The
authors:
Tarek M. Sobh, Mohamed Dekhil, Thomas C. Henderson, and Anil Sabbavarapu
Source:
Department
of Computer Science and Engineering
University of Bridgeport
Bridgeport, CT 06601, USA
and
Department of Computer Science
University of Utah
Salt Lake City, UT 84112, USA
In this paper we will present the stages of designing and building a three-link robot manipulator prototype that was built as part of a research project for establishing a prototyping environment for robot manipulators. Building this robot enabled us determine the required subsystems and interfaces to build the prototyping environment,and provided hands-on experience for the real problems and difficulties that we would like to address and solve using this environment. Also, this robot is used as an educational tool in robotics and control classes.
Teaching robotics in most engineering schools lakes the practical side and usually students end up taking lots of theoretical background and mathematical basis, and maybe writing some simulation programs, but they do not get the chance to apply and practice what they have learned on real robots. This is due to the fact that most of the robots available in the market are either too advanced, complicated, and expensive(e.g., specialized industrial robots),or toy-likerobots which are too trivial and does not give the required level of depth or functionality needed to demonstrate the main concepts of robot design and control. One of our goals in this project, was to build a robot that is simple, flexible, and easy to use and connect to any workstation or PC, and at the same time, is capable of demonstrating some of the design and control concepts. We also tried to keep the cost as low as possible to make it available to any engineering school or industrial organization.
We consider the main contribution of this work to be building URK (Utah Robot Kit) which is a three-link robot prototype that has a small size and reasonable weight which is convenient for a small laboraclassroom. URK can be connected to any workstation or PC through the standard serial port with an RS232 cable, and can be controlled using a software controller with a graphical user interface. This software controller applies a simple PID control low for each link which doesnot require the knowledge of the robot parameters. Therfore, this software can be used to control any electro-mechanical system that can be controlled by a physical PID controller. The interface enables the user to change any of the control parameters and to monitorthe behaviorof the system with an on-linegraphs and a 3-D viewfor the robot showing the current positionof the robot.
Controlling and simulating a robot is a process that involves a large number of mathematical equations. To be able to deal with the required amount of computation, it is better to divide them into modules, in which each module accomplishes a certain task. The most important modules, as described in [2], are kinematics, inverse kinematics, dynamics, trajectory generation, and linear feedback control.
There has been a lot of research to automate kinematic and inverse kinematic calculations. A software package called SRAST (Symbolic Robot Arm Solution Tool) that symbolically solves the forward and inverse kinematics for n-degree of freedom manipulators has been developedby Herrera-Bendezu, Mu, andCain[5]. The input to this package is the Denavit-Hartenberg parameters, and the output is the direct and inverse kinematics solutions. Another method of finding symbolic solutions for the inverse kinematics problem was proposed in [11]. Kelmar and Khoslaproposed a method for automatic generation of forward and inverse kinematics for a reconfigurable manipulator system [7].
Dynamics is the study of the forces required to cause the motion. There are some parallel algorithms to calculate the dynamics of a manipulator. Several approaches have been suggested in [8, 9, 10] based on a multiprocessor controller, and pipelined architectures to speed up the calculations.
Linear feedback control is used in most control systems for positioning and trajectory tracking. Thereare sensors at each joint to measure the joint angle and velocity, and there is an actuator at each joint to apply a torque on the neighboring link. The readings from the sensors will constitute the feedback of the control system. By choosing appropriate gains we can control the behavior of the output function representing the actual trajectory generated. Minimizing the error between the desired and actual trajectories is ourmain concern. Figure 1 shows a block diagram for the controller, and the role of each of the robot modules in the system.
Figure 1 – Block diagram of the Controller of a Robot Manipulator.
Most of the feedback algorithms used in current control systems are implementations of a proportional plus derivative (PD) control. In industrial robots, a local PD feedback control law is applied at each joint independently. Some ideas have been suggested to enhance the usabilityof the local PD feedback law for trajectory tracking. One idea is to add a lag-lead compensator using frequency response analysis [1]. Another method is to buildan inner loop stabilizing controller using a multivariable PD controller, and anouter loop tracking controller usinga multivariablePID controller[12]. In general,usingalocal PD feedback controller with high update rates can give an acceptable accuracy for trajectory tracking applications. It was proved that using a linear PD feedback law is useful for positioningand trajectory tracking [6].
This project was started with the study of a set of robot configuration sand analyzed the type and amount of calculation involved in each of the robot controller modules(kinematics,inverse kinematics,dynamics, trajectory planning, feed-back control, and simulation). This phase was accomplished by working through a generic example for a three-link robot to compute symbolicallythe kinematics, inverse kine- matics, dynamics, and trajectory planning; these were linked to a generic motor model and its control algorithm. This study enabled us to determine the specifications of the robot for performing various tasks, it also helped us decide which parts (algorithms) should be hardwired to achieve specific mechanical performances, and also how to supply the control signals efficiently and at what rates.
Первым шагом в разработке регуляторов для робота-манипулятора является решение его кинематики, обратной кинематики, динамики и управления с обратной связью, которые будут использоваться. Кроме того на данном этапе должны быть определены типы входных данных и пользовательский интерфейс. Мы также должны знать такие параметры робота как: длины звеньев, их массы, моменты инерции, расстояния между суставами, конфигурацию робота, и тип сочленения. Переменные параметры должны быть заданы так, чтобы контроллер мог использоваться без каких либо изменений для различных конфигураций.
The first step in the design of a controller for a robot manipulator is to solve for its kinematics, inverse kinematics, dynamics, and the feedback control equation that will be used. Also the type of input and the user interface should be determined at this stage. We should also know the parameters of the robot, such as: link lengths, masses, inertia tensors, distances between joints, the configuration of the robot, and the type of each link (revolute or prismatic). To make a modular and flexible design, variable parameters are used that can be fed to the system at run-time, so that this controller can be used for different configurations without any changes.
The kinematics and the dynamics of the three models have been generated using some tools in the department called genkin and gendyn that take the configuration of the manipulator in a certain format and generate the corresponding kinematics and dynamics for that manipulator. For the trajectory generation, The cubic polynomials method was used. The error in position and velocity is calculated using the readings of the actual position and velocity from the sensors at each joint. Our control module simu- lated a PID controller to minimize that error. The error depends on several factors such as the frequency of update, the frequency of reading from the sensors, and the desired trajectory.
A simulation program has been implemented to study the performance of each manipulatorand the ef- fect of varying the update frequency on the system. Also it helps to find approximate ranges for the required torque and/or voltage, and to determine the maximum velocity to know the necessary type of sensors and A/D. To make the benchmarks, as described in the next section, we did not use a graphical interface to the simulator, since the drawing routines are time consuming, and thus give misleading figures for the speed.
As mentioned in Section 2.2, a simple linear feedback control law can be used to control the robot manipulator for positioning and trajectory tracking. For this purpose, a PID controller simulator was developed to enable testing and analyzing the robot behavior using this control strategy. Using this control scheme helps us avoid the complex (and almost impossible) task of determining the robot parameters for our three-link prototype robot. One of the most complicated parameters is the inertia tensor matrix for each link, especially when the links are nonuniform and have complicated shapes.
The assembly process of the mechanical and electrical parts was done in the Advanced Manufacturing Lab (AML) with the help of Mircea Cormos and Prof. Stanford Meek. In this design the last link is movable, so that different robot configurations can be used (see Figure 2).
Figure 2 – The physical three-link robot manipulator.
There are three motors to drive the three links, and six sensors (three for position and three for ve- locity), to read the current position and velocity for each link to be used in the feedback control loop.
This robot can be controlled using analog control by interfacing it with an analog PID controller. Digital control can also be used by interfacing the robot with either a workstation (Sun, HP, etc.) or a PC via the standard RS232. This requires an A/D and D/A chip to be connected to the workstation (or 4 thePC) and an amplifierthat provides enough power to drive the motors. A summary ofthis designcan be found in [3, 4].
This simulator was used to give some rough estimates about the required design parameters such as linklengths, linkmasses, updaterate, feedback gains,etc. It isalso used in the benchmarking described earlier. This simulator uses an approximate dynamic model for the robot, and it allows any of the design parameters to be changed.
A software controller was implemented for the three-linkrobot. This controller uses a simplelocal PID control algorithm, and simulates three PID controllers; one for each link. Several experiments and tests have been conducted using this software to examine the effects of changing some of the control parameters on the performance of the robot.
The control parameters that can be changed in this program are:
In these experiments, the program was executed on a Sun SPARCStation-10, and the A/D chip was connected to the serial port of the workstation. One problem we encountered with this workstation is thes low protocol for reading the sensor data, sinceit waits for an I/Obuffer to be filled before it returns control to the program. We tried to change the buffer size or the time-out value that is used, but we had no success in that. This problem causes the updaterate to be very low (about 30 times per second), and this affects the positional accuracy of the robot. We were able to solve this problem on an HP-700 machine, and we reached an update rate of 120 times per second which was good enough for our robot.
A prototype 3-link robot manipulator was built to determine the required components fo ra flexible prototyping environment for electromechanical systems in general, and for robot manipulators in particular. A local linear PD feedback law was used for controlling the robot for positioning and trajectory tracking. A graphical user interface was implemented for controlling and simulating the robot. This robot is intended to be an educational tool, therefore it was designed in such a way that makes it very easy to install and manipulate.