transducer
The other critical issue for selecting a proper sensor type is the sampling rate, which represents the scanned frequency per second for reading data from sensors that have to provide clear, continuous enough, and distortion‐free data.
To make signals distortion‐free, the Nyquist–Shannon sampling theorem states that the sampling rate for signal Fs is at least more than twice the highest frequency FB component contained in the acquired signals during machining processes so that no information gets lost by discretization [6–8]. The relationship can be defined as Fs > 2FB.
In the past, the sampling rate is restricted by the hardware specification and cost. As hardware becomes faster and cheaper, in a modern monitoring system, the sampling rate can be set at a very high frequency (>10 k Hz) to ensure that captured signals are nondistorting and can represent any situation during production since the sensor can serve as an independent unit from the manufacturing tool or device, which means that they do not decrease the production performance or disturb the machining process. However, a high Fs generates a high‐dimensionality of data size, which makes raw data hard to be used directly. Some signal processing techniques for simplifying the data will be introduced in Section 2.3. In the following, several types of sensors, such as force: strain gauge, loading: current transducer, vibration: accelerometer, temperature: thermal couple, and AE waves: AE transducer, are introduced and followed by the description of sensor fusion.
Force: Strain Gauge
Cutting force is an intuitional physic quantity that affects the machining quality and product quality the most. This force generated from the relative motions between the cutting tool and workpiece is necessary to form the shape of the workpiece, this relationship is illustrated in Figure 2.3. Any change occurred in the cutting‐tool paths, heat, weight, tension, or structure of the material during the cutting operation can be directly reflected on the cutting force. Thus, the cutting force is considered to be the best indicator to perform the monitoring of cutting‐tool/equipment condition and prediction of the workpiece accuracy.
Figure 2.3 Relative motion between a cutting tool and a workpiece.
Observing Figure 2.3, the electrical strain gauge, or the so‐called piezoresistive sensor, detects forces or strains on the target device and converts the amount of the deformation into an electrical signal. As shown in the right side of Figure 2.3, the electrical resistance element changes its resistance length once the strain gauge is elongated or contracted under the external force. Through this direct measurement, the strain gauge provides extremely accurate and rapid response of the forces and strains that happened in the movement direction of cutting‐tool. The excellent durability and stability also enable them to be operated in harsh environment. Figure 2.3 demonstrates an example of the adhesive‐backed strain‐gauge installation, which can be directly attached to the tool holder to monitor the torque and force on the cutting tool during the operations.
Many similar sensor types derived from the same working principle of strain‐gauge technology, such as strain gauge‐based pressure transducer and dynamometer are also commonly used in the machining factories. The tool dynamometer or force dynamometer is also frequently adopted as a sensor for monitoring cutting force signals. The sampling rate usually ranges from 1 to 100 kHz.
The installation of a force dynamometer is illustrated in Figure 2.4, which has to be fixed on the machining table. The performance is also sensitive to the installation direction, humidity, temperature, or long‐time usage. Yet the installation process of the dynamometer is tedious because of its intrusive nature, which may deteriorate the machining performance. In addition, a dynamometer may reduce the working space and available machining conditions so that it is not acceptable and practical in the industry.
Figure 2.4 Installation of a Dynamometer.
Loading: Current Transducer
The cutting force can be estimated indirectly by monitoring the change of the motor current since it is related to the cutting force of cutting process. Motor current is proportional to the output torque of the motor that produces the amount of the cutting forces, which provide the needed mechanical force to remove material from the workpiece. Thus, the motor current is very useful for the tool wear, breakage/failure detection, and quality prediction of workpiece.
Current measurement sensors can be used to monitor the cutting process in a manner similar to a tool dynamometer. The Hall effect current sensor, or the so‐called current transducer (CT), converts motor current into an output voltage without any physical contact through detecting changes in the electromagnetic field, which is caused by the motion electric charges. As illustrated in Figure 2.5, a CT detects the electromagnetic field in the air surrounding when the electric current passes through a power cable.
Figure 2.5 Installation of a CT.
Compared to traditional sensing techniques based on Ohm’s Law, which generates a voltage that is proportional to current by directly connecting a resistor in series with the circuits, the Hall‐effect CT sensor does not disturb the machining since it has no direct connection to the circuit that carries the current.
The installation location can be attached directly on the motor cables without fixed costs. This isolated or noninvasive advantage makes the Hall current sensor become the most popular measurement method to indirectly sense the motor power or spindle currents with the sampling rate ranging from 100
