Bring back the gazebo plugins (#1063)

The models and meshes are not included. We will need
to find an alternate way to reintegrate these and use them.

* Add simulation/gz_msgs back, and build with Gradle.

* Add back in the frc simulation plugins for gazebo.

* Add a new shared library, halsim_gazebo.

This library will become the interface between the
HAL sim layer and gazebo.

* Preserve the first channel number used in created Encoders in the Sim MockData.

This allows us to use the DIO channel number to connect with simulated encoders.

* Have the HAL Simulator set the reverse direction on creation.

This enables a simulator to be aware of the direction.

* Add a drive_motor plugin.

This is a bit of a 'magic' motor, which allows us to build robot
models that drive in a more realistic fashion.  It does this
by apply forces directly to the chassis, rather than relying on
the complex motion dynamics of a driven wheel.

This in turn allows the model to reduce wheel friction,
reducing scrub, and allowing for a more natural driving experience.

simulation/frc_gazebo_plugins/src/drive_motor/cpp/drive_motor.cpp

@@ -0,0 +1,134 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2016-2018 FIRST. All Rights Reserved.                        */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#include "drive_motor.h"
+
+#include <boost/algorithm/string/replace.hpp>
+
+GZ_REGISTER_MODEL_PLUGIN(DriveMotor)
+
+void DriveMotor::Load(gazebo::physics::ModelPtr model, sdf::ElementPtr sdf) {
+  this->model = model;
+  signal = 0;
+
+  // Parse SDF properties
+  joint = model->GetJoint(sdf->Get<std::string>("joint"));
+  if (!joint) {
+    gzerr << "Error initializing drive motor: could not get joint";
+    return;
+  }
+
+  parent = joint->GetParent();
+  if (!parent) {
+    gzerr << "Error initializing drive motor: could not get parent";
+    return;
+  }
+
+  child = joint->GetChild();
+  if (!child) {
+    gzerr << "Error initializing drive motor: could not get child";
+    return;
+  }
+
+  if (sdf->HasElement("topic")) {
+    topic = sdf->Get<std::string>("topic");
+  } else {
+    topic = "~/" + sdf->GetAttribute("name")->GetAsString();
+  }
+
+  if (sdf->HasElement("max_speed")) {
+    maxSpeed = sdf->Get<double>("max_speed");
+  } else {
+    maxSpeed = 0;
+  }
+
+  if (sdf->HasElement("multiplier")) {
+    multiplier = sdf->Get<double>("multiplier");
+  } else {
+    multiplier = 1;
+  }
+
+  if (sdf->HasElement("dx")) {
+    dx = sdf->Get<double>("dx");
+  } else {
+    dx = 0;
+  }
+
+  if (sdf->HasElement("dy")) {
+    dy = sdf->Get<double>("dy");
+  } else {
+    dy = 0;
+  }
+
+  if (sdf->HasElement("dz")) {
+    dz = sdf->Get<double>("dz");
+  } else {
+    dz = 0;
+  }
+
+  gzmsg << "Initializing drive motor: " << topic
+        << " parent=" << parent->GetName() << " directions=" << dx << " " << dy
+        << " " << dz << " multiplier=" << multiplier << std::endl;
+
+  // Connect to Gazebo transport for messaging
+  std::string scoped_name =
+      model->GetWorld()->GetName() + "::" + model->GetScopedName();
+  boost::replace_all(scoped_name, "::", "/");
+  node = gazebo::transport::NodePtr(new gazebo::transport::Node());
+  node->Init(scoped_name);
+  sub = node->Subscribe(topic, &DriveMotor::Callback, this);
+
+  // Connect to the world update event.
+  // This will trigger the Update function every Gazebo iteration
+  updateConn = gazebo::event::Events::ConnectWorldUpdateBegin(
+      boost::bind(&DriveMotor::Update, this, _1));
+}
+
+static double computeForce(double input, double velocity, double max) {
+  double output = input;
+  if (max == 0.0) return output;
+  if (std::fabs(velocity) >= max) {
+    output = 0;
+  } else {
+    double reduce = (max - std::fabs(velocity)) / max;
+    output *= reduce;
+  }
+  return output;
+}
+
+void DriveMotor::Update(const gazebo::common::UpdateInfo& info) {
+#if GAZEBO_MAJOR_VERSION >= 8
+  ignition::math::Vector3d velocity = parent->RelativeLinearVel();
+#else
+  ignition::math::Vector3d velocity = parent->GetRelativeLinearVel().Ign();
+#endif
+
+  if (signal == 0) return;
+
+  double x = computeForce(signal * dx * multiplier, velocity.X(),
+                          std::fabs(maxSpeed * dx));
+  double y = computeForce(signal * dy * multiplier, velocity.Y(),
+                          std::fabs(maxSpeed * dy));
+  double z = computeForce(signal * dz * multiplier, velocity.Z(),
+                          std::fabs(maxSpeed * dz));
+
+  ignition::math::Vector3d force(x, y, z);
+#if GAZEBO_MAJOR_VERSION >= 8
+  parent->AddLinkForce(force, child->RelativePose().Pos());
+#else
+  parent->AddLinkForce(force, child->GetRelativePose().Ign().Pos());
+#endif
+}
+
+void DriveMotor::Callback(const gazebo::msgs::ConstFloat64Ptr& msg) {
+  signal = msg->data();
+  if (signal < -1) {
+    signal = -1;
+  } else if (signal > 1) {
+    signal = 1;
+  }
+}

simulation/frc_gazebo_plugins/src/drive_motor/headers/drive_motor.h

@@ -0,0 +1,126 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2016-2018 FIRST. All Rights Reserved.                        */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#pragma once
+
+#include <string>
+
+#include <gazebo/gazebo.hh>
+#include <gazebo/physics/physics.hh>
+#include <gazebo/transport/transport.hh>
+
+#include "simulation/gz_msgs/msgs.h"
+
+/**
+ * \brief Plugin for simulating a drive motor
+ *
+ * This plugin attempts to overcome a limitation in gazebo.
+ * That is, most normal FRC robots rely on wheels that have good
+ * traction in one direction, and less traction in the opposite
+ * direction.
+ *
+ * Gazebo does not model that well (in fact, drive wheels are
+ * quite hard to simulate).
+ *
+ * So this plugin subscribes to a PWM output signal and applies
+ * a force to the chassis at the proscribed point in hopefully
+ * the correct direction.  The SDF model can then have lower friction,
+ * and it should turn more naturally.
+ *
+ * This plugin also attempts to simulate the limitations of a drive
+ * motor, most notably the maximum speed any given motor can spin at.
+ * The initial implemention is quite naive; just a linear reduction
+ * in force as a product of velocity/max velocity.
+ *
+ * Nicely, this plugin let's you generate a force in any of
+ * three axes.  That is helpful for simulating a mecanum drive.
+ *
+ * This plugin subscribes to a topic to get a signal in the range
+ * [-1,1]. Every physics update the joint's torque is set as
+ * multiplier*signal*direction.
+ *
+ * To add a drive motor to your robot, add the following XML to your
+ * robot model:
+ *
+ *     <plugin name="my_motor" filename="libdrive_motor.so">
+ *       <joint>Joint Name</joint>
+ *       <topic>~/my/topic</topic>
+ *       <multiplier>Number</multiplier>
+ *       <max_speed>Number</max_speed>
+ *       <dx>-1, 0, or 1</dx>
+ *       <dy>-1, 0, or 1</dy>
+ *       <dz>-1, 0, or 1</dz>
+ *     </plugin>
+ *
+ * - `joint`: Name of the joint this Dc motor is attached to.
+ * - `topic`: Optional. Message type should be gazebo.msgs.Float64.
+ *            A typical topic looks like this:
+ *              /gazebo/frc/simulator/pwm/<n>
+ * - `multiplier`: Optional. Defaults to 1.  Force applied by this motor.
+ *            This is force in Newtons.
+ * - `max_speed`:  Optional. Defaults to no maximum.
+ *            This is, in theory, meters/second.  Note that friction
+ *            and other forces will also slow down a robot.
+ *            In practice, this term can be tuned until the robot feels right.
+ * - `dx`:  These three constants must be set to either -1, 0, or 1
+ * - `dy`:  This controls whether or not the motor produces force
+ * - `dz`:  along a given axis, and what direction.  Each defaults to 0.
+ *          These are relative to the frame of the parent link of the joint.
+ *          So they are usually relative to a chassis.
+ *          The force is applied at the point that the joint connects to
+ *          the parent link.
+ */
+class DriveMotor : public gazebo::ModelPlugin {
+ public:
+  /// \brief Load the dc motor and configures it according to the sdf.
+  void Load(gazebo::physics::ModelPtr model, sdf::ElementPtr sdf);
+
+  /// \brief Update the force on the parent of the joint from each timestep.
+  void Update(const gazebo::common::UpdateInfo& info);
+
+ private:
+  /// \brief Topic to read control signal from.
+  std::string topic;
+
+  /// \brief The pwm signal limited to the range [-1,1].
+  double signal;
+
+  /// \brief The robot's maximum speed
+  double maxSpeed;
+
+  /// \brief The magic drive force multipliers. force=multiplier*signal
+  double multiplier;
+
+  /// \brief The directional constants limited to -1, 0, or 1.
+  double dx;
+  double dy;
+  double dz;
+
+  /// \brief The joint that this motor drives.
+  gazebo::physics::JointPtr joint;
+
+  /// \brief The parent of this joint; usually a chassis
+  gazebo::physics::LinkPtr parent;
+
+  /// \brief The child of this joint; usually a wheel
+  gazebo::physics::LinkPtr child;
+
+  /// \brief Callback for receiving msgs and storing the signal.
+  void Callback(const gazebo::msgs::ConstFloat64Ptr& msg);
+
+  /// \brief The model to which this is attached.
+  gazebo::physics::ModelPtr model;
+
+  /// \brief Pointer toe the world update function.
+  gazebo::event::ConnectionPtr updateConn;
+
+  /// \brief The node on which we're advertising.
+  gazebo::transport::NodePtr node;
+
+  /// \brief Subscriber handle.
+  gazebo::transport::SubscriberPtr sub;
+};