Development Of a Simple Robot Arm Using Servo Motors

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1 Development Of a Simple Robot Arm Using Servo Motors June 2000 Oguz ASLANTÜRK, Research Assistant at Hacettepe University Dept. of Computer Science & Engineering aslantur@hacettepe.edu.tr Ahmet MUTLU Research Assistant at Hacettepe University Dept. of Computer Science & Engineering ahmetm@hacettepe.edu.tr

2 ROBOTICS The term robot was first introduced into vocabulary by the Czech playwright Karel Capek in his 1920 play Rossum s Universal Robots, the word robota being the Czech word for work. Since then the term has been applied to a great variety of mechanical devices, such as teleoperators, underwater vehicles, autonomous land rovers, etc. Virtually anything that operates with some degreee og autonomy, usually under computer control, has at some point been called a robot. The type of robot that is a computer controlled indestrial manipulator is essentially a mechanical arm operating under computer control. Such devices, though far from the robotics of science fiction, are neverthless extremely complex electro-mechanical systems whose analytical description requires advanced methods, and which present mant challenging and interesting research problems. An officical definition of such a robot comes from the Robot Institute of America (RIA) : A robot is a reprogrammable multifunctional manipulater designed to move material, parts, tools, or specialized devices through variable programmed motions for the performance of a variety of tasks. Several milestones on the road to present day robot technology are listed below : 1947 the first servoed electric powered teleoperator is developed 1948 a teleoperator is developed,ncorporating force feedback 1949 research on numerically controlled miling machines is initiated 1954 George Devol designs the first programmable robot 1956 Joseph Engelberger, a Columbia University physics student, buys the rights to Devol s robot and founds the Unimation Company 1961 the first Unimate robot is installed in a Trenton, New Jersey plant of General Motors (to tend a die casting machine) 1963 the first robot incorporating force feedback information is developed 1971 the Stanford Arm is developed in Stanford University 1973 the first robot programmind language (WAVE) is developed at Stanford 1974 Cincinnati Milacron introduces the T 3 robot with computer control 1975 Unimation Inc. registers its financial profit 1976 the Remote Center Compliance (RCC) device for part insertion in assembly is developed at Draper Labs in Boston 1978 Unimation introduces the PUMA robot, based on designs from a General Motors study

3 1979 the SCARA robot design is introduced in Japan 1981 the first direct-drive robot is developed at Carnegie-Mello University COMMON KINEMATIC ARRANGEMENTS Manipulator : Manipulator is the mechanical unit which performs the movement function in the robot. It consists of a series of mechanical links and joints capable of producing controlled movement in various directions. The manipulator is composed of the main frame (the arm) and the wrist, each having three degrees of freedom, or axes of motion. In practise, manipulators are usually designed with at least a broad class of application in mind, such as welding, materials handling, and assembly. These applications largely dictate the choice of various design parameters of the manipulator, including its kinematic structure. Robot manipulators can be classified by several criteria, such as their geometry, or kinematic structure, the type of application for which they are designed, the manner in which they are controlled, etc. Most industrial manipulators at the present time have six or fewer degrees-of-freedom. These manipulators are usually classified kinematically on the basis of the arm or first three joints, with the wrist being described seperately. The majority of these manipulators fall into one of five geometric types : 1. Articulated (RRR) or Jointed : three rotary axes 2. Spherical (RRP) : one linear and two rotary axes 3. SCARA (RRP) 4. Cylindrical (RPP) : two linear and one rotary axes 5. Cartesian (PPP) : three linear axes ARTICULATED CONFIGURATION The articulated manipulator is also called a revolute or anthropomorphic manipulator. Two common revolute designs are the elbow type manipulator and the parallelogram linkage. In these arrangements joint axis z 2 is parallel to z 1 and both z 1 and z 2 are perpendicular to z 0. The elbow manipulator configuration provides for

4 relatively large freedom of movement in a compact space. The parallelogram linkage, although less desxtrous typically than the elbow manipulator configuration, neverthless has several disadvantages that make it an attractive and popular design. The most notable feature of the parallelogram linkage configuration is that the actuator for joint 3 is located on joint 1. Since the weight of the motor is born by link 1, links 2 and 3 can be made more lightweight and the motors themselves can be less powerful. Also the dynamics of the parallelogram manipulator are simpler than those of the elbow manipulator, thus making it easier to control. Other common ways to classify robots are by their power source, application area, and method of control. Power Source: Typically, robots are either electrically, hydraulically, or pneumatically powered. Hydraulic actuators are unrivaled in their speed of response and torque producing capability. Therefore hydraulic robots are used primarily for lifting heavy loads. The drawbacks of hydraulic robots that they tend to leak hydraulic fluid, require much more peripheral equipment, such as pumps, which also requires more maintenance, and they are noisy. Robots deriven by DC- or AC-servo motors are increasingly popular since they are cheaper, cleaner and quieter. Pneumatic robots are inexpensive and simple but cannot be controlled precisely. As a resulti pneumatic robots are limited in their range of applications and popularity. Application Area: The largest application area of robots is in assembly. Therefore, robots are often classified by application into assembly and non-assembly robots.

5 Assembly robots tend to be small, electrically driven and either revolute or SCARA in design. The main non-assembly application areas to date have been in welding, spray-painting, material handling, and machine loading and unloading. Method of Control: Robots are classified by control method into servo and nonservo robots. Non-servo robots are essentially open-loop devices whose movement is limited to predeterminedmechanicval stops, and are useful primarily for materials transfer. Servo robots use closed-loop computer control to determine their motion and are thus capable of being truly multifunctional, reprogrammable devices. CONTROL LOOPS OF ROBOTICS SYSTEMS Control systems can operate either in an open loop or in a closed loop. In open-loop control systems the output has no effect upon the input. As an example of an openloop system, one can assume that a constant voltage is applied to an electric motor and consequently the motor rotates. The speed of the motor s shaft is the output, and the supplied voltage is the input. A load on the motor will cause a the speed decrease, a situation which cannot be remedied since the input voltage is not affected by the speed variations. Systems in which the output affects the input to the controlled element are called closed-loop control systems. Open-loop robots use stepping motors for driving the axes. A stepping motor is a device whose output shaft rotates through a fixed angle in response to an input pulse. Stepping motors provide the simplest means of converting electrical pulses into proportional angular movement, and as a result represent a relatively inexpensive solution to the control problem. Since there is no feedback from the shaft

6 position, positioning accuracy is solely a function of the motor s ability to step through the exact number of pulses provieded at its input. Describing different methods of classifying robot systems, we tried to make reader have enough knowledge of robotics so that he clearly understands the type of the robot system that we develop. It is an i. articulated, ii. electrically powered, iii. non-assembly, iv. non-servo (open-loop) robot system.

SYSTEM An Intel 8085 based controller system is designed in order to control a simple robot arm model, which consists of 3 joints. Joints are standard servo motors.

7 SYSTEM An Intel 8085 based controller system is designed in order to control a simple robot arm model, which consists of 3 joints. Joints are standard servo motors. Servo motors used in radio controlled models (cars,planes etc.) are very useful in many kinds of small robotics experiments because they ase small, compact and quite inexpensive (about 15 US dollars). The servo motors itself have built in motor, gearbox, position feedback mechanism and controlling electronics. It can be controlled to move any position just by using simple pulse controlling. Standard Servo Motor Servo motors have three wire interface for controlling and power supplying. The wires are colored using following color code: BLACK Ground YELLOW Control pin RED +4.8V power supply (+5V works well in this) Controlling of the servo motors are used using pulse controlling on control pin. The control pulse is positive going pulse with length of 1 to 2 ms which is repeated about times a second. You can check the details in the figure below:

At about 50 Hz, a TTL pulse between 1 and 2 millisecond is sent to the servo. The length of the pulse tells the servo at which angle it should put its action lever. 1 ms means 0, 2 ms means 120. 1.5 ms means 60, 1.

8 At about 50 Hz, a TTL pulse between 1 and 2 millisecond is sent to the servo. The length of the pulse tells the servo at which angle it should put its action lever. 1 ms means 0, 2 ms means ms means 60, 1.2 ms means 24, and so on... (Servos are counter-reactive: they will try to maintain the lever at the asked angle, whatever the forces acting upon it.) As a standard servo for model kits does not have any sort of memory, it has to receive its control pulse very often (About every 20ms). When receiving no pulse, it won t use it s motor any more, and just follow the mechanical forces acting on it. Receiving too many pulses, it will go crazy. The controlling scheme is very easy to impelement with some electronics. A 8155 static read/write memory with I/O ports and timer can manupilate both signalling and timing. The controller connection diagram is below. A 8085AH microprocessor with 2732 EPROM and 8155 static RAM, I/O and timer are used. (CE0 comes from device select logic, TIMER comes from 8155)

9 Processor operates at 3.07Mhz. So a 6.144Mhz crystal is connected to X1 and X2 inputs, since clock logic is internal to the 8085AH. Reset operation is controlled by a switch so there is no power-on reset circuit. System is reset by inputting a low signal via RESET for at least 500 nanoseconds. Therefore, when system is powered up, a zero-going and then a one-going pulse must provided manually from the switch. RESET OUT signal is provided which goes to reset Only A0-A7 is used for addressing 2732 (32Kbit = 4Kbyte), enabling an address space of 256 bytes is a custom circuit designed specifically for 8085 microprocessor. Provides 256 bytes of static read/write memory, two or three parallel I/O ports and a programmable timer. System RAM and I/O ports are on this chip.therefore, it must be selected whenever a port is accessed or a RAM location is accessed. Device select logic is based on the assumptation that, 2732 EPROM is mapped to addresses through 000-0FFH, 8155 static RAM is mapped to 100-1FFH. Connection diagram is as below: Whenever A8 is 1 (a RAM access is being done) or whenever an I/O operation is performed via an IN or OUT instruction (IO/M = 1), 8155 is enabled by CE1 and when an address lower then 100H (that is, A8 = 0) is accessed, 2732 is enabled by CE has 6 ports addressable with AD0, AD1 and AD2 while IO/M = 1 and chip enable true (CE1 = 0). AD0 AD1 AD2 Port Status / Command Register Port A Port B Port C

1 0 0 Counter/Timer register, low-order byte 1 0 1 Counter/Timer register, high-order byte Command register at 00H, Port A at 01H, Port B at 02H, Port C at 03H, Counter low order byte at 04H and

10 1 0 0 Counter/Timer register, low-order byte Counter/Timer register, high-order byte Command register at 00H, Port A at 01H, Port B at 02H, Port C at 03H, Counter low order byte at 04H and Counter high order byte at 05H. Timer is programmed for outputting a zero-going pulse every 0.1 ms. This pulse is connected to trap input of 8085, in order to generate a trap interrupt every 0.1 ms which provides a real-time clock for system software. Timer gets input from 8085 s clock which is at 3.07Mhz. So, every 320ns, counter is decremented by one. Counter registers are set to 139H (313 decimal). Thus, counter reaches to zero after 313x320 = ns = 0.1ms, by generating a pulse on TIMER OUT pin. TIMER OUT 0.1 ms Each time the pulse goes to zero and stay there for one full clock cycle, a nonmaskable trap interrupt ocuurs. Port A is used as control signals of motors. PA0 controls motor1, PA1controls motor2 and PA2 contols motor 2. System uses these pins as input to motors. There is no feedback mechanism supplied from motors. Control scheme is open-loop.

11 SOFTWARE System is closed to interaction. Robot arm is programmed to perform a specific action. All three motors, say joints, keeps their action level periodically at 3 angle values : 0,60 and 120 degrees. There are two codes in 2 EPROMs. The first one makes motors move concurrently. Three motors (joints) makes the same rotations at the same time. The second one is more clever then the first. Motor1 (rotary base joint), moves between 3 points of its trajectory: start position, middle positoin and end position. Each time it stops, other motors makes a full rotation to end position and back to start position at the same time. These control schemes have no practical benefit. This is just a simulation of its movement and control. This code section makes the initializations of timer, system stack and motor positions. ORG 040H START: MVI A, B ; Disable Rst 7.5 Rst 6.5 and Rst 5.5 SIM LXI SP,01FAH LXI H,0102H MVI M,0AH ; Set Stack ; Initial motor positions, ; 10 x 0.1 ms, 1ms signal, start position ; all motors have same positions at any time MVI A, B ; Stop timer, set port A,B output OUT 00H ; 8155 Control register at 00H MVI A,39H ; Set timer to generate a zero-going pulse ; every 0.1 ms OUT 04H ; on timer-out pin of 8155, which is ; connected to trap ; interrupt pin of MVI A, B ; High-order byte, set timer mode 3 OUT 05H ; Timer high-order byte at 05H MVI A, B ; Start timer OUT 00H ; Control register at 00H This part controls the motor signals. Here, the second control scheme is given in which, motors have different motion trajectories. The below state diagram explains how the system decides which motor to rotate for how much degrees; Motor1 60 degree Motor 2,3 120dgr. 1.5 sec 1.5 sec 2,3 120dgr. back L1: 1.5 sec

12 L1_0: STATENZ: ROTATE: L2: REFRESH: U0: U1: MVI B,60 CALL WAIT25MS ; Refresh motor signals every 25ms CALL REFRESH DCR B JNZ L1 _0 ; Update motor position every 60x25=1500 ms MVI A,00H ; At state 0, CMP C JNZ STATENZ ; Rotate upper motors LXI H,102H ; Rotate around y axis (motor1) MVI E,05H ; Rotation value (5x0.1ms = 0.5ms, 60 degree) CALL ROTATE MVI C,02 ; Go to state 2 JMP L1 DCR C LXI H,103H ; Motor 2 and 3 MVI E,0AH CALL ROTATE JMP L1 MVI A,14H ; Rotation value (10 x 0.1 = 1ms,120 degree ; to end or start position) ; Calculates the new positions ; If at end position (20 x 0.1 = 2ms, ; if at 120 degree) CMP M JNZ L2 MVI M,0AH ; Go to start position (10 x 0.1 = 1ms, 0 ; degree) RET MOV A,E ADD M MOV M,A RET ; If not at end position, rotate according to ; value in E ORG 0C0H ; Refresh servo signals for them to keep ; their positions againts external forces MVI D,00H MVI A, B ; Motor1 OUT 01H ; Output logic 1 on PA0 for 1-2 ms LXI H,102H MOV A,M CMP D JNZ U0 MVI A,00H OUT 01H MOV D,00H MVI A, B ; Motors 2 and 3 OUT 01H ; Output logic 1 for 1-2 ms on PA1 and PA2 LXI H,103H MOV A,M CMP D JNZ U1 MVI A,00H OUT 01H

13 WAIT25MS: WAIT: RET MVI D,00H MVI A,0FFH ; Generate a delay for 25 ms CMP D ; Wait for D register to reach 255 ; which means that trap interrupt occurred ; 255 times. 255 x 0.1 = 25.5 ms passed JNZ WAIT RET ORG 24H INR D RET ; Trap handler. occurs every 0.1 ms ; Increment D each time int. occurs

14 References 1. Microprocessor and Peripheral Handbook, Intel Corp &8-Bit Microprocessor Handbook, Adam Osborne, Gerry Kane, Osborne/McGraw-Hill From web; Robot Dyanamics and Control, Mark W. Spong, M. Vidyasagar, 1989, by John Wiley & Sons, Inc. 5. Robotics for Engineers, Yoram Koren, 1985, by McGraw-Hill

Introduction to Robotics

Introduction to Robotics Analysis, systems, Applications Saeed B. Niku Chapter 1 Fundamentals 1. Introduction Fig. 1.1 (a) A Kuhnezug truck-mounted crane Reprinted with permission from Kuhnezug Fordertechnik