A-9. Chapter 2. Material flow. Chapter 2. Festo Didactic Mechatronics

Transcription

1 A-9 Material flow Festo Didactic Mechatronics

2 A General Definition of terms Material flow is the linking of all processes for the acquiring, processing, machining and distribution of material goods within defined areas. An important aspect of the definition of the term is its limitation to material goods, therefore excluding the transport of energy or of information. However, material goods are not restricted solely to materials forming part of the production process, i.e. raw material, semi-finished and finished products, but also other materials such as, for instance, waste, pallets and packaging. Roughly speaking differentiation is made in material flow between handling, conveying and transporting. Handling Conveying Continuous conveyor Handling refers to all motion sequences used for the starting or ending of production processes and also of transporting and storage. This includes, for instance, the insertion of a workpiece in a workpiece retainer or the stacking of workpieces at a storage place. Handling therefore includes all material flow processes taking place at a workstation. Conveying is the movement in horizontal or vertical direction via limited distances and is therefore generally restricted to in-plant processes. Examples are: The supply of screws by means of a vibratory bowl feeder and the transporting of vehicle bodies by means of overhead conveyors. These examples immediately highlight an important difference: in the first example, a continuous conveyor is used. Continuous conveyors operate continuously (at least over an extended time period). Mechatronics Festo Didactic

3 A-11 The second example involves an intermittent-flow conveyor. Each cabin of the overhead conveyor has its own timetable, to which it operates, with alternating travel operation, empty running and stops. Steady-flow conveyors are generally more economical to operate than intermittent-flow conveyors. Being of identical dead weight, these have greater conveyor capacity whilst requiring less drive power. This is partly due to the continuous operating mode, thereby eliminating the continuous starting and decelerating of the drive, handling equipment and material to be conveyed. On the other hand, intermittent-flow conveyors are frequently more flexible in application. As shown by the example, these are predominantly used for heavy individual loads. Conveyors often have yet a secondary function resulting from the dwelltime of the material being conveyed. For example, in the case of a refrigerated conveyor, parts cool down to a point where they reach the temperature required for further processing. Conveyors are also used as buffers in order to harmonise the working cycle of several processing stations. The term transporting describes the movement of goods across larger, generally horizontal distances. Transporting takes place on roads rail and more rarely on waterways. As such, transport mainly involves external, non-operational movement. Owing to its nature, transport is intermittent, since the use of vehicles is necessary for transporting. Differentiation is made between material to be conveyed in so far as this has a significant effect on the method of conveying or transport. Bulk material constitutes a load consisting of a large number of small items, e.g. screws, rivets, and also plastics granular material or sand. Bulk materials always require an enclosing container although, occasionally, it is possible to convey these in pipelines, similar to fluids. Fluid materials are generally transported in silo containers. However, to meet internal conveying requirements, pipelines are used. Packaged goods are unit loads which can be established according to the number of items e.g. machine parts. Bulk materials may also be treated as packaged goods, if these are packed in boxes or sacks. Intermittent-flow conveyor Transporting Material to be conveyed Bulk materials Fluid materials Packaged goods Festo Didactic Mechatronics

4 A-12 Material flow stage 1 Material flow stage 2 Material flow stage 3 Material flow stage 4 Qualitative assessment of materials flow Analysis of material flow The terms handling, conveying and transporting are contrasted by the grading into different stages of material flow. The first stage of material flow includes transport between the factory and its suppliers or customers. This stage of material flow involves locational planning, which does not form part of MPS training and is therefore not discussed here. The second stage of material flow includes movement within the factory site between the various sectors of the operation, e.g. factory buildings. Factory planning again takes into account material flow and evolves an appropriate building plan. Again, this stage of material flow will not be dealt with at this point. The third stage of material flow includes the movement between the individual departments of an operational area and, within the departments, the movements between the various workstations, machine groups and storage areas, etc. This stage can be dealt with as part of MPS. The fourth stage of material flow involves movement on the workstation itself. This stage deals primarily with handling equipment for the automation of material flow on the workstation. This represents a major aspect of MPS. In order to determine the optimum layout of equipment and the respective handling equipment involved, plus the possibly required storage and buffer stores, it is necessary to establish the material flow. The first step towards this involves the structure of the material flow. When designing a project, the following questions should be answered for every product: What equipment is connected with what other equipment? In what order is the equipment started? The answers to these questions provide a graph for each product as shown in the following example: Mechatronics Festo Didactic

5 A-13 Fig. 2.1: Graphic representation of material flow for one product In practice, a production facility will not just produce one product, but a multitude of products which, however, will run through the same equipment. In this case, the following representation arranged according to equipment is indicated: Fig. 2.2: Material flow sorted according to equipment for two products and one module. Left as per sequence, right taking into consideration the location of the equipment Festo Didactic Mechatronics

6 A-14 Quantitative assessment of material flow Direct recording of material flow Indirect recording of material flow ABC analysis Although a graphic representation of material flow gives some idea about the paths of material flow, it does not give any indication of the incidence of transport. Only when the number of goods to be conveyed within a specific unit of time is known, plus the required means conveyance and additional information about weight and dimensions, is it possible to optimise material flow. Direct recording of material flow takes place during the production process, in that employees keep a list at the individual stations. Because of the interruption of the normal production process as a result of this activity, direct recording of material flow should be avoided. Indirect recording of material flow is the result of the product spectrum of the production operation and the actual number of items over a representative period of time (e.g. a production week). The component parts and modules are determined on the basis of parts lists of each product and their overall number extrapolated within the time period considered. The schedules of job operations produce the structure of the materials flow and this then permits the numerical calculation of the material flow between the equipment. With indirect recording of material flow, care must be taken to ensure that information is not recorded directly in a period of seasonal high or low values. Particular care is also indicated during production of highly seasonal products, such as Christmas goods. In production operations which have a very large product spectrum, it will not be possible to record all products. A better method, other than statistical is to take into account particularly "important" products. This can be determined by means of ABC analysis, which is an economics procedure. Products are sorted according to a criteria, e.g. according to profit achieved. Products with the highest profits are listed on the left and those with the lowest on the right. This scale of product order is represented graphically, in that the profit of each article is added to the profit calculated thus far. If the same profit applies for each product, this will result in a straight line. However, in practice, the line is characteristically curved, which indicates that, for instance, with 20% of products. 80% of profit is already being achieved. This 20% of products represents the "important" ones and must be taken into account particularly when planning the material flow. Mechatronics Festo Didactic

7 A-15 Fig. 2.3: Selection of representative products by means of ABC analysis Once the extent of material flow has been established on all conveying distances, the structural representation can be entered, in that either the numbers are entered on the connection line or the lines drawn in corresponding width or number. For quantitative material flow, a matrix form is also frequently used. A material flow matrix is a square arrangement of cells. The equipment is entered on the lefthand side and upper edge. For the sake of simplicity, the numbers 1 to 6 are used in this example. The lines (legend left) mean the starting point, the columns (legend top), the destination points of the conveying distance. The incidence of transport is entered in the cells. We shall use the following example, which has already been used in the structural representation: Conveying distance of section A: pieces Conveying distance of section B: pieces Conveying distance of section C: pieces Matrix representation Festo Didactic Mechatronics

8 A-16 Fig. 2.4: Material flow matrix to from A: 100 B: 50 C:150 B: 50 C: 150 A: 100 C: 50 B: 50 B: 50 5 A: The conveying of 100 parts A from operational equipment 1 to operational equipment 3, for instance, is entered in the third column of the first line. Please note that the outward and return travel between two lots of operational equipment such as in section B, are to be entered in different halves of the matrix. Where identical parts are involved, the number of parts on an identical distance may be combined in order to establish the overall incidence. In the case of conveyors, capable of outward and return conveying, e.g. overhead conveyors, a differentiation between the outward and return distance is immaterial; since the greater of the two transport requirements are used. Mechatronics Festo Didactic

9 A-17 During the operational implementation, an ideal plan should first be drawn up, starting from the premises of minimum transport requirement and therefore minimal material flow costs. Taking into account the given parameters (such as already existing buildings, equipment, site conditions, etc.), this will form the basis for the real plan. Proceeding along the lines of the intuitive process first of all, a quantitative material flow plan is drawn up. In contrast with fig. 1.3, fig. 1.6 indicates the extent of material flow by means of the number of lines. The operational equipment is then arranged in such a way as that as few connections as possible cross and to keep connections with a large number of lines are as short as possible. Operational implementation Intuitive process Fig. 2.5: Optimisation of material flow according to intuitive process (left: initial situation, right: result). The equipment is then entered in the factory building plans taking into account their size and the space available. In the triangular method, the building plan is covered with a triangulation system. Each node is a possible location for equipment. The material flow matrix is used in triangular form (i.e. the total of outward and return travel is entered in a matrix element. The matrix element with the highest number determines the two devices with the most intensive exchange of material. These are placed at two neighbouring nodes. For all remaining equipment, the total of the matrix elements in relation to the two already placed devices is calculated. The one with the highest total is positioned directly near the first two, thus completing a triangle. This same procedure is followed until all the equipment has been placed. Triangular method Festo Didactic Mechatronics

10 A-18 Numerical method If this procedure does not provide a conclusive outcome, the calculation can be narrowed down, where i is the total of products from the extent of material flow m ij and the distance s ij to all previously placed equipment j (and not just the neighbouring equipment) is calculated: i t s m i ij ij j 1 In the case of more complex installations, computer-aided optimisation material flow is indicated. Although it is possible to achieve precise calculations, the amount of calculation nevertheless increases enormously with the number of equipment used. This is why approximation methods are used in practice. With the interchanging method, you start with a random distribution of equipment. Pairs of equipment are then interchanged up until the entire material flow incidence can no longer be significantly reduced. If used constructively, one operational device after another is placed, assuming the most optimum position in relation to the equipment already placed. Mechatronics Festo Didactic

11 A Handling According to VDI guideline 2860, handling is a subsection of material flow and has been defined as follows: "Handling" is the creation, defined changing or temporary maintaining of a specified spatial configuration of geometrically defined bodies within a reference coordinate system. Additional conditions such as, for instance, time, quantity and motion path may be specified." The definition does not lay down any stipulations regarding the execution of handling be it manual or mechanical. However, the following is intended to examine the automation of handling, whereby mechanical handling will be in the foreground of discussion. Differentiation between handling devices is made according to whether these are fixed- programmed or freely programmed. In the case of fixed-programmed handling devices, the motion if the device is defined structurally and can only be changed with a certain amount of inconvenience. This has, for instance, been defined by means of the stroke movement of pneumatic or hydraulic cylinders, by cam discs or limit switches of electromotive drives. Fixed-programmed handling devices are used as pick-and-place robots in single-purpose systems in large volume and mass production. Terminology Fixed-programmed handling devices Fig. 2.6: Typical motion sequence of pick-and-place robot Festo Didactic Mechatronics

12 A-20 Path optimisation Pick-and-place robots use simple kinematics. In order to approach any given point within a space, three options of movement are always required: three translatory movements (straight linear displacement), two translatory and one rotational movement, one translatory and two rotational movements or three rotational movements. The options of movement (even if a translatory movement is involved), are known as axes. In practice, pick-and-place robots are frequently able to operate with less than three axes, if these are installed in such a way that the starting point and terminal point of the required movement is on a path with one or two axes. Fig. 2.6 illustrates a typical motion sequence using two axes, i.e. a translatory and a rotational axis. To keep the period of the pick-and-place process as short as possible, the path is optimised. Fig. 2.7, for instance, suggests that there is no need for the empty gripper to be lifted as during the return movement as during the go movement. The vertical movement has three terminal points. With a pneumatic or hydraulic drive, this can be realised by means of series connected cylinders. With an electrical drive, limit switches are positioned at the appropriate positions, whereby a middle switch needs to be traversed. With an electrical drive via cam discs or with a controllable motor (e.g. stepper motor), the path can be precisely predetermined. In such cases, the execution of the partial movements can be overlapping. The displacement-time diagram (see fig. 2.7) clearly illustrates the time saving between sequential and overlapping control. Mechatronics Festo Didactic

13 A-21 Fig. 2.7: Overlapping control of motion from fig. 2.6, plus displacement-time diagram Similarly, the control of a pick-and-place device is generally simple. Basic logic operations, e.g. on electrical or pneumatic basis, are adequate for the realisation of such simple sequence controls. However, the development of electromotive drive technology and the availability of small controllers is gradually reducing the need for fixed programmed solutions. Handling devices must be equipped with grippers, capable of gripping, moving and releasing the workpiece. Grippers either establish a frictionlocking or interlocking connection to the part. With the exception of the handling of standard parts interlocking grippers are always special solutions. Mechanical grippers are mainly driven pneumatically. With smaller loads, frictional locking is effected by means of spring force. The pneumatic drive opens the gripper and releases the workpiece. This ensures that in the event of failure of the controller or compressed air supply, the part will not be dropped. Pincer grippers have two swiveling fingers, which are fairly versatile in application. By contrast, parallel grippers have two parallel moving fingers. Vacuum grippers hold the workpiece by means of vacuum in one or several suction cups. A workpiece must have smooth surfaces for the use of vacuum grippers. A vacuum pump is required for the supply of vacuum grippers. Pick-and-place robot controllers Grippers Mechanical grippers Vacuum grippers Festo Didactic Mechatronics

14 A-22 Electromagnetic grippers Modular system Freely programmable handling devices Industrial robots Magnetic grippers are used to grip soft-magnetic workpieces with the help of electromagnets. Critical is the permanent magnetisation, which cannot be entirely eliminated even in the case of soft-magnetic workpieces. In order to release a workpiece safely, magnetic grippers must be briefly pressurised with a short pulse in reverse polarity or AC voltage. When examining simple handling tasks, it is possible to detect a basic similarity time and again. The obvious answer is therefore for industry to offer modular systems. Individual modules are available in various sizes and of different functionality whereby it is possible, for instance, to use arms of different length or to chose between a vacuum and pincer gripper. Freely programmable handing devices differ from fixed-programmed handling devices as far as two characteristics are concerned: The control of the axes permits not only the approaching of a few end positions, but also a targeted approach of any number of intermediate positions, whereby any point within the range of the handling device can be reached. The motion sequence is not hard-wired, but stored in the main memory of the control computer. In this way, the motion sequence can be changed without mechanical intervention. In flexible production cells or transfer lines, where different workpieces need to be handled by identical handling devices, it is even possible to switch the control computer between several prepared motion sequences. Freely programmable handling devices with a greater number of axes (depending on delimitation, five or six) are known as industrial robots. Similar to pick-and-place robots, industrial robots require three axes to transport a workpiece to a given point, the so-called main axes or arm axes. However, a further three axes are required in order to swivel the workpiece in the required direction, i.e. the hand joint axes. In practice, hand joint axes are always rotational axes. Robots are categorised into types, depending on how the main axes are divided into translatory and rotational movements. Mechatronics Festo Didactic