WEBVTT 00:00.110 --> 00:06.680 The orders to control library has been designed for the development of any control system for robots 00:06.680 --> 00:07.910 and Ros two. 00:08.420 --> 00:15.410 For this reason, this library offers various interfaces and configurations to adapt to the different 00:15.440 --> 00:16.940 needs of each robot. 00:16.970 --> 00:23.960 In particular, this library offers three different interfaces that corresponds to three different control 00:23.960 --> 00:24.800 logics. 00:24.830 --> 00:32.660 The control of an actuator in fact can be implemented for position, velocity or force or torque. 00:33.630 --> 00:40.410 The position control is exactly what we will be using in this course to control our robot. 00:40.860 --> 00:47.820 Controlling an actuator for position means implementing a control logic that moves a certain system 00:47.820 --> 00:54.900 from its current position to a desired position and potentially following a specific trajectory during 00:54.900 --> 01:01.410 these movement, reaching intermediate configurations between the initial and the final position. 01:02.310 --> 01:10.290 Let's consider a real case where a motor that controls a link of a robotic arm needs to rotate to reach 01:10.320 --> 01:12.330 a position of 90 degrees. 01:12.960 --> 01:19.140 Let's also assume that the motor is currently stationary at the zero degree position. 01:19.860 --> 01:26.280 The error variable, which is provided as an input to the control system is calculated as the difference 01:26.280 --> 01:27.840 between the desired position. 01:27.840 --> 01:31.230 So 90 degrees and the current position of the motor. 01:31.230 --> 01:32.490 So zero degrees. 01:32.880 --> 01:37.500 Therefore the error variable is also equal to 90 degrees. 01:38.530 --> 01:45.430 This value is passed as an input to the control system, which applies a specific logic to calculate 01:45.430 --> 01:51.670 the commands to be sent to the motor, to reach the desired position and minimize the error variable. 01:52.400 --> 01:57.140 So the motor that controls the robot link starts to rotate. 01:57.140 --> 02:03.650 And in the next time step when the control loop is executed again, it will have rotated by a certain 02:03.650 --> 02:04.190 angle. 02:04.580 --> 02:12.650 Let's assume, for example, that it has rotated by 30 degrees, so the error variable is recalculated 02:12.650 --> 02:19.700 as the difference between the target position, which still remains 90 degrees and the current position 02:19.700 --> 02:22.700 of the motor, which now is 30 degrees. 02:22.940 --> 02:29.280 Therefore, in this iteration of the loop, the error variable corresponds to 60 degrees. 02:29.300 --> 02:35.150 Once again, this error variable is passed as an input to the control system which calculates the new 02:35.150 --> 02:37.520 commands to be sent to the robot. 02:38.030 --> 02:44.510 Once again, the motor executes the command received from the control system and moves towards the desired 02:44.510 --> 02:45.110 pose. 02:45.650 --> 02:48.890 At the next time step the control loop repeats. 02:48.890 --> 02:55.860 So this time the motor is at the 60 degrees position in the error variable is recalculated as the difference 02:55.860 --> 02:57.390 between the desired position. 02:57.390 --> 02:59.940 So 90 degrees and the current position. 02:59.940 --> 03:03.450 So 60 degrees resulting in 30 degrees. 03:03.540 --> 03:06.900 We can see that with each control loop iteration. 03:06.930 --> 03:13.200 The error variable is effectively decreasing, indicating that the system is approaching the desired 03:13.200 --> 03:13.980 position. 03:13.980 --> 03:18.510 So the arm continues to move and a new control loop begins. 03:18.510 --> 03:22.320 So the current position of the motor is 90 degrees. 03:22.350 --> 03:28.470 The error variable is also recalculated and represents the difference between the desired position. 03:28.470 --> 03:32.880 So 90 degrees and the current position that is also 90 degrees. 03:32.880 --> 03:35.760 And therefore in this case the error variable is zero. 03:36.720 --> 03:43.140 However, depending on the implemented control system, even when the error variable reaches zero and 03:43.140 --> 03:48.720 the system reaches the desired position, the control system is not turned off or stopped. 03:48.750 --> 03:56.700 It continues its execution to correct any external disturbance, such as gravity, acting on the robotic 03:56.700 --> 03:59.700 arm which can push the robots downwards. 04:00.390 --> 04:06.030 The control system tasks in this case is to maintain the arm stable in the current position. 04:06.910 --> 04:14.080 In addition to position control, the Ros2 Control Library also supports the velocity control. 04:14.780 --> 04:17.780 In this type of control, the goal changes. 04:17.900 --> 04:24.950 Instead of having a motor to reach a specific position, we want it to start moving to start rotating 04:24.950 --> 04:26.780 with a certain velocity. 04:27.140 --> 04:33.770 Therefore, the input to the control system is represented by the desired velocity and the error variable 04:33.770 --> 04:39.050 is calculated as the difference between the desired velocity and the current velocity of the motor. 04:39.230 --> 04:46.070 And so the control system sends commands to the actuator to rotate at the desired velocity in order 04:46.070 --> 04:48.110 to minimize the error variable. 04:49.030 --> 04:56.650 Furthermore, the Ros2 control also supports force or torque control applied to the motors. 04:57.540 --> 05:04.020 This type of control is useful, for example, in manipulation application where fragile objects needs 05:04.020 --> 05:06.210 to be grasped without breaking them. 05:06.690 --> 05:12.120 In this case, our position control could apply too much pressure and break the object. 05:12.120 --> 05:14.550 Or it could apply not enough pressure. 05:14.550 --> 05:15.750 And so drop it. 05:16.200 --> 05:21.180 In this case, a force torque control is used where the input variable is. 05:21.180 --> 05:25.560 For example, the force to be applied in order to grasp an object. 05:25.560 --> 05:31.740 And the error variable is calculated as the difference between the desired force and the current force 05:31.740 --> 05:33.750 that the gripper is applying. 05:33.750 --> 05:40.230 And so the control system ensures that the gripper is closed with the right force in order to grasp 05:40.230 --> 05:43.170 and manipulate the object without breaking it.