Transporting Workpieces around FMS | Industries | Engineering

There are various ways of transporting workpieces around FMS in the industries. The ways are: 1. Material Handling Systems 2. Automated Guided Vehicle System (AGVS) 3. Automatic Monorail Systems (AMS) 4. Magazines and Conveyoring Systems 5. Monitoring Systems 6. Hybrid (Symbolic-Numeric) Expert System. 

Way # 1. Material Handling Systems:

Material handling systems (MHS) have an important role in the operation of flexible manufacturing systems. MHS integrates the subsystems and the connected systems of the FMS. Physically the FMS and the storage systems are integrated with a MHS. The information system integrates FMS, MHS and CAD on an information level.

The analysis and the design of the material handling of a flexible manufacturing system is a complex job. MHS requirements include the transportation of part, raw material, final product, fixtures, tools, pallets and auxiliary materials. The transportation has to be solved not only between manufacturing cells but between the FMS and the warehouse too.

The material handling components of MHS are automated guided vehicle (AGVS), stacker cranes, conveyors, etc.

Material handling systems can be divided into two groups:

(i) Serial access transport systems (e.g. Conveyor System).

(ii) Direct or random access transport systems (e.g. AGVS).

The random access transport systems being more flexible, can simplify dynamic scheduling and flexible control of the system.

A computer controlled MHS offers a wide range of solutions for the FMS. An inadequately designed MHS can significantly limit an otherwise highly flexible manufacturing environment.

Simulation in MHS Design:

This provides the basic technique for planning MHS. Simulation is a symbolic representation on a computer for evaluating stochastic problems. Simulation is a very effective technique for dynamic analysis of FMS, but it does not have optimising ability.

The main purposes of simulation are:

(i) To ensure that there are no fundamental weaknesses in the design.

(ii) To identify the bottlenecks in the system.

(iii) To examine scheduling and sequencing procedures (alternatives)

(iv) Various types of design analysis tools are.

(v) Queuing network models (useful in initial stages of the project).

(vi) Network or graphical input simulation methods

(vii) Generalised simulators

(viii) Full scale simulation models (emulation models)

It is generally observed that users have limited expertise to make the simulation software really effective.

A good simulation program must satisfy following requirements:

(i) Enable FMS models to be easily constructed and changed

(ii) Be user friendly (interactive, with animation, etc.)

(iii) Be modular

(iv) Accurately reflect the system and its constraints.

(v) Provide a variety of performance measures and statistical data output.

Artificial Intelligence (AI) Tools for MHS Planning:

AI (the subfield of computer science concerned with designing of intelligent computer systems—that is, systems that exhibit the characteristics associated with intelligence in human behaviour—understanding language, learning, reasoning, solving problems, etc.) uses symbolic processing, knowledge base and special engineering techniques. Describing complex relations among various symbolic objects is easier using AI techniques.

Expert (or knowledge-based) systems are high—performance programs that deal in specialised areas of different problems (knowledge domains) and use knowledge and inference procedures to solve them.

Expert systems can be thought of as a model of the expertise of the field’s actual best human experts. Expert systems can be applied very usefully in solving practical problems in the manufacturing environments including MHS.

Coupling AI and Simulation Techniques

Although the techniques of AI and simulation are different, still they have a similarity that both are dealing with models. Simulation models are generally description and need human help for the solution of the task. AI models are constructive ones and can solve real problems automatically.

These two models can be combined advantageously by embedding, in parallel, in cooperation, and as intelligent front-end.

Way # 2. Automated Guided Vehicle System (AGVS):

The conventional conveyor system many times are inadequate to satisfy the requirements of plants where production is carried out through interconnected work cells and where flexibility and rapid change-over times are of primary importance.

AGVS offer a viable solution for such needs. AGVS possess intrinsic flexibility, and capability of asynchronous operations and easiness of integration with other automatic devices like robots, CNC, automatic storage/ retrieval systems.

These systems find applications for:

(i) Distribution,

(ii) Assembly, and

(iii) Manufacturing.

As regards distribution, AGVS are used to transport materials between storage areas and production lines and vice versa. These also substitute the manned vehicles and thus offer advantages like better rationalisation of the loading/unloading operation through the control system. These are also adaptable to hostile environments and provide more efficient floor area utilisation.

For assembly purpose, AGVS are provided with on board platform and these unify the two functions of transport and working place. These offer the advantages of capability to organise jobs in parallel instead of in-line, to decouple each station cycle time from the plant’s production rate, and to easily mix automatic and manual workstation with different work organisation.

For production (in FMS), AGVS are used for transportation of parts and devices. These are able to meet, with their intrinsic flexibility, the many different needs arising from the overall process, to accomplish the simultaneous machining of different parts following different routes.

The vehicles and control systems have different features and characteristics depending on the type of application and the jobs to be done. However the basic building blocks (hardware and software) remain similar in order to achieve a high degree of standardisation in the overall architecture. The carriers of AGVS are different in mechanical strength. On board actuators may be roller beds, telescopic forks, lifting table, etc., to load/unload components.

The microprocessor based control system and its associated main components like antennas, communication devices, operator panel, etc. are more or less same.

A large number of AGVS may be used in a factory. These move and cover entire area of the plant under the control of computer.

A battery charging and maintenance area is provided in the plant. The AGVS are automatically sent to this zone when the battery charge level is below some warning limit. In some system, batteries are recharged along the circuit, at each waiting station or accumulation point (opportunity charging).

The overall control system comprises of several PLCs to control vehicle traffic and to provide the required exchange of signals between the AGVs and the operating machines. A host computer is used for the overall system management, mission execution and warehouse load optimisation cycles.

Types of loads for which AGVs are suited:

Automated Guided vehicles (AGVs) are designed for automatic, efficient and flexible execution of transport tasks in which a variety of loads (workpieces, fixtures, empty pallets, tool dispensers) have to be transported at irregular intervals and with varying frequencies to and from a fairly large number of locations (input/output stations, set-up areas, machining centres and auxiliary machines, wait stations and storage areas).

Small workpieces with machining times of only a few minutes require a more or less continuous form of transport like conveyors. But AGV would be suitable for small workpieces which are set-up and machined in groups (several parts per pallet). Large workpieces (4 tons or more) involving machining time of hours can be best transported by forklift truck. AGVs are suited for majority of workpieces with weights ranging from 500 to 3000 kg and machining times between 15 minutes to 2 hours.

Requirements of AGVs:

As far as possible a single type of AGV should be used in a plant and it should be capable of transporting all types of loads like machine pallets, wooden pallets, tool dispenses, etc.

Further AGVs should be capable of both vertical and horizontal transfer of load, and the load transfer should be automatic. For vertical transfer (as for machine pallets and wooden pallets), the AGVs are equipped with lifting platforms. At machining centres the workpieces mounted on machine pallets are transferred horizontally between the AGV and automatic pallet changer. AGVs may be either side- loading (from left or right depending on installation layout) or rear loading. Typical transfer mechanisms are chain retractor, hydraulic piston arm and worm wheel conveyor.

Special positioning equipment with high positioning accuracy is necessary where very careful and precise handing is required.

Control and Coordination:

The activity of AGVs are controlled and coordinated by a central computer. The AGV transport tasks are handled by a dedicated micro-computer which is integrated into the overall FMS control hierarchy by means of an on-line link. The transport computer has to handle problems like: management of transport commands generated by the host computer/operator, identification and tracking of individual loads throughout the installation; AGV scheduling, i.e. dynamic assignment of transport tasks to free AGVs; AGV path finding, route optimisation and traffic regulation (avoidance of collision, etc.).

Role of AGVs in Extending Flexibility of FMS Systems:

AGVs increase flexibility of FMS in regard to:

(а) Plant Integration:

AGVs provide the integrating links between all production facilities like machining centres, deburring and other special machines, washing plants, tool changing areas, set-up stations, buffer stations and storage areas.

(b) Layout Arrangement:

Transport routes can be laid out to accommodate any arrangement of production facilities.

(c) Process Sequences:

AGVs can transport workpieces through variable sequences of machining and other operations. Machines out of order can be automatically bypassed.

(d) Transport Capacity:

AGVs can be dynamically scheduled to meet the current transport loads in different areas.

(e) Working Time:

AGVs can support unmanned shifts thereby permitting round the clock operation.

(f) Load Handling:

AGVS can handle a variety of loads like machine pallets, wooden pallets, tool dispensers, etc.

(g) Accessibility:

AGVs do not require fixed installation and can share transport routes with forklift trucks etc., and keep machines freely accessible.

(h) Breakdowns:

By having a redundant AGV, breakdown is immaterial.

(i) System Extension:

Layout can be easily extended to link additional machines, etc.

Way # 3. Automatic Monorail Systems (AMS):

The automated monorail is proving more and more an excellent material handling solution whenever fast point to point parts supply is needed without occupation of space on the floor. As compared with other overhead systems the supporting structures are inobtrusive, light and adaptable even to low and simple buildings.

Way # 4. Magazines and Conveyoring Systems:

Either conveyor belts or magazines could be used for handling blanks and finished parts.

Various systems in common use are:

(i) Pallet Conveyor:

Specially adapted pallets which can hold one or more parts are used. Such a system can be used for different types of chuck parts and shaft parts. The pallet conveyor can be used as a part magazine for a single machine or for interlinking machines with an intermediate buffer of parts.

(ii) Steel Band Conveyor:

It is used for chuck parts, blanks in the form of blocks cut from bar stock. This system enables easy change overs between different dimensions of blanks, and pallets are not necessary since the part is transported on the steel band itself. However its use is limited for the parts which can stand in a stable way on their end surfaces.

(iii) Feed-Table:

Like the pallet conveyor it can be used for different types of parts. It can be used as a part magazine for a single machine and an intermediate conveyor enables several machines to be interlinked.

The feed-table can be served by an auto-guided vehicle. This type of transport system can be used to join together several machine tools and other machine groups to create a more extensive FMS installation.

Way # 5. Monitoring Systems:

Various means usually incorporated are:

(i) Correct Clamping:

This check is necessary to ensure that the workpiece is correctly set up in the chuck. In this respect the positions of the chuck jaws and quill are checked. Cleaning of chuck jaws and correct pushing of components against chuck jaws deserve attention.

(ii) Measurement Control:

A variety of electronic measuring devices are available for checking the finished part. Measured values are transmitted to the control equipment and the computer will continuously correct the tool setting so that specified tolerances are produced.

(iii) Tool Tip Measuring:

Automatic tool tip measuring is used for:

(a) Forwarding information to the control equipment concerning the tool setting length (useful when using spare tools),

(b) Checking the status of cutting edge, (degree of wear, and if it is intact),

(c) Indirectly measuring the workpiece.

(iv) Programmable Wear Time of the Tools:

Computer keeps a check on the tool engagement time and takes action when the maximum wear time has elapsed. The operation is not stopped in between for this reason.

(v) Cutting Force Monitoring:

Tool breakage can be discovered by measuring the feed force.

(vi) Collision Free Zones for the Computerised Part Changer:

The objective of these zones is to prevent, during manual runs, the shuttle carriage from colliding with any extra equipment which has been mounted and which is located within the travel range of the carriage.

Way # 6. Hybrid (Symbolic-Numeric) Expert System:

Hybrid knowledge based systems for analysing material flow and planning the MHS of an FMS are available.

The purposes of such a system are:

(i) Analysis of material flow.

(ii) Selection of type of MHS.

(iii) Planning the cell layout.

(iv) Selection of the type of transportation equipment.

(v) Route planning of the transportation equipment system.

(vi) Definition of the number of transportation equipment, pallets, stations, etc.

(vii) Definition of the capacity of in-process storage and buffers.

Such a system handles following two types of data:

(i) Permanent data stored in its data-or knowledge base.

a. Technical and geometric parameters of machine tools, transportation equipment.

b. Standard layout variations,

c. The parameters of pallets, fixture, tools.

d. Decision rules for selecting material handling system, transportation equipment.

e. Optimisation rules, strategies.

f. Control strategies, etc.

(ii) Input data for the actual problem.

a. Production requirements for each part-family.

b. Process plans.

c. Physical parameters of each part-type.

d. Manufacturing cell configurations.

e. Workshop building geometry.

f. Cost data, etc.

This system can make decisions based on rules, heuristics or simulation results, and can control the simulation module, too.

The planning process consists of following five phases:

(i) Layout planning.

(ii) Routine problem solution.

(iii) Global planning.

(iv) System simulation.

(v) Evaluation, documentation.