DESIGN AND DEVELOPMENT OF A PROGRAMMABLE LOGIC CONTROLLER BASED INDUSTRIAL ROBOT FOR SORTING ITEM BOXES.
ABSTRACT: This paper presents the design and experimental implementation of a robotic system that can sort various item- boxes in terms of sizes. The sorted item boxes are dispatched to dedicated containers according to the sizes. This paper describes the physical construction of the robotic system, the hurdles which may be faced while constructing, and possible solution for such constraints. A sorting algorithm is also formulated and is tested by embedding it in a programmable logic controller (PLC). A number of sensors and actuators are deployed that interact with the programmable logic controller to successfully operate the system. Results show that the constructed robotic system can work for long hours, provide satisfactory sorting which results in rapid and safe operation.
Keywords: Conveyor belt robotic system, industrial control, parts sorting, PLC control
Robotic systems have become an essential work-horse for modern industrial setups as these tend to reduce cost of the operation and production time while maintaining a high degree of precision handling [1-2]. A variety of robotic systems have been invented that perform different tasks.
Some robotic systems are designed to perform one specific task, while some robotic systems are designed to handle multiple tasks. A robotic system always has a controller that controls the tasks to be performed .
Conveyor belts find a large number of applications in the industry. These applications include material transportation, material handling, material sorting, and material categorizing (good and bad items). In addition, temperature processing of products can also be done on conveyor belts system e.g., baking, drying, re-shaping, etc. Modern conveyor belts systems provide rapid packing of goods that are done while the product is being transported on conveyor belt. In addition, production line assembly, and quality control checks are also done on conveyor belt to save time and space [3-5].
In this work, a robotic system is proposed that sorts and transports item-boxes on a conveyor belt system. Such robotic systems find many applications in the industry including motion control , parcel handling by postal couriers, dispatching of goods manufactured in a factory, etc. Scope of Work
The scope of work is to construct a robotic system that sorts item-boxes with respect to three different sizes. Figure 1 presents the task of the robotic system to be developed, where three different size item-boxes are shown; each item-box is to be placed in separate containers according to the size. Item- boxes of three sizes small, medium, and large are to be sorted in three individual containers U, V, and W, respectively. Table 1 presents the dimensions of each item-box. Additional requirement includes a fool-proof system that provides rapid sorting with less wear-and-tear to the robotic system itself, and minimum damage to the item-boxes.
Proposed Robotic System Setup
Figure 2 presents a 3-dimensional diagram of the proposed robotic system. The main parts of the robotic system include two conveyer belts, i.e., conveyer belt-A and conveyer belt- B, and two motors motor-A and motor-B, that enable conveyer belts to operate. Conveyer belt-A with the help of Motor-A, will operate only in forward direction. However, conveyer belt-B with the help of motor-B can operate in both forward and reverse direction. Each conveyer belt is equipped with a number of rollers that keep the item-boxes stable and upright, while being transported. The rollers are installed in between an individual conveyer belts and are free to rotate in either direction.
The proposed robotic system is equipped with a variety of automation components, which include input sensors from X1 to X5, and output signals from Y1 to Y3. The input sensors are used to detect either the size of the item-boxes or the location of the item-boxes while being sorted in the proposed robotic system. Different types of input sensors, from X1 to X5, have been used according to different requirements at various stages of the proposed robotic system. The input sensors include infra- red (IR) sensors, opto-coupler sensors, and limit switches. Each sensor is discussed in detail in a later section. The signals from the input/output sensors or actuators communicate with the programmable logic controller (PLC) [7-8].
The PLC is the main controller that executes a programed algorithm to control system. The PLC consists of an arithmetic logic unit (ALU), RAM and ROM memory units, a number of input and output interfaces. To control the robotic system an algorithm is required. This algorithm takes the form of a software program, and is usually written in the PLC memory unit. A variety of PLCs are available which differ in terms of execution-speed, number of input and output interfaces, and memory size. The PLC used in this robotic system is the Allen Bradley(r) Micro Logix 1500 PLC which is equipped with 12 digital inputs and 12 digital relay based outputs. The PLC is programmed by RS logix 500 and communicates using the industry standard RS-232 protocol. The outputs of the PLC are control signals Y1, Y2, Y3, Y4, and Y5. The output control signal Y1 starts or stops the motor-A.
When the control signal from Y1 is logic one, the motor-A runs in forward direction, and when it is logic zero the motor-A stops. The output signal Y2 controls the forward and backward movement of a pusher. The pusher is a mechanical device that is used in sorting medium and large item-boxes. The pusher pushes the items-boxes from conveyer belt-A to conveyer belt-B and returns to its original position. The output signals Y3 controls the horizontal and vertical movement (HVM) arm , and is used when small size item-boxes are to be sorted. The HVM arm has grips that takes hold of an item box, rotates its body and drops the item box in a container for small item boxes. Details of the HVM- arm will be presented in following sections.
The output signal Y3 and Y4 control the start/stop and forward/reverse direction of motor-B, respectively. When output signal Y4 is logic one the motor-B will start, and when it is logic zero the motor will stop. Likewise, when output signal Y5 is logic one the motor-B will run in forward direction and when it is logic zero the motor-B will run in reverse direction.
Robotic Automation Procedure (Control Algorithm) Reference to figure 2, an item-box to be sorted is fed at point A. An infrared sensor X1 located at point A detects the arrival of an unsorted item box and communicates the information to the PLC. The PLC upon receiving the information of an unsorted item-box entrance, commands the motor-A to operate with the help of control signal Y1. Since conveyor belt-A is coupled to motor-A, therefore as motor-A operates the conveyor runs forward and brings the unsorted item-box to point B. At point B, an array of opto-coupler sensors, collectively labeled as X2, are located. At position B, the height of the item-box will be evaluated by input sensors X2. The height information of the item-box will be provided to the PLC.
If the item-box is of small size then the PLC will allow the item-box to travel from point B to D. The arrival of the tem- box at point D will be detected by an infrared sensor X3. At point D, the PLC control signal Y5 will activate the HVM arm to close its grips by doing so the HVM arm takes hold of the item-box. Following, the PLC signal Y6 commands the HVM-arm to rotate its body to the position itself over the location of container-U. When the HVM-arm is over container-U, the PLC again with the help of control signal Y5 will control HVM-arm to open its grips so that the small item-box falls in container-U. Finally, the PLC with the help of control signal Y6 commands the HVM-arm to return to point D.
At position B, if the item-box is medium or large size, then the PLC will transfer the item-box from conveyor belt-A to belt-B when the item-box reaches point C. This transfer of item boxes will be done by the pusher. PLC control signal Y2 will enable the pusher. However, before the pusher arm, an infrared sensor (not shown in fig.1) is located. This infrared sensor senses the position of an item-box is present in front of the pusher, so that when the item-box is brought in front of the pusher i.e., at point C, at that time the pusher can operate and transfer the (only medium or large) item-box from conveyor belt-A to belt-B. The item-boxes are transferred from conveyor belt-A to belt-B i.e., from point C to E.
At point E, if the item-box is medium-size, then the PLC with the help of control signals Y4 and Y5 will operate the conveyor belt-B (using motor-B) in forward direction so that the medium-size item-box is brought to point F. At point F an infrared red sensor X4 senses the arrival of item-box and provides the information to the PLC. However, the conveyer belt-B is kept running so that the medium-size item-box falls into container-V.
In the case a large-size item-box is present at point E, then the PLC with the help of control signals Y4 and Y5 operates the conveyor belt-B (using motor-B) in reverse direction so that the large-size item-box is brought to point G. At point G an infrared red sensor X5 senses the arrival of item-box and provides the information to the PLC. However, the conveyer belt-B is kept running (in reverse direction) so that the large- size item-box falls into container-W.
Figure 3 presents in detail the control flow algorithm of the proposed robotic system. A software program based on the control flow algorithm has been embedded and executed in the PLC.
Hardware Implementation of the Proposed Robotic System
This section presents the hardware implementation of the proposed robotic system. The development and application of various components, devices, input sensors, output actuators, integrated circuits (ICs), are also presented.
A. The Robotic System Components
As mentioned above, the robotic system consists of two belts each operated by independent DC motors. A pusher to transport the item-boxes from conveyor belt-A to belt-B, is operated with the help of relays and pneumatic pistons. Conveyor belt-A is located at a higher height as compared to conveyor belt-B. The conveyor belt-A is linked to conveyor belt-B by a sliding channel. Any item-box present on the sliding channel will be transferred from belt-A to belt-B. By this way, the pusher pushes the medium or large item-boxed onto the sliding channel, where the item-boxes are transferred from conveyor belt-A to conveyor belt-B. The small size item-boxes are sorted with the HVM-arm. The HVM-arm has a gripper, vertical pneumatic piston, and a DC motor. The gripper grips the item-boxes with the help of pneumatic pistons, whereas the DC motor rotates the whole body of the HVM-arm back and forth.
B. Input components
Two types of sensors are used in the project that are infrared sensors and limit Switches. Infrared sensors are electronic- sensors that detect infrared light radiating from an infrared source. Hence, infrared sensors are used in a pair, i.e., the transmitter that transmit the infrared light and the receiver that detects the infrared light. Infrared sensors have been employed due to their low cost, rapid response, and ease of installation. The input signal from the infrared sensor is compared to a reference signal using a LM324 comparator. The comparator provides a 5V signal when an item is present between transmitter and receiver, and if there is no item between the transmitter and receiver the comparator provides a 0V signal. The signal from the comparator is communicated to the PLC through an opto-coupler interface. Limit Switches are electronic switches that detect the arrival or dispatch of an object from a location. The limit switches have been used in the HVM-arm to detect the location of the arm.
Table 1 Dimensions of Item-Boxes
C. Output components
The PLC controls the indication lights, conveyor belt motors, pusher, and the HVM-arm, with the help of output-interfaces including relays and solenoid valves. Relays are used to operate the indication lights, motor, and the DC motor of the HVM-arm. The solenoid valves are used in combination with relays to operate the pusher and HVM-arm. When solenoid valve is energized it operates a pneumatic piston connected to its output. These pneumatic pistons are used to control the pusher and HVM-arm gripper opening and closing.
D. Interfacing with the PLC
The PLC controls the robotic system according to the embedded software. The input signals to the PLC are isolated from the robotic conveyor system using opto-couplers. The isolation is done to protect the PLC and to shift the voltage level required by the PLC. For this purpose, opto-coupler IC PC-817 is used, that provides isolation and converts 5V from the comparator LM324 to 24 volts required by the PLC. The output of the PLC controls the indicator LEDs, Motors for conveyor belts, pusher, and the HVM-arm. However, motors for conveyor belts are operated at 12V DC while the motor for HVM-arm operates at 5V DC. The PLC cannot provide enough power to run the motors; therefore the PLC is interfaced to motors with the help of relays. A combination of relays is used for the operation of motors conveyor belts to move in forward or reverse direction, the pusher, and HVM- arm.
Driving and controlling the DC motors
Two DC motors are used in the robotic system for moving the conveyor belt-A and -B. The length of conveyor belt-A is longer as compared to the length of conveyor belt-B, therefore, motor-A has to drive a larger load. Due to this reason motor-A is selected to carry a larger load. The large starting current of motor-A is provided by a 7812 dc voltage regulator with a current booster BJT transistor MJ2955. For the HVM-arm, a simple DC motor is used instead of a stepper motor as a stepper motor require more current than a DC gear motor . However, with the DC motor, two limit switches are used to limit the range of angle of rotation.
This experimental robotic system provides an efficient, fast and reliable way to sort random incoming of item-boxes by a dual conveyer belt system, flexible. The proposed robotic system takes approximately 3.5s to complete a sorting process. The robotic structure is retune-able for variations in the sizes of item boxes. In addition, the infrared sensors are adjustable to desired height. Error of sorting boxes to the wrong container is almost negligible.
The threshold voltage for the detection of any part is adjustable by just varying the value of the variable resistor. The angle of rotation of the HVM arm is not fixed and can be varied by changing the position of initial and maximum point limit switches.
As any item box advances from one stage to another, the current state of the robotic system (location of box) is stored and the pervious state is over-written in the PLC memory. As a result, in the case of power loss or interruption, the system retains the current state in its memory and on resuming the power the system will resume its operation.
A few limitations of the proposed robotic system are presented as follows;
i. Item shapes have to be square since sphere shaped items may roll unwantedly on the conveyor belt. In addition, sphere shaped items may not be gripped by the HVM arm.
ii. The robotic system can sort items on further division of size, however, in such case volume and space required for the robotic system will increase.
iii. Depending on the size of motors item boxes of not more than 5kg in weight may be sorted.
iv. A limited number of input and output sensors can be connected to the PLC. The quantity of sensors depends on the capacity of the PLC. However, more than one PLCs can be arranged to create a PLC network, and multitasking may be introduced for detail controlling.
This research paper shows a technique to sort and classify item-boxes according to the size with the help of a robotic system. The proposed robotic system has successfully shown that sorting of item boxes can be achieved in a small period of time (i.e., approximately 3.5s). The main parts of robotic system include two motors, two conveyor belts, a HVM arm, input infrared sensors, and a PLC. The location of sensors on the conveyer belt can be re-adjusted and the robotic system can be made compatible with the changes of item boxes. By applying additional sensors and conveyor belts, the robotic system can be enhanced to sort item boxes on the bases for even more different box sizes.
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|Date:||Aug 31, 2016|
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