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https://github.com/nilseuropa/gazebo_ros_motors

Motor simulation plugins for Gazebo - ROS
https://github.com/nilseuropa/gazebo_ros_motors

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Motor simulation plugins for Gazebo - ROS

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# Gazebo ROS motor plugins



This repository contains currently two motor plugins, one with an ideal speed controller and one without a controller that models a DC motor. *Planning to extend the package with AC, BLDC, stepper and servo motor models as well...*



![](doc/plugins.png)



## DC motor plugin



This plugin is trying to estimate a parametric DC motor by the solution of it's electro-mechanical differential equations, exposing the virtual motor's current consumption and torque interfaces as topics. It has been designed to allow motor control system development in ROS nodes utilizing the physics engine of Gazebo.



![](doc/dc_curves.png)



### Electro-mechanical model



![](doc/MCC.png)



![](doc/variables.png)



From Kirchoff's voltage law:



![](doc/kirchoff.svg)



By Newton's law:



![](doc/newton.svg)



Electro-mechanical coupling:



![](doc/back_emf.svg)



![](doc/torque.svg)



Where ![](doc/ke.svg) is the electromotive constant and back EMF (e) is proportional to the shaft angular velocity, and ![](doc/kt.svg) is the torque constant while the torque (T) is proportional to actual motor current. The mechanical power produced by the DC motor is ![T_m\omega = K_Ti\omega](doc/mech_pwr.svg). The electric power ![P_e = vi](doc/elec_pwr.svg) delivered by the source goes into heat loss in the resistance ![R](doc/res.svg), into stored magnetic energy in the inductance ![L](doc/ind.svg) and the remaining quantity ![iK_e\omega](doc/rem.svg) is converted in mechanical energy ![T_m\omega](doc/mech_en.svg). It leads to ![T_m\omega = K_Ti\omega = K_ei\omega](doc/teq.svg) whether ![K_T = K_e = K_\phi](doc/keq.svg) *( Chiasson 2005 ).* *Model description by [INSA Lyon.](http://www.ctrl-elec.fr/en/ctrl-elec/motor-control/dc-motor-control/84-2/)*



### Simulator interface



The controlled input for the DC motor model is the **normalized** **voltage** that is applied to the motor's electrical connectors, environmental inputs are the shaft velocity and the external load torque on the shaft. Output is the electromotive force produced by the virtual motor.



```c++

applied_torque.Z() = Km * i_t * gear_ratio_; // motor torque T_ext = K * i * n_gear

this->link_->AddRelativeTorque(applied_torque);

joint_->GetForceTorque( 0u ); // get external load

joint_->GetVelocity( 0u ); // get shaft angular velocity

```



#### Advantages of applying torque on the link



- Supported on all physics engines

- Object feels the force that accelerates it

- Can undershoot or overshoot the target velocity *( ideal for controller development )*

- Shaft's damping and friction can be set as joint properties



### Usage



Just use the **dc_motor** macro in a descriptor file as if it were a joint:



```xml

  

  

    

    

    

  

```



Plugin parameters are loaded from a **yaml** file *( defaults are close to a 24V / 450W wheelchair motor )*:



```yaml

# topics

publish_velocity: true

publish_encoder:  false

publish_current:  true

publish_motor_joint_state: false

update_rate: 100.0

# motor model

motor_nominal_voltage: 24.0 # Volts

moment_of_inertia: 0.001 # kgm^2

armature_damping_ratio: 0.0001 # Nm/(rad/s)

electromotive_force_constant: 0.08 # Nm/A

electric_resistance: 1.0 # Ohm

electric_inductance: 0.001 # Henry

# transmission

gear_ratio: 20.0

joint_damping: 0.005

joint_friction: 0.01

# shaft encoder

encoder_ppr: 4096

velocity_noise: 0.0

```



#### Dynamic reconfiguration



Aforementioned DC motor parameters can be set through dynamic reconfiguration server as well.



![](doc/noise.gif)



#### Parameter check



If the parameters would produce an instable state in the solvers update function the plugin is going to restrict the changes to the last stable parameter set. The solvers stable parameter space looks like this - assuming the armature damping and moment of inertia is unchanged:



![](doc/dc_plugin_stable.png) 



#### ROS Topics



**Subscriber**



- **/motor/command**  -- normalized voltage, scales to **motor_nominal_voltage** parameter



- **/motor/supply_voltage** -- overwrites* the **motor_nominal_voltage** parameter runtime ( **V** )



  **To be used with [Gazebo ROS Battery plugin](https://github.com/nilseuropa/gazebo_ros_battery)*



**Publishers**



- **/motor/velocity** -- motor shaft velocity *(encoder side, before gearbox)* **( rad / sec )**

- **/motor/encoder** -- encoder counter 

- **/motor/current** -- electrical current flowing on the coil



## ODE joint motor plugin



This plugin is using the Open Dynamics Engine's joint motor interface, that implements an ideal speed controller. The reason behind a single velocity encoded motor was to allow the development of parametric multi-actuated kinematic models *(E.g. 3 omni wheel geometry )* without the need to tune a controller.![](doc/ode_controlled.png)



### Simulator interface



The plugin's input is the desired angular velocity of the motor joint, and the maximum force the ODE joint-motor may apply during a time step as parameter. 



```c++

joint_->SetParam("fmax", 0, motor_fmax_); // set max. force applied per time step

joint_->SetParam("vel",  0, input_); // set shaft angular velocity 

joint_->GetVelocity( 0u )*encoder_to_shaft_ratio_; // read joint velocity 

```



### Usage



Just use the **joint_motor** macro in a descriptor file as if it were a joint:



```xml

  

  

    

    

    

  

```



Plugin parameters are loaded from a **yaml** file:



```yaml

# ODE joint motor

ode_joint_motor_fmax: 50.0

ode_joint_fudge_factor: 1.0

# transmission

joint_damping: 0.005

joint_friction: 0.01

# shaft encoder

encoder_ppr: 4096

encoder_to_shaft_ratio: 1.0

```



More details on *fmax* and *fudge factor* can be found in the [ODE user guide](https://ode.org/ode-latest-userguide.html#sec_7_5_0). 



#### ROS Topics



**Subscriber**



- **/motor/command**  -- desired joint angular velocity **( rad / sec )**



**Publishers**



- **/motor/velocity** -- motor shaft velocity *(encoder side, before gearbox)* **( rad / sec )**

- **/motor/encoder** -- encoder counter 



##### Authors



Marton Juhasz [nilseuropa](https://github.com/nilseuropa) and Gergely Gyebroszki [gyebro](https://github.com/Gyebro)