CONTENTS
· Project Concept
· Work Cell Layout
· Cell Operation
. Video
· Components of System
· Program Code
· Applications
· Advantages
Project Concept:
The project is designed as an important part of material handling system. The aim of the work cell is to separate out parts of different widths and placed them into one of the storage bins or to dispense them to separate sections of the following manufacturing process. In an automation process this functionality is often required. A process station may produce parts of different dimensions. According to the requirement of the complete manufacturing process the parts with different dimensions should be sent to different work stations. In some industries both the stations may be whole separate work cells that do not depend on each other in the sense that the proceeding station is not provided any data regarding the dimensions of the variety of the parts processed by the previous station. In such a case this work cell is useful in connecting the two process station rather then modifying the present work cells.
This work cell can also be presented as initial stage of an industrial process where the raw materials are fed to the different processing station based on the dimensions of the raw part.
The project is designed by taking an overview of such kinds of processes mentioned above and tries to achieve maximum possible speed measured as parts per minute i.e. the number of parts the cell can deliver to the proceeding station assuming that there is always availability of parts at the input of this cell.
Work Cell layout: The cell consists of a conveyor system that contains a conveyor belt, digital sensors to detect the presence of part at different positions on the conveyor belt and a width gauge to measure the width of the parts. A robot is present at the other end of the conveyor belt where the parts end their journey while being detected by digital sensors and width measured by width gauge. Two storage bins are present near the robot to collect two types of parts that are conveyed. The conveyor system and the robot derive all the signals from the controller which is connected to the personal computer that runs the software WALLI 3 for the manufacturing cell. A CNC machine is also present in the work cell that may be used for drilling and milling purpose.
Cell Operation: Parts of different widths are conveyed in series through a conveyor belt driven by a servo motor and controller by software through Serpent 1EC controller. As the parts advance on the belt, they are detected by digital sensor 1 at a particular position on the conveyor belt. A digital width gauge is placed on the conveyor system at a distance of 90mm (approx.) from the digital sensor 1. As the part is detected by digital sensor 1 the controller gives a signal to the software via the controller. The software then executes a statement to advance the part on conveyor by 90mm and then stops the conveyor. The width gauge that was previously calibrated (when the operation began) measures the width of the part and sends the reading to the software. The width gauge derives all the signals namely START, READ and OUT from the controller via the software. Till this stage the software has the reading of the width of the part and can now decide the placement of the part into one of the two storage bins depending on the width of the part. Next the conveyor starts until the part reaches the digital sensor 2. At this stage the software command stops the conveyor and gives a signal to the Serpent 1 Robot to pick up the part and place it in one of the storage bins. The conveyor may also reject the part in case the part is not within the range of any of the two part dimensions that are to be sorted. This is achieved by moving the conveyor through specific distance so the part is automatically thrown off the conveyor. The motion of the robot to pick up the part and place in the bin was programmed using the teach pendent. So depending on the width of the part the software give the instructions to the robot to place the part into one of the bins. If the part is outside the range of the two specified parts, the conveyor advances and drops the part out into the garbage.
Description of different components of the Cell:
Serpent 1 Robot:
Serpent 1 is a 2 axis SCARA robot with 4 degrees of freedom that provides the pick and place operation for storage of parts in the storage bins. The robot derives all the control signals from the serpent 1 EC controller which in turn gets all the control signals from the WALLI3 software on computer via RS-485 communication. The robot consists of two links and at the end is the end effectors connected to the Z-axis mechanism. The end effecter has a wrist that provides the roll motion. The gripping is achieved using vacuum in the end effecter. The robot can be controlled either using the software control from the computer screen or using a teach pendent. All the motors in the robot have a feedback that delivers the accurate position of all the axes and end effecter to the computer software screen. The robot achieves a zero position when the system starts. The user can then put the robot to user defined zero or initial position, with respect to this position the robot is programmed. Thus to program the robot of pick and place operation the following steps are followed:
1. Define an initial position for the robot by putting it into a particular position. Save this position.
2. From the software window select editàinsert, and then using teach pendent move the robot to a specified position. The software window shows the coordinates of all the axis of the robot are it is operated. Once the desired position is reached it is automatically saved in the window unless it is changed by the user.
3. To add a new position repeat step 2.
The following are the controls provided on the teach pendent to control the robot.
AXIS 1: Operates the link 1 of the robot connected to joint 1.
AXIS 2: Operates the link 2 of the robot connected to joint 2.
Z AXIS: Operates the link 3 in up or down direction.
WRIST: Operates the link 3 in clockwise or anti-clockwise direction (Provides Rotation)
GRIP: Operates the gripper as Open or close.
Conveyor System with Sensors:
Conveyor system consists of approximately 1 meter of conveyor belt driven through a motor. The signals to motor are given though the controller via software instructions. The commands are given on the serial terminal of the conveyor control system. The conveyor can also be started or stopped directly from the sensors by connecting the output of the sensors to the respective (start/stop) terminal of the conveyor control. Parts of different widths are conveyed along the conveyor to the other end. There are two infra red sensors located in the path of the of the conveyor belt. Whenever the part is in the front of the sensor the output off the sensor is Logic 0 (OFF), this data is sent to the computer via the controller and hence a particular action can be taken by the software control. For instance the user may program the system to pick up the part when the part reaches sensor 2 position. A width gauge placed on the conveyor belt between sensor 1 and sensor 2 measures the width of the part and sends the reading to the computer. The software can now take appropriate action depending on the width of the part.
WALLI 3 Software:
Work-cell Amalgamated Logical Linguistic Instruction
This software provides the complete platform to develop a FLEXIBLE manufacturing cell using all the components of the FMS. It is a Windows -98 based software that provides the interfaces to all parts of the WALLI FMS cell through RS-485 communication. The following steps indicate how the above manufacturing cell was created by adding different components of the WALLI FMS cell.
File -> New Project -> “Specify Project Name” (To create new Project)
Cell -> Add Serpent 1 Robot (To add Serpent 1 robot to project)
Cell -> Add Conveyor System (To add conveyor system to project)
Cell -> Add digital sensor (To add sensors to the project)
As there can be more then one digital sensors in the system, each sensor should be connected to a different port of the controller and connect (wired) accordingly in the hardware. These connections should also be specified in the software while adding the sensors in the project.
Once the particular components are added to the project they are visible in the cell definition in the software window. The current position of all the cell components is also visible on the computer screen as current status of the components is read by the software online. Thus the software indicates the present position of the Serpent 1 Robot with respect to all the axes. It also indicates the status of the digital sensors as “Digital Sensor 1 Off” and so on.
The cue card contains all the commands that are required to program the FMS cell. The project contains a MAIN program that’s specifies all the commands to the FMS cell operation. In addition to the main program the user may define addition sub-programs (subroutine) for separate functions. Defining a sub-routine helps in the parallel execution of more then one component of the cell by executing the main program and the sub-routine in parallel.
Auxiliary Conveyor belt to Dispense parts to the conveyor system:
This conveyor is used for the intake of the part onto the conveyor system. This conveyor is referred as auxiliary conveyor as its main function is to provide support to the main conveyor system to make the process completely autonomous and to maintain a minimum distance between two parts on the main conveyor system. Auxiliary conveyor does not have any electrical connection to the work cell. It is a complete separate entity, driven through separate power supply. It derives the signal for its operation from a limit switch that is connected to the width gauge. Thus when the width gauge measures the width of a part, it pusses this limit switch. As the switch is connected to the motor of the auxiliary conveyor, the auxiliary conveyor launches a new part on the conveyor system. This allows a minimum distance to be maintained between two parts on the conveyor system, the distance being the difference between the width gauge and the place where auxiliary conveyor connects to the conveyor system.
Program Code:
MAIN Program:
LINE COMMAND
CALIBERATE DIGITAL WIDTH GAUGE 1
START START CONVEYOR 1
WAIT UNTIL DIGITAL SENSOR 1 IS OFF
HALT CONVEYOR 1
MOVE CONVEYOR 90 MM
HALT CONVEYOR 1
GAUGE WIDTH 1
START CONVEYOR 1
WAIT UNTIL DIGITAL SENSOR 2 IS OFF
HALT CONVEYOR 1
2 IF GAUGE WIDTH 1 IS BETWEEN 0 AND 22.0 MM THEN GOTO 4
3 IF GAUGE WIDTH 1 IS BETWEEN 23.0 AND 29.0 MM THEN GOTO 5
MOVE SERPENT 1 EC THROUGH POSITION 1 TO 3
START PARALLEL SUB ROUTINE CONV
MOVE SERPENT 1 EC THROUGH POSITION 4 TO 7
WAIT UNTIL DIGITAL SENSOR 2 IS OFF
GOTO 2
4 MOVE CONVEYOR 1 90 MM
GOTO START
5 MOVE SERPENT 1 EC THROUGH POSITION 1 TO 3
START PARALLEL SUB ROUTINE CONV
MOVE SERPENT 1 EC THROUGH POSITION 8 TO 12
WAIT UNTIL DIGITAL SENSOR 2 IS OFF
GOTO 2
SUBROUTINE PROGRAM
SUBROUTINE CONV
LINE COMMAND
START WAIT UNTIL DIGTAL SENSOR 2 IS on
START CONVEYOR 1
WAIT UNTIL DIGITAL SENSOR 1 IS OFF
HALT CONVEYOR 1 MOVE CONVEYOR 1 90 MM
HALT CONVEYOR 1
GAUGE WIDTH 1
START CONVEYOR 1
WAIT UNTIL DIGITAL SENSOR 2 IS OFF
HALT CONVEYOR 1
END PARALLEL SUBROUTINE 1
Applications:
The system can be used in the following industrial processes:
1. Separate raw material at the beginning of the process and feed the materials to different lines based on their width.
2. At quality control station to separate parts and define them into different grades, and reject parts that do not pass the quality control.
Advantages:
The designed system has the following advantages:
1. Parallel Operation: The system consists of two steps. The first is to measure the width of the parts which is achieved using the conveyor system and sensors. The second is to place these parts into one of the storage bins based on the measured width which is achieved using the serpent 1 robot. To make the system efficient and speed up the process, these to steps are carried out in parallel. So when the robot picks up the part from the end of the conveyor and proceeds to place it into the storage bin, the conveyor system advances and measures the width of the next part I the queue. This is achieved using the parallel execution of the main program and the subroutine for the conveyor system.
2. Feedback System: The system is a closed loop feedback system that indicates the position of the part at different stages. Thus using digital sensor 1, the presence of the part is detected at position before the width gauge and hence appropriate signal is sent to the width gauge as to when the width is to be measured. The digital sensor 2 indicates the presence of the part at the end position of the conveyor which is also the pick up point for the robot. Thus the robot does not execute the program to pick up until the part of until this sensor gives the output. Neither does the conveyor advances unless the part is picked up by the robot. This is important because if the part is not picked up and the conveyor advances, the part may be thrown off the conveyor and may cause destruction in the working area.
