolic function

$$\alpha =\text{tanh}\left(\frac{4\Delta p}{\Delta {p}_{\text{Threshold}}}\right)\xb7\text{tanh}\left(\frac{4\omega}{{\omega}_{\text{Threshold}}}\right),$$

where:

*Δp*_{Threshold}is the specified value of the**Pressure drop threshold for motor-pump transition**block parameter.*ω*_{Threshold}is the specified value of the**Angular velocity threshold for motor-pump transition**block parameter.

The leakage flow rate is computed from efficiency tabulated data through the equation

$${q}_{\text{Leak,Motor}}=\left(1-{\eta}_{\text{v}}\right)q,$$

in motor mode and through the equation

$${q}_{\text{Leak,Pump}}=-\left(1-{\eta}_{\text{v}}\right){q}_{\text{Ideal}}.$$

in pump mode, where:

*η*_{v}is the volumetric efficiency obtained through interpolation or extrapolation of the**Volumetric efficiency table, e_v(dp,w)**parameter data.

Similarly, the friction torque is computed from efficiency tabulated data through the equation

$${\tau}_{\text{Friction,Motor}}=\left(1-{\eta}_{\text{m}}\right){\tau}_{\text{Ideal}},$$

in motor mode and through the equation

$${\tau}_{\text{Friction,Pump}}=-\left(1-{\eta}_{\text{m}}\right)\tau .$$

in pump mode, where:

*η*_{m}is the mechanical efficiency obtained through interpolation or extrapolation of the**Mechanical efficiency table, e_m(dp,w)**parameter data.

**Case 3: Loss Tabulated Data**

```
Analytical or tabulated
data
```

and the **Leakage and friction parameterization**parameter is set to

```
Tabulated data
— volumetric and mechanical losses
```

, the leakage flow
rate equation is
$${q}_{\text{Leak}}={q}_{\text{Leak}}\left(\Delta p,\omega \right).$$

and the friction torque equation is

$${\tau}_{\text{Friction}}={\tau}_{\text{Friction}}\left(\Delta p,\omega \right),$$

where *q*_{Leak}(*Δp*,*ω*) and *τ*_{Friction}(*Δp*,*ω*) are the volumetric and mechanical losses, obtained through
interpolation or extrapolation of the **Volumetric loss table,
q_loss(dp,w)** and **Mechanical loss table, torque_loss
(dp,w)** parameter data.

**Case 4: Efficiency Physical Signal Inputs**

If the active block variant is `Input efficiencies`

,
the leakage flow rate and friction torque calculations are as described for
efficiency tabulated data (case 2). The volumetric and mechanical efficiency
lookup tables are replaced with physical signal inputs that you specify through
ports EV and EM.

**Case 5: Loss Physical Signal Inputs**

If the block variant is `Input losses`

, the leakage
flow rate and friction torque calculations are as described for loss tabulated
data (case 3). The volumetric and mechanical loss lookup tables are replaced
with physical signal inputs that you specify through ports LV and LM.

### Characteristic Curve Visualization

If the block variant is set to ```
Analytical or tabulated
data
```

, you can plot a variety of performance, efficiency, and loss
curves from simulation data and component parameters. Use the context-sensitive menu
of the block to plot the characteristic curves. Right-click the block to open the
menu and select **Fluids** > **Plot characteristic**. A test harness opens with instructions on how to generate the curves.

### Assumptions

Fluid compressibility is negligible.

Loading on the motor shaft due to inertia, friction, and spring forces is negligible.

## Ports

### Input

### Conserving

## Parameters

## Extended Capabilities

## Version History

**Introduced in R2006b**