Added PD torque controller in joint space (#336)

The pd controller in joint space will accept joint targets and compute
joint torques based on PD control scheme.

Co-authored-by: Mads Holm Peters <mahp@universal-robots.com>
Co-authored-by: Felix Exner <feex@universal-robots.com>

Author: 3 people

doc/architecture/script_command_interface.rst

@@ -25,6 +25,8 @@ At the time of writing the ``ScriptCommandInterface`` provides the following fun
 - ``setGravity()``: Set the gravity vector for the robot.
 - ``setTcpOffset()``: Set the TCP offset of the robot.
 - ``setFrictionScales()``: Set viscous and Coulomb friction scale factors for direct torque control.
+- ``setPDControllerGains()``: Set PD gains for the PD control loop running in the external control script.
+- ``setMaxJointTorques()``: Set Max joint torques for the PD control loop running in the external control script.
 
 Communication protocol
 ----------------------
@@ -58,6 +60,8 @@ The robot reads from the "script_command_socket" expecting a 32 bit integer repr
            - 9: setGravity
            - 10: setTcpOffset
            - 11: setFrictionScales
+           - 12: setPDControllerGains
+           - 13: setMaxJointTorques
    1-27   data fields specific to the command
    =====  =====
 
@@ -152,6 +156,25 @@ The robot reads from the "script_command_socket" expecting a 32 bit integer repr
    6-9    sensor_cog in m, displacement from the tool flange (3d floating point)
    =====  =====
 
+.. table:: With setPDControllerGains command
+   :widths: auto
+
+   =====  =====
+   index  meaning
+   =====  =====
+   1-6    Kp gains for each joint, used in the PD control loop running in the external control script (floating point)
+   7-12   Kd gains for each joint, used in the PD control loop running in the external control script (floating point)
+   =====  =====
+
+.. table:: With setMaxJointTorques command
+   :widths: auto
+
+   =====  =====
+   index  meaning
+   =====  =====
+   1-6    max_joint_torques for each joint, used to clamp the torques between +-max_joint_torques before applying them to the robot in the PD control loop running in the external control script (floating point)
+   =====  =====
+
 .. table:: With setGravity command
    :widths: auto
 
@@ -170,6 +193,8 @@ The robot reads from the "script_command_socket" expecting a 32 bit integer repr
    1-6    TCP offset as [x, y, z, rx, ry, rz] given in the flange coordinate system in SI units (meters and radians, floating point)
    =====  =====
 
+
+
 .. table:: With setFrictionScales command
    :widths: auto
 

examples/CMakeLists.txt

@@ -62,3 +62,7 @@ target_link_libraries(instruction_executor ur_client_library::urcl)
 add_executable(external_fts_through_rtde
 external_fts_through_rtde.cpp)
 target_link_libraries(external_fts_through_rtde ur_client_library::urcl)
+
+add_executable(pd_controller_example
+pd_controller_example.cpp)
+target_link_libraries(pd_controller_example ur_client_library::urcl)

examples/pd_controller_example.cpp

@@ -0,0 +1,225 @@
+// this is for emacs file handling -*- mode: c++; indent-tabs-mode: nil -*-
+
+// -- BEGIN LICENSE BLOCK ----------------------------------------------
+// Copyright 2025 Universal Robots A/S
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are met:
+//
+//    * Redistributions of source code must retain the above copyright
+//      notice, this list of conditions and the following disclaimer.
+//
+//    * Redistributions in binary form must reproduce the above copyright
+//      notice, this list of conditions and the following disclaimer in the
+//      documentation and/or other materials provided with the distribution.
+//
+//    * Neither the name of the {copyright_holder} nor the names of its
+//      contributors may be used to endorse or promote products derived from
+//      this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+// ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+// CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+// SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+// CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+// ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+// POSSIBILITY OF SUCH DAMAGE.
+// -- END LICENSE BLOCK ------------------------------------------------
+
+#include "ur_client_library/example_robot_wrapper.h"
+#include "ur_client_library/ur/dashboard_client.h"
+#include "ur_client_library/ur/ur_driver.h"
+#include "ur_client_library/types.h"
+#include "ur_client_library/ur/instruction_executor.h"
+
+#include <iostream>
+#include <memory>
+#include <cmath>
+#include <signal.h>
+
+using namespace urcl;
+
+const std::string DEFAULT_ROBOT_IP = "192.168.56.101";
+const std::string SCRIPT_FILE = "resources/external_control.urscript";
+const std::string OUTPUT_RECIPE = "examples/resources/rtde_output_recipe.txt";
+const std::string INPUT_RECIPE = "examples/resources/rtde_input_recipe.txt";
+
+std::unique_ptr<ExampleRobotWrapper> g_my_robot;
+
+void signal_callback_handler(int signum)
+{
+  std::cout << "Caught signal " << signum << std::endl;
+  if (g_my_robot != nullptr)
+  {
+    // Stop control of the robot
+    g_my_robot->getUrDriver()->stopControl();
+  }
+  // Terminate program
+  exit(signum);
+}
+
+struct DataStorage
+{
+  std::vector<urcl::vector6d_t> target;
+  std::vector<urcl::vector6d_t> actual;
+  std::vector<double> time;
+
+  void write_to_file(const std::string& filename)
+  {
+    std::fstream output(filename, std::ios::out);
+    output << "timestamp,target0,target1,target2,target3,target4,target5,actual0,actual1,actual2,"
+              "actual3,actual4,actual5\n";
+    for (size_t i = 0; i < time.size(); ++i)
+    {
+      output << time[i];
+      for (auto& t : target[i])
+      {
+        output << "," << t;
+      }
+      for (auto& a : actual[i])
+      {
+        output << "," << a;
+      }
+      output << "\n";
+    }
+    output.close();
+  }
+};
+
+bool pd_control_loop(DataStorage& data_storage, const std::string& actual_data_name,
+                     const comm::ControlMode control_mode, const urcl::vector6d_t& amplitude)
+{
+  const int32_t running_time = 13;
+  double time = 0.0;
+
+  // Reserve space for expected amount of data
+  data_storage.actual.reserve(running_time * 500);
+  data_storage.target.reserve(running_time * 500);
+  data_storage.time.reserve(running_time * 500);
+
+  urcl::vector6d_t actual, target, start = { 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 };
+
+  rtde_interface::DataPackage data_pkg(g_my_robot->getUrDriver()->getRTDEOutputRecipe());
+
+  bool first_pass = true;
+  while (time < running_time)
+  {
+    const auto t_start = std::chrono::high_resolution_clock::now();
+    // Read latest RTDE package. This will block for a hard-coded timeout (see UrDriver), so the
+    // robot will effectively be in charge of setting the frequency of this loop.
+    // In a real-world application this thread should be scheduled with real-time priority in order
+    // to ensure that this is called in time.
+    if (!g_my_robot->getUrDriver()->getDataPackage(data_pkg))
+    {
+      URCL_LOG_WARN("Could not get fresh data package from robot");
+      return false;
+    }
+    // Read current joint positions from robot data
+    if (!data_pkg.getData(actual_data_name, actual))
+    {
+      // This throwing should never happen unless misconfigured
+      std::string error_msg =
+          "Did not find '" + actual_data_name + "' in data sent from robot. This should not happen!";
+      throw std::runtime_error(error_msg);
+    }
+
+    if (first_pass)
+    {
+      target = actual;
+      start = actual;
+      for (size_t i = 0; i < start.size(); ++i)
+      {
+        start[i] = start[i] - amplitude[i];
+      }
+      first_pass = false;
+    }
+
+    for (size_t i = 0; i < target.size(); ++i)
+    {
+      target[i] = start[i] + amplitude[i] * cos(time);
+    }
+
+    // Setting the RobotReceiveTimeout time is for example purposes only. This will make the example running more
+    // reliable on non-realtime systems. Use with caution in productive applications.
+    bool ret = g_my_robot->getUrDriver()->writeJointCommand(target, control_mode, RobotReceiveTimeout::millisec(100));
+    if (!ret)
+    {
+      URCL_LOG_ERROR("Could not send joint command. Make sure that the robot is in remote control mode and connected "
+                     "with a network cable.");
+      return false;
+    }
+
+    // Increment time and log data
+    const auto t_end = std::chrono::high_resolution_clock::now();
+    const double elapsed_time_ms = std::chrono::duration<double, std::milli>(t_end - t_start).count() / 1000;
+    time = time + elapsed_time_ms;
+    data_storage.actual.push_back(actual);
+    data_storage.target.push_back(target);
+    data_storage.time.push_back(time);
+  }
+
+  return true;
+}
+
+int main(int argc, char* argv[])
+{
+  // This will make sure that we stop controlling the robot if the user presses ctrl-c
+  signal(SIGINT, signal_callback_handler);
+
+  urcl::setLogLevel(urcl::LogLevel::INFO);
+
+  // Parse the ip arguments if given
+  std::string robot_ip = DEFAULT_ROBOT_IP;
+  if (argc > 1)
+  {
+    robot_ip = std::string(argv[1]);
+  }
+
+  const bool headless_mode = true;
+  g_my_robot = std::make_unique<ExampleRobotWrapper>(robot_ip, OUTPUT_RECIPE, INPUT_RECIPE, headless_mode);
+
+  if (!g_my_robot->isHealthy())
+  {
+    URCL_LOG_ERROR("Something in the robot initialization went wrong. Exiting. Please check the output above.");
+    return 1;
+  }
+
+  {
+    auto robot_version = g_my_robot->getUrDriver()->getVersion();
+    if (robot_version < urcl::VersionInformation::fromString("5.23.0") ||
+        (robot_version.major > 5 && robot_version < urcl::VersionInformation::fromString("10.10.0")))
+    {
+      URCL_LOG_ERROR("This example requires a robot with at least version 5.23.0 / 10.10.0. Your robot has version %s.",
+                     robot_version.toString().c_str());
+      return 0;
+    }
+  }
+
+  auto instruction_executor = std::make_shared<urcl::InstructionExecutor>(g_my_robot->getUrDriver());
+
+  URCL_LOG_INFO("Move the robot to initial position");
+  instruction_executor->moveJ(urcl::vector6d_t{ 0, -1.67, -0.65, -1.59, 1.61, 5.09 }, 0.5, 0.2, 5);
+
+  DataStorage joint_controller_storage;
+
+  // Once RTDE communication is started, we have to make sure to read from the interface buffer, as
+  // otherwise we will get pipeline overflows. Therefor, do this directly before starting your main
+  // loop.
+  g_my_robot->getUrDriver()->startRTDECommunication();
+  URCL_LOG_INFO("Start controlling the robot using the PD controller");
+  const bool completed_joint_control =
+      pd_control_loop(joint_controller_storage, "actual_q", comm::ControlMode::MODE_PD_CONTROLLER_JOINT,
+                      { 0.2, 0.2, 0.2, 0.2, 0.2, 0.2 });
+  if (!completed_joint_control)
+  {
+    URCL_LOG_ERROR("Didn't complete pd control in joint space");
+    g_my_robot->getUrDriver()->stopControl();
+    return 1;
+  }
+
+  return 0;
+}

examples/script_command_interface.cpp

@@ -89,6 +89,13 @@ void sendScriptCommands()
                                                      { 0.8, 0.8, 0.7, 0.8, 0.8, 0.8 });
       });
     }
+    run_cmd("Setting PD controller gains", []() {
+      g_my_robot->getUrDriver()->setPDControllerGains({ 500.0, 500.0, 300.0, 124.0, 124.0, 124.0 },
+                                                      { 44.72, 44.72, 34.64, 22.27, 22.27, 22.27 });
+    });
+    // The following will have no effect on PolyScope < 5.23 / 10.10
+    run_cmd("Setting max joint torques",
+            []() { g_my_robot->getUrDriver()->setMaxJointTorques({ 27.0, 27.0, 14.0, 4.5, 4.5, 4.5 }); });
   }
   URCL_LOG_INFO("Script command thread finished.");
 }

include/ur_client_library/comm/control_mode.h

@@ -54,7 +54,8 @@ enum class ControlMode : int32_t
   MODE_TOOL_IN_CONTACT =
       7,            ///< Used only internally in the script, when robot is in tool contact, clear by endToolContact()
   MODE_TORQUE = 8,  ///< Set when torque control is active.
-  END               ///< This is not an actual control mode, but used internally to get the number of control modes
+  MODE_PD_CONTROLLER_JOINT = 9,  ///< Set when PD control in joint space is active
+  END  ///< This is not an actual control mode, but used internally to get the number of control modes
 };
 
 /*!
@@ -65,8 +66,8 @@ class ControlModeTypes
 public:
   // Control modes that require realtime communication
   static const inline std::vector<ControlMode> REALTIME_CONTROL_MODES = {
-    ControlMode::MODE_SERVOJ, ControlMode::MODE_SPEEDJ, ControlMode::MODE_SPEEDL, ControlMode::MODE_POSE,
-    ControlMode::MODE_TORQUE
+    ControlMode::MODE_SERVOJ, ControlMode::MODE_SPEEDJ, ControlMode::MODE_SPEEDL,
+    ControlMode::MODE_POSE,   ControlMode::MODE_TORQUE, ControlMode::MODE_PD_CONTROLLER_JOINT
   };
 
   // Control modes that doesn't require realtime communication

include/ur_client_library/control/script_command_interface.h

@@ -222,6 +222,32 @@ class ScriptCommandInterface : public ReverseInterface
                          const vector3d_t& sensor_measuring_offset = { 0.0, 0.0, 0.0 },
                          const vector3d_t& sensor_cog = { 0.0, 0.0, 0.0 });
 
+  /*!
+   * \brief Set gains for the PD controller running in the external control script. The PD controller computes joint
+   * torques based on either tcp poses or joint poses and applies the torques to the robot using the direct_torque
+   * function. The gains can be used to change the response of the controller. Be aware that changing the controller
+   * response can make it unstable.
+   * The PD controller can be used without explicitly defining those gains, as it contains a set of default gains for
+   * each robot model.
+   *
+   * \param kp A vector6d of proportional gains for each of the joints in the robot.
+   * \param kd A vector6d of derivative gains for each of the joints in the robot.
+   *
+   * \returns True, if the write was performed successfully, false otherwise.
+   */
+  bool setPDControllerGains(const urcl::vector6d_t* kp, const urcl::vector6d_t* kd);
+
+  /*!
+   * \brief Set the maximum joint torques for the PD controller running in the external control script. The PD
+   * controller will clamp the torques between +-max_joint_torques before aplying them to the robot using the
+   * direct_torque function.
+   *
+   * \param max_joint_torques A vector6d of the maximum joint torques for each of the joints.
+   *
+   * \returns True, if the write was performed successfully, false otherwise.
+   */
+  bool setMaxJointTorques(const urcl::vector6d_t* max_joint_torques);
+
   /*!
    * \brief  Returns whether a client/robot is connected to this server.
    *
@@ -264,6 +290,8 @@ class ScriptCommandInterface : public ReverseInterface
     SET_GRAVITY = 9,                ///< Set gravity vector
     SET_TCP_OFFSET = 10,            ///< Set TCP offset
     SET_FRICTION_SCALES = 11,       ///< Set viscous and Coulomb friction scales for direct_torque
+    SET_PD_CONTROLLER_GAINS = 12,   ///< Set PD controller gains
+    SET_MAX_JOINT_TORQUES = 13,     ///< Set max joint torques
   };
 
   /*!