ABSTRACT

Hazardous or uncomfortable working conditions, repetitive tasks, difficult handing, and multi-shift operations demands use of robots in industrial situations. They are greatly applied in production situations. Such as materials transfer, machine loading, welding, sprays coating, process operations, assembly, and inspection. Material transfer robot includes pick and place operations. Computer Aided Manufacturing (CAM) systems involves the use of industrial robots to perform a portion of the parts handing chore. Pick and place robots are used to move work parts between the machine and to orient and locate the work parts for processing at the machines. This paper describes a simple low-cost pick and place type robotic system is designed to transfer the work parts from one place to another. Pick and place operations are controlled through PC by using add-on card. One can pick the object and place at a position decided by user using PC. Software is developed for deciding angle through which the robot arm moves pick and place positions and activation of sensor.

Introduction

• The completely automated and integrated environment consists of computers, PLCs, NCs, CNCs or DNC machine robots, database management system (DBMS), LAN based instrumentation and graphical user interface (GUI) based software.

• Robot is an integral part of the computer aided manufacturing (CAM) systems for providing flexibility, high productivity and improving the quality product consisting of faster operations and hazards situations.

Motivation

• Robots are ideally suited for any repetitive job, which is regarded as too boaring or too tedious for human operators.

• For such situations pick and place robot is an ideal solution where more flexibility is not desired. This paper describes fabrication of a simple robot for a small production line or work-cell².

Designing of pick and place robot

• A robot is a re-programmable, multifunction manipulator designed to move material, and parts, tools or specialised devices through variable programmed motions for the performance of a variety of tasks².
• Another generalised definition of robot is that an industrial robot is a general purpose, programmable machine possessing certain anthropomorphic characteristics.
• Design and fabrication of a robot is mainly divided into three major aspects namely: Mechanical, Electronics, and Interfacing to PC

Mechanical aspects:

• The main mechanical part in pick and place robotic system is to fabricate a robotic arm.
• Basically the robotic arm is designed for the application such that it can hold both the solenoid actuator and the object.
• One end of the arm is fixed which is mechanically coupled to the shaft of the stepper motor so that it can be moved in angular direction.
• The second end of the arm consists of solenoid actuator mechanically coupled to the arm, which is used to pick and place the object from one position to the other at desired angle.
• The stepper motor with robotic arm is mounted on a metallic base-table. The torque of the motor is 1/2 kg. The counterweight is added to balance the mechanical system.

Electronics aspect:

• For the controlled movement of robotic arm at a desired position, electronic control circuit is designed.
• It consists of stepper motor controller IC297 and dual full bridge driver IC298³.
• Stepper controller generates four phase drive signals for stepper motor in PC controlled application.
• The motor can be driven in half step and full step mode. On chip PWM chopper circuit permits switch mode control of the current in the winding. A feature of this device is that it requires external clock, mode input, and direction signals.
• The phase sequence is generated internally. Hence programming burden is highly reduced for interfacing purpose.
• IC L298 is high voltage and high current, dual bridge driver.
• It accepts TTL logic levels at the input and it can drive inductive load.
• The bridge is grounded through current sense resistors.
• Current mode PWM maintains the torque at high speed.
• Stepper motor of rating 12V/500mA is selected for the present application.
• The controller supply is +5 V and driver supply is +12 V. The diodes are used as a feedback diode to return the energy to the supply in the off time of PWM.
• These fast switching diodes also protect the driver transistors during off time.

Interfacing to PC:

• Figure-2 shows the interfacing block diagram of the pick and place robotic system.
• System consists of digital input output (DIO) PC add-on card, stepper motor controller circuit and robotic arm. DIO card, PCL227 supplied by Dynalog Systems⁴ is used in the present system.
• The control software ROBO is designed developed and implemented so as to integrate, mechanical, electronics and computer interface cards for a simple assembly line.
• Figure-3 indicates flowchart of the control software ROBO. The flowchart itself is self-explanatory. After initializing the hardware, the pick and place positions, and the actuation current of the electromagnetic actuator are decided.

Results and Discussion

• Pick and place robot has been designed to move the work parts from one position to another.
• The system is implemented with a proper integration of the respective mechanical, electronics and computer hardware and software.
• This software has been developed under Turbo C/C++ environment.
• It is completely tested for consistency and reliability for the precise movement of the arm.
• The pick and place positions can be precisely selected with minimum angle of rotation of ± 0.9⁰.
• It has been observed that by changing the activation current of the electromagnetic actuator, the amount of weight that could be picked up also varies linearly.
• By setting the actuation current, different objects having different weights could be sorted out.
• Selection of specific object from a lot of objects can also be implemented. This work in this direction is in progress.

References:

1. B. Hodges, "Industrial Robots", Jaico publishing house (1993).
2. Groover, Mikell P.Groover, Zimmers, JR. "CAD/CAM : Computer-Aided Design and Manufacturing ",Printice Hall of India Private Ltd.( 1992).
3. SGS-THOMSON MICROELECTRONICS "Industrial and Computer Pheripheral Ics " DATA BOOK, (Oct. 1988).
4. Installation and Operating Manual of PCL-227 DYNALOG MICRO SYSTEMS PVT.LTD. (1996).