LECTURE № 2

MECHATRONICS: PRODUCTS AND SYSTEMS IN MANUFACTURING

2.1. COMPUTER NUMERICAL CONTROL (CNC) MACHINES

CNC machine is the best and basic example of application of Mechatronics in manufacturing automation. Efficient operation of conventional machine tools such as Lathes, milling machines, drilling machine is dependent on operator skill and training. Also a lot of time is consumed in workpart setting, tool setting and controlling the process parameters viz. feed, speed, depth of cut. Thus conventional machining is slow and expensive to meet the challenges of frequently changing product/part shape and size.

Computer numerical control (CNC) machines are now widely used in small to large scale industries. CNC machine tools are integral part of Computer Aided Manufacturing (CAM) or Computer Integrated Manufacturing (CIM) system. CNC means operating a machine tool by a series of coded instructions consisting of numbers, letters of the alphabets, and symbols which the machine control unit (MCU) can understand. These instructions are converted into electrical pulses of current which the machine’s motors and controls follow to carry out machining operations on a workpiece. Numbers, letters, and symbols are the coded instructions which refer to specific distances, positions, functions or motions which the machine tool can understand.

Fig. 2.1.1. Comparison between a conventional machine tool and

a CNC machine tool

CNC automatically guides the axial movements of machine tools with the help of computers. The auxiliary operations such as coolant on-off, tool change, door open-close are automated with the help of micro-controllers. Figure 1.2.1 shows the fundamental differences between a conventional and a CNC machine tool. Manual operation of table and spindle movements is automated by using a CNC controllers and servo motors. The spindle speed and work feed can precisely be controlled and maintained at programmed level by the controller. The controller has self diagnostics facility which regularly alarms the operator in case of any safety norm violation viz. door open during machining, tool wear/breakage etc. Modern machine tools are now equipped with friction-less drives such as re-circulating ball screw drives, Linear motors etc. The detail study of various elements of such a Mechatronics based system is the primary aim of this course and these are described at length in the next modules.

2.2. TOOL MONITORING SYSTEMS

Uninterrupted machining is one of the challenges in front manufacturers to meet the production goals and customer satisfaction in terms of product quality. Tool wear is a critical factor which affects the productivity of a machining operation. Complete automation of a machining process realizes when there is a successful prediction of tool (wear) state during the course of machining operation. Mechatronics based cutting tool-wear condition monitoring system is an integral part of automated tool rooms and unmanned factories. These systems predict the tool wear and give alarms to the system operator to prevent any damage to the machine tool and workpiece. Therefore it is essential to know how the mechatronics is helping in monitoring the tool wear. Tool wear can be observed in avariety of ways. These can be classified in two groups (Table 2.2.1).

Table 2.2.1

Tool monitoring systems

Direct methods	Indirect methods
Electrical resistance	Torque and power
Optical measurements	Temperature
Machining hours	Vibration and acoustic emission
Contact sensing	Cutting forces and strain measurements

Direct methods deal with the application of various sensing and measurement instruments such as micro-scope, machine/camera vision; radioactive techniques to measure the tool wear. The used or worn-out cutting tools will be taken to the metrology or inspection section of the tool room or shop floor where they will be examined by using one of direct methods. However, these methods can easily be applied in practice when the cutting tool is not in contact with the work piece. Therefore they are called as offline tool monitoring system. Figure 2.2.1 shows a schematic of tool edge grinding or replacement scheme based on the measurement carried out using offline tool monitoring system. Offline methods are time consuming and difficult to employ during the course of an actual machining operation at the shop floor.

Indirect methods predict the condition of the cutting tool by analyzing the relationship between cutting conditions and response of machining process as a measurable quantity through sensor signals output such as force, acoustic emission, vibration, or current.

Fig. 2.2.1. Off-line and on-line tool monitoring system for tool edge grinding

Figure 2.2.1 shows a typical example of an on-line tool monitoring system. It employs the cutting forces recoded during the real-time cutting operation to predict the tool-wear. The cutting forces can be sensed by using either piezo-electric or strain gaugebased force transducer. A micro-processor based control system continuously monitors «conditioned» signals received from the Data Acquisition System (DAS). It is generally programmed/trained with the past recorded empirical data for a wide range of process conditions for a variety of materials. Artificial Intelligence (AI) tools such as Artificial Neural Network (ANN), Genetic Algorithm (GA) are used to train the microprocessor based system on a regular basis. Based on this training the control system takes the decision to change the tool or gives an alarm to the operator. Various steps followed in On-line approach to measure the tool wear and to take the appropriate action are shown in Figure 2.2.3.

A lot of academic as well as industrial research has been carried out on numerical and experimental studies of design, development and analysis of «Tool Condition Monitoring Systems». Readers are suggested to browse various international journals such as International Journal of Advanced Manufacturing Technology (Springer), International Journal of Machine Tool and Manufacture; International Journal of Materials Processing Technology (Elsevier), etc. to learn more about these techniques.

Fig. 2.2.3. Steps followed in an indirect tool monitoring system

Nowadays customers are demanding a wide variety of products. To satisfy this demand, the manufacturers «production» concept has moved away from «mass» to small «batch» type of production. Batch production offers more flexibility in product manufacturing. To cater this need, Flexible Manufacturing Systems (FMS) have been evolved.

As per Rao, P.N. [3], FMS combines microelectronics and mechanical engineering to bring the economies of the scale to batch work. A central online computer controls the machine tools, other work stations, and the transfer of components and tooling.

The computer also provides monitoring and information control. This combination of flexibility and overall control makes possible the production of a wide range of products in small numbers. FMS is a manufacturing cell or system consisting of one or more CNC machines, connected by automated material handling system, pick-and-place robots and all operated under the control of a central computer. It also has auxiliary sub-systems like component load/unload station, automatic tool handling system, tool pre-setter, component measuring station, wash station etc. Figure 2.2.4 shows a typical arrangement of FMS system and its constituents. Each of these will have further elements depending upon the requirement as given below.

A. Workstations

· CNC machine tools

· Assembly equipment

· Measuring Equipment

· Washing stations

B. Material handing Equipment

· Load unload stations (Palletizing)

· Robotics

· Automated Guided Vehicles (AGVs)

· Automated Storage and retrieval Systems (AS/RS)

C. Tool systems

· Tool setting stations

· Tool transport systems

D. Control system

· Monitoring equipments

· Networks

It can be noticed that the FMS is shown with two machining centers viz. milling center and turning center. Besides it has the load/unload stations, AS/RS for part and raw material storage, and a wire guided AGV for transporting the parts between various elements of the FMS. This system is fully automatic means it has automatic tool changing (ATC) and automatic pallet changing (APC) facilities. The central computer controls the overall operation and coordination amongst the various constituents of the FMS system. Video attached herewith gives an overview of a FMS system.

Fig. 2.2.4. A FMS Setup

The characteristic features of an FMS system are as follows:

1. FMS solves the mid-variety and mid-volume production problems for which neither the high production rate transfer lines nor the highly flexible stand-alone CNC machines are suitable;

2. Several types of a defined mix can be processed simultaneously;

3. Tool change-over time is negligible;

4. Part handling from machine to machine is easier and faster due to employment of computer controlled material handling system.

Benefits of an FMS:

· Flexibility to change part variety

· Higher productivity

· Higher machine utilization

· Less rejections

· High product quality

· Reduced work-in-process and inventory

· Better control over production

· Just-in-time manufacturing

· Minimally manned operation

· Easier to expand

2.3. COMPUTER INTEGRATED MANUFACTURING (CIM)

In the last lecture, we have seen that a number of activities and operations viz. designing, analyzing, testing, manufacturing, packaging, quality control, etc. are involved in the life cycle of a product or a system (see Figure 1.1.4). Application of principles of automation to each of these activities enhances the productivity only at the individual level. These are termed as «islands of automation». Integrating all these islands of automation into a single system enhances the overall productivity. Such a system is called as «Computer Integrated Manufacturing (CIM)».

The Society of Manufacturing Engineers (SME) defined CIM as CIM is the integration of the total manufacturing enterprise through the use of integrated systems and data communications coupled with new managerial philosophies that improve organizational and personal efficiency.

CIM basically involves the integration of advanced technologies such as computer aided design (CAD), computer aided manufacturing (CAM), computer numerical control (CNC), robots, automated material handling systems, etc. Today CIM has moved a step ahead by including and integrating the business improvement activities such as customer satisfaction, total quality and continuous improvement. These activities are now managed by computers. Business and marketing teams continuously feed the customer feedback to the design and production teams by using the networking systems. Based on the customer requirements, design and manufacturing teams can immediately improve the existing product design or can develop an entirely new product. Thus, the use of computers and automation technologies made the manufacturing industry capable to provide rapid response to the changing needs of customers.

2.4. INDUSTRIAL ROBOTS

Industrial robots are general-purpose, re-programmable machines which respond to the sensory signals received from the system environment. Based on these signals, robots carry out programmed work or activity. They also take simple independent decisions and communicate/interact with the other machines and the central computer. Robots are widely employed in the following applications in manufacturing:

a) Parts handling: it involves various activities such as:

· Recognizing, sorting/separating the parts;

· Picking and placing parts at desired locations;

· Palletizing and de-palletizing;

· Loading and unloading of the parts on required machines.

b) Parts processing: this may involves many manufacturing operations such as:

· Routing;

· Drilling;

· Riveting;

· Arc welding;

· Grinding;

· Flame cutting;

· Deburring;

· Spray painting;

· Coating;

· Sand blasting;

· Dip coating;

· Gluing;

· Polishing;

· Heat treatment.

c) Product building: this involves development and building of various products such as:

· Electrical motors;

· Car bodies;

· Solenoids;

· Circuit boards and operations like;

· Bolting;

· Riveting;

· Spot welding;

· Seam welding;

· Inserting;

· Nailing;

· Fitting;

· Adhesive bonding;

· Inspection.

Further detail discussion on various aspects of industrial robots such as its configuration, building blocks, sensors, and languages has been carried out in the last module of this course.

2.5. AUTOMATIC QUALITY CONTROL AND INSPECTION SYSTEMS

Supply of a good quality product or a system to the market is the basic aim of the manufacturing industry. The product should satisfy the needs of the customers and it must be reliable. To achieve this important product-parameter during a short lead time is really a challenge to the manufacturing industry. This can be achieved by building up the «quality» right from the product design stage; and maintaining the standards during the «production stages» till the product-delivery to the market.

A number of sensors and systems have been developed that can monitor quality continuously with or without the assistance of the operator. These technologies include various sensors and data acquisition systems, machine vision systems, metrology instruments such as coordinate measuring machine (CMM), optical profilometers, digital calipers and screw gauges etc. Now days the quality control activities are being carried out right from the design stage of product development. Various physics based simulation software is used to predict the performance of the product or the system to be developed. In the manufacture of products such as spacecrafts or airplanes, all the components are being critically monitored by using the digital imaging systems throughout their development.

In the next module we will study the various sensors, signal conditioning devices and data conversion devices which are commonly used in mechatronics and manufacturing automation.