2016-10-07
Production Management
Flexible Manufacturing Automation (6) 10. FMS supervising and diagnostics system 11. FMS availability
Wacław Skoczyński room 2.17 B-4, tel. 71 320 26 39
[email protected]
Reasons for monitoring modern manufacturing systems • Machine tools operate with speeds that do not allow manual intervention. • Manufacturing systems have become larger in scale, and monitoring of such large-scale systems is already beyond the capability of human beings. • Increase of labor costs and the shortage of skilled operators calls for operation of the manufacturing system with minimum human intervention. • Ultra-precision manufacturing can only be achieved with the aid of advanced metrology and process monitoring technology. • The use of sophisticated machine tools requires the integration of monitoring systems to prevent machine failure. • Heavy-duty manufacturing processes with higher energy consumption should be conducted with minimum human intervention, from the safety point of view. • Environmental consciousness in the manufacturing of today requires monitoring emissions from the process.
Possible forms of direct acting in case of a machine tool: • Machine tool stoppage - the simplest and most radical action • The part program changing - it results in automatic adaptation of machining tasks to actual technological possibilities, changed e.g. due to damages • Correction of tool setting-up - aimed to eliminating, or diminishing of the disturbances influence on the machining accuracy • Changing the tool trajectory - it can take place in case of form-closed control • Changing the movement speed - e.g. the feed or spindle speed, made with the aim to change the cutting parameters • Compensation of disturbances influences • Exchanging the elements unable to work, into efficient one - e.g. exchanging the worn-out tools.
Course Outline: 1. Introduction 2. Need for Flexible Manufacturing Systems 3. Organizational categories of flexible manufacturing 4. Functional structure of flexible manufacturing system 5. Machining subsystem 6. Tool management 7. Workpiece subsystem 8. FMS logistic system (automated material movement and storage) 9. FMS information system 10. FMS supervising and diagnostics system 11. FMS availability
10. FMS supervising and diagnostics system The main goal of this system is: „achievement of required manufacturing accuracy and assurance of high system utilization rate in appearance of disturbances”. Realization of supervising and diagnostics system tasks requires: - Determining of factors and parameters, which values will be monitored, - Choice and application of suitable sensors, - Choice of suitable strategy. In manufacturing systems with metal-cutting machine tools they are monitored: 1 – tool condition, 2 – machine tool functioning, 3 – machining process, 4 – workpiece accuracy.
Types of monitoring systems : • Protective • Corrective • Optimizing. The protecting system prevents failures (serious damage causing breaks, or disturbance in manufacturing process). The protecting system includes diagnostic system detecting the appearing irregularities, e.g. the wear, or damage of tools. The correcting system maintains the selected values in determined, allowed range. It functions based on measurement of disturbances or part machining results. The optimizing system automatically changes the process parameters to obtain the most advantageous value of selected quality characteristic.
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The main modules of the monitoring and diagnostic system: 1. Sensor’s set, 2. Monitoring module, 3. Operation unit. The sensors are the sources of signals enabling to obtain information about the system state. They should be situated possibly near the monitored object, should be distinguished by a great sensibility on the variations of the followed-up parameter and resistant to disturbances. In the monitoring module there take place the evaluation of primary processed signals obtained from the measurement sensors and transferring the information to the machine tool control system. The operation unit enables communication of the user with the monitoring system. The realization of monitoring and diagnostics system tasks requires: • Determining of factors and parameters, which values will be monitored, • Choice and application of suitable sensors, • Choice of suitable strategy.
Flame Detector Detektor płomienia Smoke CzujnikSensor dymu
Figure 10.1. Groups of factors influencing the manufacturing system function
a) Czujnik temperaturySensor i wilgotności Czujnik Sensor prędkości Temp./Humidity Speed
Czujnik Sound level poziomu hałasu
Czujnik ciśnienia Miernik Currentprądu Sensor Pressure Sensor DetektorDioxide dwutlenkuGas węgla (czujnik obciążenia) Carbon Sensor
sensor
(load sensor) Torque sensor Czujnik momentu
Sensor obrazu Image Sensor
Base sensor I
O Transducer
Measured quantity
Electrical signal
Czujnik emisji AE Sensor
zużycia narzędzia
Edge Position Sensor Sensor położenia krawędzi Czujniktype drgań Vibration Accelerometer Ogranicznik Limit Sensor Czujnik Sensor sejsmiczny Seismic
Sensor Czujnik Precision/Thermal odkształceń cieplnych Sensor
Czujnik Force/Torque (current) siły/momentu (prądu) Czujnik położenia Position Sensor Czujnik siłySensor zacisku Clamping Force Tool Damage Sensor Czujnik zniszczenia narzędzia
b)
Sensor
Czujnik temperatury Lube Oil/Coolant smaru/cieczy obróbkowej Temperature Sensor Miernik poziomu Level Meter
Figure 10.2. Abundance of sensors for manufacturing system monitoring, after T.Morivaki [8]
Strain gauge sensors allow the measurement of the elongation of the element.
In the elastic range it is, under the law of Hooke, directly proportional to the load. The basis for the physical functioning of strain gauge is a change of crosssection of resistance wire, which is accompanied by his elongation. At low elongation, with a change in cross-section, resistance varies linearly in accordance with the relationship:
Integrated sensor I
Measured quantity
c)
O Electrical signal
Intelligent sensor I
Measured quantity
Processor
Sensor Tool Wear Sensor
Czujnik Dust mgły Sensor Czujnik Chip Monitoring monitorowania wiórów
Transducer
dotykowy
Preamplifier
Machined Sensor chropowatości Surface powierzchni Roughness obrobionej Sensor
CzujnikSensor Touch
Transducer
pH Sensor Sensor pH
obecności smaru
Czujnik temperatury Temperature Sensor
Preamplifier
Detektor Lube Oil Detector
CzujnikDistribution rozkładu Temp. temperatury Sensor
converter A/D transducer A/C
akustycznej (AE)
O Information
Figure 10.3. Sensors integration degrees
Under the notion of sensor, there is understood a technical appliance acquiring physical and chemical states and also their temporal, or space changes, and converting them into signals (mostly electrical) suitable to processing. Miniaturization of electronic systems allows in many cases to integrate in one encapsulation several functions. Actually, there can be distinguished sensors of three integration levels: Base sensor - contains only converter reacting on the measured quantity and converting it in electric signal, Integrated sensor - it realizes the functions of both the converter, as well as primary signal processing (preamplifier), Intelligent sensor - an integrated sensor equipped with a system processing the signal and converting it in information suitable to the role which it plays in the system.
Piezoelectric sensors - use the phenomenon of the appearance the electric charge that occurs in some materials under mechanical stress. The piezoelectric elements are obtained by pressing ceramic materials such as lead zirconate titanate or by cutting the plates from a uniform crystal piezoelectric material (tourmaline, quartz). The charge formed on the surface of the piezoelectric is proportional to the applied load.
R l R l where: R – resistance, l – length,, κ – sensivity factor, ε – unit elongation.
Strain gauge probe
piezoelectric milling dynamometer
piezoelectric accelerometer
piezoelectric pressure transducer
piezoelectric torque meter
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A temperature sensor consists of two basic physical types: • Contact Temperature Sensor Types –are required to be in physical contact with the object being sensed and use conduction to monitor changes in temperature. • Non-contact Temperature Sensor Types –use convection and radiation to monitor changes in temperature. Thermocouple is an electrical device consisting of two different conductors forming electrical junctions at differing temperatures. A thermocouple produces a temperature-dependent voltage as a result of the thermoelectric effect. Thermistor - is a special type of resistor which changes its physical resistance when exposed to changes in temperature. Quartz thermometer - measures temperature by measuring the frequency of a quartz crystal oscillator. The oscillator contains a specially cut crystal that results in a linear temperature coefficient of frequency, so the measurement of the temperature is essentially reduced to measurement of the oscillator frequency. Thermographic camera - is a device that forms an image using infrared radiation, similar to a common camera that forms an image using visible light.
Functions realized using complex signal processing in an intelligent sensor or in measurement system: 1. Static correction, such as: • Linearization and characteristics shift, • Scaling, • Elimination of systematic errors (e.g. temperature dependent), • Statistical processing, • Approximation by given function. 2. Dynamic correction, e.g.: • Taking in consideration system transmittance, • Comparison with introduced model. 3. Evaluation of complex quantities, such as: • Surface master (image identification), • Frequency spectrum (identification of tool breakage, or wear), • Status of sensor (self-monitoring).
Monitoring strategies
LV 1 - collision LV 2 - tool breakage LV 3 - tool wear Cutting force signal
LV 4 - lack of tool
t
Figure 10.4. Tool condition monitoring with four limits of cutting force value
The strategy of multi-limits values are more often used and allows taking in consideration different cases which may take place during the process realization. Figure shows four-limits values of the cutting force, which may be significant for the tool condition monitoring.
Thermocouple
Resistance thermometer
Quartz thermometer
Monitoring strategies: • One-limit value • Multi–limits values • Master (running limit) • Variable master (self teaching) • Multi-parametric evaluation. Strategy of one-limit value - consists in comparison of value of signal measure with the evaluated, most often empirical, the limit value. The exceeding there of upwards (or in certain cases downwards), means disturbance of monitored process course, or damage of some element of the machine tool and initiates the appropriate system reaction.
Monitoring strategies
There are often significant changes in the parameter characterizing process during its monitoring, for example, changes of cutting forces while milling. In such cases, the running limit as a field of tolerance is applied to the standard course of the average value of this parameter determined for stable conditions, such as cutting with a sharp tool. The alarm signal is triggered when the measured signal exceeds the tolerance field (it could mean the tool Supervision of the tool with the master (running limit) strategy breakage). (milling using cutter head)
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Monitoring strategies
Workpiece monitoring The measurements of machined parts may be made either in the working area of the machine tool, or beyond the machine on the special stations provided with automated measurement equipment. The measurements in the working space of machine tool are made with the use of measurement probes. The contact probes are in turn divided into impulse (also called switch-over), which generate by the contact a signal causing the reading from the measurement systems of the machine tool NC axes, the values of coordinates of this point and measuring are provided with measuring transducer. The complex and time-consuming measurements are realized outside of the machine tool, on special measuring stations. There can be used different automated measuring equipments on these stations, both, the universal as well as the special ones. The special equipment are adapted to a defined part spectrum.
Principle of the system multi-parametric evaluation g (c) - the boundary function separating the areas allowable from unacceptable conditions
Function principle of the electro-contact probe consists in them, that a moving object, running into the probe gauging point, sticks it out opening one of the three series connected contacts. After breaking electric circuit on the transducer output appears a signal, which initiates reading of measured points coordinates and stoppage of NC axes driving. Measurement probes became a standard equipment of centers and flexible manufacturing cells. They fulfill the following functions: • Checking the workpiece dimensions, • Inspection of tool condition, • Part identification, • Inspection of part setting in order to correct errors of fixing its on the palette and palette clamping on the table, Figure 10.5. Two types of probes: a) impulse, b) measuring • Correction of position errors of rotating table.
b)b)
a)a)
mechanical contact
piezoelectric sensors
The principle of operation electro-contact probe
Improved construction of the electro-contact probe is a probe with auxiliary piezoelectric transducers arranged like electro-contacts circumferential at 120o. If the measure pressure exceeds 0,01 N caused deformation of one of the piezoelements generates voltage impulse releasing reading of the point coordinates. These coordinates are stored. Further displacement of object relative to the probe causes increase of pressure up to several hundredths of newtons and opening of one of contacts, what generates the second signal, so called confirming. This signal initiates stoppage of NC axes drive.
Figure 10.6. Configuration of sensors in a piezoelectric probe
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Measurement of piston on the CNC machine using 3D tool probe Checking the workpiece during processing
Cutting tool condition monitoring The direct methods are based on the measurement, either directly geometric features of cutter (its edges and surfaces), or the quantities directly depending on their changes. They better reflect the real state, but are often difficult in realization. In the indirect methods however, there are measured quantities, which characterize other phenomena, which are influenced by the cutting edge wear. They are then based on the measurement of this wear results. These methods are usually technical simpler, but their results are burdened with uncertainty associated with appearing measuring disturbances evoked by the influences of other factors onto the level of measured signal. The most known indirect methods are the following: • Measurement of cutting temperature and thermoelectric power • Measurement of roughness changes, or workpiece dimensions • Measurement of vibrations and noise • Measurement of cutting force and derivatives quantities • Measurement of power consumption or motor power • Measurement of acoustic emission • Measurement of tool working time.
Kinematic contact tool setter (simple length and radius checking and broken tool detection)
Kinematic contact tool setter (simple length and radius checking and broken tool detection)
Exemplary touch probes for rotating tools and turning tools installed on CNC machine tools
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The laser system for tool monitoring in machining centers
Supervision of tools and machining process
Detection of broken tool damage using Renishaw probe
The principle of operation of the induction sensor
Drill Bit Breakage Detection Sensor
Exemplary dynamometer with tool holders for 3-component cutting force measurement during turning Possible sensors positions for tool monitoring in CNC lathes Nordmann Tool Monitoring http://www.toolmonitoring.com/praesentation.html
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Fi [N]
Strain gauges 11 tensometry
Fr
+Us
2400
Fv
1800
Voltage napięcie Us
Us
t czas
Time
Fa 1200
800
tensometry Strain gauges 22 T
0
0,2
0,4
0,6
0,8
1,0
~
-
mostek pom. 1 Measuring bridge 1
VBc [mm]
T
The dependence of the cutting force components Fi on on tool flank wear VBC
~
+
-
mostek pom. Measuring bridge 2 2
The principle of indirect measurement of cutting force
Prędkości skrawania Cutting speeds vc v od 150 150 m/min from m/min do 300 m/min
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Supervising and diagnostics functions for machine tools Automatic supervising and diagnostics for machine tools has to fulfill two basic functions: • Ensuring the proper operation of the machine and the whole of its additional equipment. • Ensuring the proper quality of the workpieces.
AE×10 6
to 300 m/min
Diagnostics of machine tools and systems automate their work can be divided into: • Diagnostics of operation correctness, • Diagnostics of machine tool state. 0
100
200
300
400
VB c [ m]
500
The dependence of the acoustic emission (measured by the sum of the pulses) on wear of the cutting edge: ΣAE - the sum of pulses of acoustic emission, VBC - blade tool flank wear
Abnormalities in machining process Supervision and diagnostics machining process serves to secure machines and tools from the consequences of abnormalities occurring during the process of cutting. These abnormalities are: • Excessive vibration (especially the appearance of self-excited vibration) • Abnormal form of chip • Collisions of moving machine units • Disturbances in the supply of cutting fluids.
The detection of a specific system failure can only be either signaled (for example by acoustic signal), or may be triggered procedure for automatic removal of reasons (on-line diagnostics).
Diagnostic methods related to change of chip form: • •
•
•
thermal imaging cameras - registration thermal chips radiation (stacking chips are a source of increased emission of thermal radiation) acoustic emission sensors - time signal is a pulse (caused by breaking the chip); for short chip signal waveform has the pulse form and continuous chip correspond to waveforms with relatively small changes in amplitude, dynamometers - there is a clear difference in frequency spectrum of the feed cutting force component at continuous chip and in the presence of chip breaking, CCD camera with image processing system for automatic detection of emerging continuous chips.
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prędkość skrawania = 350 m/min Cutting speed vc = 350vcm/min posuw f =revolution 0,16 mm/ob r Feed per f = 0.16 mm/rev głębokość skrawania Depth of cut ap = 2mmap = 2 mm materiał obrabiany 20MoCr4 Workpiece material:: 20MoCr4 sensor AE umieszczony na AE sensors placed on housingkorpusie of the turret głowicy rewolwerowej
AE-RMS(voltage) (napięcie) Digital wyj. output cyfrowe AE-RMS
2,5 V
Continous chips wiór ciągły
1,5 1,0 2,5
0,5 0 0 1,0 V 0,5 0 0
0,05
0,10
0,15
0,20
0,25
0,30
0,35
0,40
0,45 s 0,50
wiór przerywany Segmental chips 0,05
0,10
0,15
0,20
0,25 0,30 Czas Timet t [s]
0,35
0,40
0,45 s 0,50
Collision detection
The waveform signal of acoustic emission (AE) depending on the chip form
The kinematical system of measuring machine consists of three mutually perpendicular NC axes creating the coordinate system x, y, z. It is then analogical to kinematical system of a milling machine with the difference, that in place of tool, there is installed the measuring head. The basic types of measuring machines are the following: • Bridge-type, • Outrigger, • Column, • Crossbar.
Figure 10.7. Bridge-type vertical coordinate measuring machine (CMM)
The CMMs are characterized by many advantages, such as high accuracy, universality, flexibility, effectiveness, possibility of fast processing and evaluation (from different points of view) of measurement results, and ease to obtain broad measuring documentation.
Measurements using a coordinate measuring machine
11. FMS availability They are, to characterize the system capacity and efficiency, the following terms introduced: • technical availability – Dt, and • real available time using – Wr. The technical availability means difference between total available machine work time and by technical reasons caused failure times to total available machine work time ratio. The technical reasons are shared into direct, connected with disturbances in functioning of machine tool units and indirect, connected with disturbances in functioning of other FMS facilities. In the practice real available time used in a FMS is smaller then its technical availability as result of failure times due to managerial reasons. Measurements of gears with a coordinate measuring machine
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Dtc
Tcc T ptc T psc Tcc
Total time available = 100%
For a defined work cycle, e.g. whole day (three shifts) of one machine tool (machining centre), the technical availability is expressed by the formula:
where: Dtc – technical availability of one machining centre Tcc – working time potential of centre Tptc – standstill times generated by direct technical causes Tpsc – standstill times generated by indirect (system) technical causes.
Managerial disturbances in the company
Start-up phase Technical disturbance s Related to FMS managerial disturbances
The real utilization of one manufacturing centre may then be estimated from the equation:
where: Wrc – real utilization of one machining centre Tpoc – standstills time generated by organization causes
e.g. - lack of material - maintenance - personnel mistakes
Real using of available system work time
Technical availability
Figure 11.1. Sankey diagram of FMS available total time using
100
System failure
93% 83%
%
Real using of FMS work time
70
Technical availability
60 50 40 30
technical availability real available time using
20 10 0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Work days
Average
Figure 11.2. Example of diagram presenting the technical availability and real FMS work time using
Flexible manufacturing categories
3 – shift work Praca 3 - zmianowa 100
% 90 80
Available work time using
Availability
80
70
2 – shift work Praca 2 - zmianowa
60 50 40 30 20 10 0
Figure 11.4. Available work time using by different flexible manufacturing categories
stand-alone NCmachine
machining centre
managerial failures
set-up part changing
swervice
testing
FMS
processing programs realisation
Technical reasons
Managerial reasons
Electrical systems
Mechanical systems
Equipment
Production planning
Manufacturing
Drives End position switches Wires Connecting clamps Probe Plugs Amplifiers Measuring systems Data input Data output
Drives Transmissions Slidebars Hydraulic systems Couplings Bearings Pneumatic systems Spindle Lead-screws Tool changer Pallet changer
Coolants Means of measuring Processing programs Robots Fixtures Cutting tools
Programming Program failures Part fixturing planning Tool demand planning Service and repair equipment Part Design mistakes
Personnel mistakes Positioning failures Measuring failures Statutory breakes Maintenance Repairs Set-up changes Awaitings Monitoring Tool missing Part missing
Figure 11.3. Classification of reasons of disturbances by the FMS operation
Planning of Flexible Manufacturing System Flexible manufacturing systems are regarded as one of the most efficient methods to employ in reducing or eliminating problems in the manufacturing industries. Achievement of FMS benefits depends decisively on correctness of decisions made by the system planning. The planning process should begin with the answer to the following questions: • What are the real manufacturing targets the company should be aiming for by purchasing an FMS? • How will be determined the economical efficiency of system in use? • Where is the achievement of main savings expected? • Which compromises are acceptable? • Is the investment costs limit determined and is it known to all project team members? • Where are the allowed saving possibilities in case of investment costs limit exceeding? • Are the staff training and maintenance concern costs taken into account?
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Flexible Manufacturing Systems Planning
Part Volume and Variety
Means of Production
Transport System
Tool Management
Productivity
Control and Programming
Monitoring System
Benefits
Complexity
FMS Machine Tools
Other
Type of Material Transport
Supervising
Transport Tasks
Control
Programming
Quality Assurance
Communication System
Figure 11.5. The main problem areas to take in consideration by process of FMS planning
Investment costs
Availability
Flexibility
Figure 11.6. Dilemmas of flexible manufacturing automation
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