The control of parallel robots using intelligent algorithms

The control of parallel structures using intelligent algorithmsRadu CORCODEL 2nd year, Industrial Robots Dept., Faculty of Machine Building, Technical University of ClujNapoca Cristian CAIA 2nd year, Machine Building Dept., Faculty of Machine Building, Technical University of ClujNapoca Scientific coordinators: Conf. dr. ing. Radu BĂLAN Department of Mechanisms, Precision Mechanics and Mechatronics Drd. ing. Sergiu-Dan STAN Department of Mechanics and Programming Abstract RC-2 is an intelligent 2 DOF parallel robot that can predict bad trajectories and reconfigure itself to avoid them. The command console interactively simulates and controls the motion of the structure by means of a logic LPT-based algorithm. Such algorithm, developed in Borland Delphi, named SMART, is multiplexed directly on the port and doesn’t need a complicated electronic interface, as a com-based program requires. SMART simulates possible trajectories and, according with the real one, generates collision free paths and so flawless motion is achieved. A linear amplifier powers the coils of the motors, turning them one way or another. The amplifiers are designed in a “D”-type amplification class and have a yield of about 95%. The robot is using two unipolar floppy disk-drive motors, turning 1.8º in micro-step command. This kind of command gives the entire structure high accuracy and great mobility and thus, the motion of the end-effector is optimized. Keywords: parallel robots, interactive simulation, SMART©, LPT, stepper motors I. INTRODUCTION Most robots are basically serial or parallel, but several architectures can produce a hybrid robot, including end effectors attachments, robotic hands, multiple limb robots or a few special architecture robots. These systems are usually developed in an attempt to take advantage of the benefits of both types of robotic structures [1], [8]. A parallel robot can but use a 32-bit processor as the main CPU. Numerous works investigating parallel mechanisms stress out the various advantages of them [7]. Fig. This architecture supports much more powerful application software than with a microcontroller. Parallel manipulators show better stiffness. [6]. or Java frameworks such as J22EE and J2ME. medical applications and others. A parallel robot is a closed chain mechanism. As the general industry has matured. Where historically a microcontroller would be found at the heart of a robot. The usages of this kind of motors are imposed by means of correlation of each kinematic chain [9]. parallel robots are using DC motors in peak torque. II. unipolar steppers are used where high power is needed. frameworks and APIs such as Microsoft’s Windows APIs and . Consequently. [8]. Today.1 Two DOF parallel robot . most robotic products require a great deal of custom software components. especially the ability to run today’s 32-bit operating systems [2]. For example. for functionally ranging from graphical user interfaces (GUI). such as in machine tools. which have great angular precision and relatively high torque-toweight ratio. frameworks and software components are available to the robotic industry. The drawing below shows the concept of such a robot.support greater loads without significant deflections than a comparably sized serial robot [4]. network connectivity and databases access. The extensibility of a parallel robot can be extended by using dedicated tools such as micro grippers or micro dispensing systems. Unipolar motors have the great advantage of a high torque. Usually. a common scenario today is to see such processors dedicated to perform certain functions within the robot. yet they are already becoming a niche in several robotics research areas. along with their applications. compromises are inevitable when using a fixed geometry robot fore some tasks. Robots are used to perform various tasks involving complex manipulations and interactions with their environment. connected together by at least two independent parallel kinematic chains that perform certain operations within their workspace. assembly lines. Standardized objects. Choosing a type of stepper is according to a specific job that the motor has to perform. TWO DOF PARALLEL ROBOT Parallel robots that have two degrees of freedom are usually built on a console named framework and have two motors that drive a five-bar linkage [1]. they can be classified in two categories: unipolar and bipolar steppers. The compelling price-performance ratio of computing technologies derived from the PC industry is increasingly influencing the embedded system market. They can operate at higher velocity and acceleration due to lightweight materials of the moving parallel structure and heavy motors and gears that build up the supporting frame [5].Net frameworks. which consist of two platforms (base and end-effectors). and robotics is no exception. Regarding the constructions of stepper motors. [10]. while bipolar motors are used where the space reserved for them is very small. positioning accuracy and loadcarrying capacity then serial manipulators. Parallel robots are relatively new trend in the field of robotics. while bipolar motors are much smaller at a given torque. it has increasingly come to employ standardized software objects. Several robots can be combined in a working cell offering solutions to a variety of tasks in micro assembly [2]. The inverse kinematic problem provides the relation between the Cartesian coordinates of the gripper. and contains some particularities which are to be discussed in the following part. is transformed into degrees of rotation on which the actuators should turn. the precise coordinates on which the gripper should move. via a dedicated program. Initialize Printer. the RC-2 control program gives the LPT the following data string: Step 1: 00000001 Step 2: 00000011 Step 3: 00000010 Step 4: 00000110 Step 5: 00000100 Step 6: 00001100 Step 7: 00001000 (3) Fig. one at a time. it was kept in mind that the motors are turning 3. Basically. Thus it can be used to generate complicated data streams that the COM port would only do with a microcontroller. It controls the structure using a graphical user interface (which is to be referred in the following claims by GUI) that enables a firm control of the rotations of the two motors.8º per step in microstep sequence. Further more. Busy. Doing so. the control program (which is to be referred from this point on as RC-2) was developed to use the LPT port which has greater speed and complexity in the commands given then the communication port.The gripper. Fig. The movement of the gripper is determined by the generalized coordinates of the motor rotors (q1. is determined by two coordinates (X. Thus. Y). the LPT port transmits through eight data bits and three control ports (Select Input. Acknowledge) [11]. They can be powered in two different modes. The structure in discussion uses unipolar stepper motors. or 1. The program developed to do so. q1 = π + 2arctg q 2 = 2arctg ( a + l1) 2 b 2 − ( a − l1) 2 2 −b 2 2 + arctg y x−c y x (1) (2) ( a + l 2) b 2 − ( a − l 2) −b 2 + arctg By using this equations. in a clock-wise or counter-clock-wise sequence. Another purpose of this parallel robot is to reduce the time from the moment when the operator gives a certain command to the moment it is actually executed by the structure. the data from the PC is inserted directly into the power amplifiers which make the motors turn.3 Internal schematic of the LPT port As shown in the schematic above. It receives signal via five channels (Error. was designed in Borland Delphi 7. placed on the conjunction of the upper tendons. One of them is to power-up the four coils. Figure three shows the internal structure of the parallel printer port.6º per step in normal step sequence. q2). the GUI was created using micro-step sequence. and Auto line feed). the miniaturization is even greater. Select. Paper end.2 The micro-step sequence of a peak torque motor . When developing the GUI. For high precision in movement and low deploying errors. or powering in a micro-step sequence as shown in the image below [3]. if a structure is using a notebook. and the generalized coordinates of the actuators. Thus the robot doesn’t need a restart or reposition by hand. the algorithm is reconfiguring the trajectory using the second solution. The algorithm uses the resources of the system to calculate optimal trajectories and by comparing them with the theoretical path. the program changes into auto mode. SMART© is a logical algorithm that identifies the position of the two lower tendons by a logic condition. it transmits an error message. where the coordinates are entered in dedicated fields.Step 8: 00001001 until the motor has rotated a certain angle. because the equation implies two solutions. or 30 rpm to 1. Also. The program has a feature that supports both automatic positioning and manual override. It has a virtual 2D matrix that allows it to anticipate further collisions or bad trajectories that end up in a singular point. 1. to change from auto to manual mode. one should press the execute button. then the solutions are regarded as invalid.4 The graphic interface of the RC-2 control program When the program is executed. the singular points are eliminated by .8° (4) then starts a repetitive data string in accordance with (3). It simply moves freely in the workspace. To execute the desired movement. by doing so.5 The bypass of a dead point The algorithm that is responsible for these actions is called SMART© (Simulate then Move And Reconfigure Trajectory). computes a collision free trajectory. and if at least one of them coincides. In order to do so.5 rpm. SMART© will always find two such points. The program calculates the number of cycles using the formula: swapping the dial in which said elements are. Around it. The auto/man button can be pressed anytime. The solutions are then compared with the theoretical one. The nodes of the matrix are in fact the points that generate easily errors. By pressing the auto/man button. When the solution does not coincide. It varies between 5ms to 100 ms. and it doesn’t take them in consideration. Then it covers the circumference of the circle and in every point checks if the distance from that particular point to the gripper is equal to the length of the upper limbs. there are four buttons for deploying the gripper up-down and left-right with a step of 5mm. the manual override mode appears. Fig. and in case of surpassing the workspace. NC = qi . the point is in the workspace. The speed of the robot is determined by the pause between the steps. The design of the source file makes RC-2 accommodate with sudden changes of the coordinates imposed. SMART© is also designed to choose between the results found. SMART© draws two circles with their centre in the axis of the two motors and radius equal to the length of the lower limbs. Thus the error will be Fig. When the robot is closing up to these points. Now all the commands are made freely with the mouse. When the logical condition is true that means that the point is not less but the conjunction of the upper and the lower limb. that means that the solutions are true and further more. while the power level is supported by an “A” class-like cell. It was discovered that this class has great throughput power at low frequencies. when closing up to them due to a repetitive procedure that analyzes every point and surrounding points in the workspace. because of the high complexity of the LPT port. A power amplifier is needed to increase the power that the parallel port delivers. especially below 10Hz.9W). LPT ports are working with 5V at a current of approximately 56 mA. the motors could be powered one in the normal sequence. the amplification of the cell equals with the sum of the separate amplification given by the transistors. Each bit corresponds to a coil of the motor. If the system does not have a mathematic coprocessor. The 1 to 4 bits commands the first motor (on the left). Because the structure has two motors. Stepper motors are working at 12 V. To use SMART© at its full efficiency the system should be at least a P3 at 600MHz. The hybrid was produced while trying to improve the yield of “A” class amplifiers. Using this display. When powering up the coils. it jumps backwards (also known as recoil). When using the actual amplifier. For experiments it was used a LED console which gave the precise data stream delivered on the printer port. then it’s recommended to install an emulator that simulates the functioning of a coprocessor using a mire part of the main processor. known for their low internal noise. By using a series of transistors connected in a Darlington design. the motors are working independently because the data strings of the LPT port are very complex (0 to 255 symbols every second). and the other in micro-step sequence. For improving the linear amplification. The combining of the two classes are made to take advantage of the major qualities that these types have. In rest it remains into “0”. It is also the gateway that commands the LPT port and thus the motors via power amplifiers. the associated bit turns into logic “1”. and when it’s displaced by one step. The nodes are generating themselves. When doing so. the motor does one of the followings: when the alignment of the rotor teeth is displaced by two steps. the peak speed of the structure was calibrated. Microcontrollers have the great disadvantage that they always start over the sequence when a new turning of the rotor is imposed. the rotor teeth would simply demagnetize. That involves a RMS power of 280 mW. SMART© is dividing the port in two stand-alone ports.avoided. Developing the algorithm could not be done without the PC’s 32-bit main processor and mathematic coprocessor because of the need of a little more processing power than a microcontroller could offer. The display is powered between 3V and 12V directly from the PC’s power supply (black and red wire). because using an inappropriate speed. depending on the task. and when the coil needs to be powered. The signal coming from the source is first filtered and digitalized using a “C” class method. the motor is gemmed for one step due to high intensity magnetic fields. Another advantage of the routine is that it remembers the last power sequence and continues from it. It was also helpful to test the functionality of several LPT ports apparently having problems transmitting data streams. . the display could be connected in parallel to monitor the activity of the port in real time. The robot in discussion is using a multistage “D” class power amplifier. wile the 5 to 8 bits commands the second motor (on the right). The “D” class is a hybrid between “A” class and “C” class. or if the source is a notebook then the power source can be a 9V battery. and usually consumes a current of about 160mA (thus the power needed is 1. Such option is not available but it is an idea for the future versions of SMART©. The matrix is virtual because is not exactly a table-like design. the transistors chosen for this display were BC 413. When using for the first time. 6 The RC-2 parallel structure The previous image shows the parallel structure in stand-by mode. High yield power amplifiers then energize the motors which put the robot in motion. The amplifier is powered at 12V from an external power supply. it is possible to improve the dynamic of parallel structures. flawless motion. 7 PCB of the power amplifier To avoid the insight of external currents. Italy. The amplifier used supports an opto-coupler module connected in series with the communication cable coming from the LPT port.Fig. CONCLUSIONS By means of intelligent algorithms. Basically an . the robot will start instantly on executing the program. it is recommended to use the opto-coupler module. If the source is the PC’s own power supply. The value is first calculated and then it’s adjusted by experiment. The correct sequence of coils is shown following table: Coil 1 Coil 2 Coil 3 Coil 4 — — — — Red Green Brown White If the wires are connected properly. If the resistor has too great resistance. III. the LPT port gives large perspectives in using the 32-bit processors which have far greater computing power than the microcontrollers. Since a small resistance makes the transistors extremely hot and in short time. the use of an optocoupler is highly recommended. and in the same time it behaves like a simple conductor. The power amplifier used is built on the platform of the display. 2004 Fig. having an extra set of power transistors mounted in Darlington. Prisco. it is imperative to recheck the sequence of the motor coils. There is no electrical contact between them. Massimo Bergamasco. The whole assembly is mounted on four adjustable pivots to allow a firm fastening. opto-coupler is a device built on two photoelectric elements (receiver and transmitter). then the whole device has low amplification. If so. Designing such amplifiers are time consuming because of the resistors. which are the base of each transistor. Via Carducci. Further more. and so the receiver is fully isolated from the transmitter. SMART© is such algorithm that can predict some bad trajectories and correct them giving the structure a smooth. Fabio Salsedo. It has a bakelite framework and four Plexiglas limbs connected together by rotary joints. it would lead to their destruction. often greater then 12V. Design and kinematic performance evaluation of a new tendondriven planar parallel robot. REFERENCES [1] Antonio Frisoli. Giuseppe M. PISA. then the devices can be connected directly to the port. hexapods.frasco.ch [6] http://www.graduate.technion.il/Theses/ [8] http://www.uk/ [7] http://www.ac.htm [9] http://www.doc.cs. 30.harmonicdrive.[2] The Newspaper of Johns Hopkins University. 2001. 19R.ac. 2000 . no.demon.ucla.edu/ [11] Patton Electronics catalogue.co. vol.sscnet.uk/~ih/doc/stepper/ [4] http://www.de/en/ [10] http://cisstweb.csem.net/hexapod.16 [3] http://www. January 16th.ic.edu/ [5] http://www.jhu. nr.