Modern Developments in Sensor Technology and their Applications in Condition Monitoring of Manufacturing Processes A. Al-Habaibeh School of Computing and Technology The Nottingham Trent University Burton Street, Nottingham, NG1 4BU,UK
[email protected] R. Cai and M.R. Jackson Mechatronics Research Centre, Wolfson School of mechanical and manufacturing engineering, Loughborough University, LE11 3TU, UK www.mechatronics.org.uk R.M. Parkin Iensys Ltd (Intelligent Engineering Systems), UK www.iensys.com ,
[email protected] Abstract Monitoring manufacturing processes has been the focus of research in academia and industry for several decades due to expectation of high productivity, high product’s quality, and low costs. Sensors form one of the main components of a successful condition monitoring system. This paper presents a survey of the sensor technologies that have been implemented in research and industry. Advantages and limitations experienced with different technologies are discussed. The paper also outlines the modern developments and future trends in the applications of sensors in condition monitoring of manufacturing processes. Keywords: sensors, condition monitoring, machining
1. Introduction In modern competitive manufacturing industry, machining processes is expected to deliver high accuracy, improved reliability and excellent quality with reduced costs. From the technical side, new demands are being placed on monitoring systems in the manufacturing environment because of recent developments and trends in machining technology such as high speed machining, VIPER grinding, hard cutting and dry cutting. To meet such demands, the present monitoring system should become more reliable and flexible. Firstly, the monitoring system should be capable of detecting any unexpected faults which happen during manufacturing. Secondly, information regarding the process parameters obtained with the monitoring system can be used for minimising the machining cost or time by modifying the machining parameters to achieve adaptive control optimisation. Thirdly the monitoring system can gather information regarding the process that will make it possible to obtain different information that could be useful for establishing a database on the process parameters and set-up information [1]. In general terms, condition monitoring can achieve its objectives by two main directions: the first direction is to ensure good quality of products by monitoring the conditions of processes. The second is to maintain high productivity by establishing a good maintenance system of the whole machine tool to prevent any operational failure or down-time using Condition-Based Maintenance (CBM). Productivity can also be improved by embed the necessary inspection and quality control processes within the production stage. Sensors form the main component of machine/process condition monitoring system. The success of a condition monitoring system depends on the type, suitability and reliability of information captures by the sensors. According to the report "Sensor Markets 2008," by Intechno Consulting (Basel, Switzerland) [2], the sensor market is expected to reach up to $54 billion by 2008. Many different types of sensors have been commercially available, see for
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example [3]. An integrated process and machine condition monitoring has been suggested in [4]. An off-line characterisation of the machine and on-line monitoring of the machine and the process using different sensor technology has been presented. The basic principle is to integrate off-line and on-line sensory measurement of machine and process in order to assess the health of the machine and the total performance of the process. The argument is that full information regarding the manufacturing system can be obtained and fault diagnostic and prognostic can be simpler by recognising faults that are caused by the machine and other faults that are caused by the process (e.g. tool wear, tool fracture, etc.). In this paper, the focus is on the on-line sensory systems that can be used when the machine performing its operations. The on-line sensory system is normally associated with the process rather than the machine tool. However, in some cases some sensory information from the machine itself can be monitored to guarantee high standard of performance. Machining processes are very complex process affected by many factors such as, machine tool stiffness, characteristics of cutting tool and workpiece, splashed chips and coolant, machining parameters etc. The main problems associated with machining processes are: tool wear, catastrophic tool breakage, teeth breakage, collision and chatter. Also some problems and impurities in material could also cause unexpected problems. Figure 1 presents the main structure of a condition monitoring system. The signals obtained by suitable sensors and analysed for information related to the health of the process. Decisionmaking is then made to send feedback to the machine tool.
Sensory Signals
Feature extraction
Decision making
Faulty Control Feedback
Normal
Manufacturing Process Figure 1: A general structure of a condition monitoring system Sensors, therefore, are very important components since they are the first part of a condition monitoring system. A suitable sensor could be the determinant factor of the success or failure of the monitoring system. An example of condition monitoring system sensor configurations is presented in Figure 2. It presents the application of acoustic emission (AE), accelerometers, platform force dynamometer and rotating dynamometer for monitoring machining of aerospace components. This paper presents the most commonly used sensors in academia and industry and the expected future trends in the sensor technology and condition monitoring.
2. Sensors in Research and Industry This section summarizes some of the sensory systems that common sensors that have been reported in research and industry. The sensors should be simple to operate and robust due to the hostile environment. The sensors should comply with the expected requirements in manufacturing processes including measurement as close to the machining point as possible, sustaining the static and dynamic stiffness on the machine tool, maintaining the working space and cutting parameters, wear and maintenance free, easily replaceable and cost-effective, resistant to dirt, chips and mechanical, electromagnetic and thermal influences, function independent of tool and work-piece, reliable signal transmission [5].
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Machine tool spindle Force rotating dynamometer AE sensor
Platform dynamometer
Three perpendicular vibration sensors
Figure 2: A typical use of force, vibration and acoustic emission sensors in monitoring manufacturing process. 2.1 Force, torque and power transducers Force measurement is fundamentally based on the determination of a displacement. Strain gages were primarily used to analyse forces in the early fifties. The main disadvantage of this approach is the reduction of the total system stiffness since strain gauges require significant strain values to measure forces [1]. With the development of piezo electric transducers, piezo electric quartz dynamometer are one of the most popular approach to measure forces [6-8]. Tool cutting forces have been shown to be sensitive to tool wear, breakage and collision [9-11]. The main disadvantage of this approach is the installation and use of a force dynamometer in an industrial environment, although different types and sizes of force measuring platforms are available and are well protected against the environment conditions presented in industrial production. Rotating force dynamometers, see Figure 2, with speeds up to 25,000 rpm available from Kistler [12]. The advantage of a rotating dynamometer is that the sensor has the same relative place relative to the tool, however, they are more expensive than platform dynamometers. Figure 3 presents example of cutting forces of a normal and worn milling tools. Notice the difference in shape and level of the signals due to wear.
Cutting Force (N)
Normal Tool
Worn Tool Cutting Force (N)
Time (ms)
Figure 3: A typical example of cutting forces for a normal and a worn tool. Monitor power consumption represents one of the simplest methods of monitoring the machining systems [4,13,14]. The power sensor is used to measure spindle or axis drive power consumption. It provides a true measurement of power that includes voltage, current, and phase. Power measurement can be used to detect collision and faulty tools. The sensor is easily installed without machine modifications. It is useful to monitor the trend of tool wear; the main drawback is that the small change of the tool cannot be detected [15, 16]. The power signal
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could have low signal to noise ratio [4]. But, the monitoring index based on the sum of power spectrum magnitudes at low spindle frequency harmonics has been found sensitive to both cutter run out and flute chipping/breakage in milling [17]. Montronix presents a power sensor with digital gain module designed primarily for rotating tool applications [18] it has on-board micro processor controlled gain, offset, and non-linear filtering. The manufacturer claims that the features allow the sensor to work in applications using both large and small diameter tools. The sensor's ability to accurately measure very small (
