This paper presents the results of using a sensor-integrated workpiece for in situ measurement of strain during an outer-diameter cylindrical grinding process. The motivation of this work is to measure in situ process parameters using integrated sensors in a workpiece in order to characterize the manufacturing process. Resistive sensors that operate on the same principle as conventional strain gauges were fabricated on wafers made of steel using standard microtechnology and later the wafers were diced to form unique sensor-integrated steel components (sensor inlays). These inlays are embedded into a groove on the top surface of a cylindrical workpiece using epoxy adhesive. The workpiece is also made of the same steel as the sensor wafers and has similar properties due to a heat treatment process, thereby maintaining the homogeneity of the material over the whole contact area. The sensor-integrated workpiece was used to perform experiments in a Studer S41 high-performance cylindrical grinding machine. The sensor response to the internal strain was recorded during every grinding step starting from a depth of 1 mm down to 2 mm from the top surface. Such an application of sensor integration in materials for in situ process monitoring can be used in other manufacturing processes as well and this can help to observe internal loads (mechanical or thermal) in manufacturing processes.
The motivation of this work is to produce a sensor-integrated workpiece for in situ measurement of process parameters like strain and temperature during manufacturing processes. This can be achieved by embedding a sensor-integrated component (referred to as the sensor inlay hereafter) into a standard workpiece used in manufacturing processes like grinding. By integrating sensors on the workpiece, a sensorial workpiece can be created which can be used to measure in situ strain and temperature in real time by acquiring the measured data from the integrated sensors during machining. The sensor inlay and the workpiece are made up of the same material. This ensures more homogeneity and is therefore more advantageous than integrating commercial strain sensors on the workpiece. In this way, the measurement uncertainties due to a so-called “wounding” effect caused by sensor embedding can be reduced to a large extent. This is one of the major advantages of using material-integrated sensors as opposed to using commercially available sensors which are externally mounted. Also, this is one of the most effective ways of measuring the process-induced strain and temperature in the workpiece as the forces on the material are directly translated into the underlying sensor layer. Also, the sensor inlay is universal and can be used in any manufacturing process and integrated into any workpiece made of the same material. The primary goal behind producing such a sensor-integrated workpiece is to characterize various manufacturing processes. By measuring strain and temperature in situ during any manufacturing process, it may be possible to correlate the internal loads that are specific to each manufacturing process and the corresponding material modifications induced by the various internal loads like stress, strain, temperature, etc., in the machined workpiece. This concept is termed process signatures of manufacturing processes (Brinksmeier et al., 2014). In this concept, it is assumed that all manufacturing processes fundamentally cause specific combinations of internal loads which can be broadly classified into three major types: thermal, mechanical, and chemical. Each manufacturing process contains at least one or a combination of these internal material loads. Hence, measuring the process parameters like external forces will help in identifying the specific internal loads of each manufacturing process. The measured data obtained from the integrated sensors are used to feed analytical models to calculate the internal loads implied by the process. This can lead to generation of transfer functions which can be used to design specific processes or process chains through the targeted desired surface properties independently of the manufacturing technologies. This procedure is so-called inverse process design. The schematic of the concept of using a sensor-integrated workpiece in a grinding process is shown in Fig. 1.
Schematic illustration of in situ measurement with a sensor-integrated workpiece in a surface grinding process.
The application of a sensor-integrated workpiece for measuring in situ parameters in manufacturing processes is a novel concept of sensor integration in materials. Previously thin-film thermocouples were embedded in nickel substrates by electroplating for testing a welding process (Cheng et al., 2006). Strain gauges and thermocouples were together embedded in stainless steel substrates through the thin-film deposition technique, and it was tested in a milling process (Cheng et al., 2008; Choi et al., 2006). The use of a sensory workpiece for process monitoring has been reported earlier (Denkena et al., 2016) where commercial strain gauges were integrated into the workpiece. In this paper, strain sensors are fabricated directly on the workpiece material. The material used in this work for sensor fabrication is the high-grade heat treatable steel 42CrMo4 (AISI4140), which is a common material used in manufacturing processes. But this material is very prone to oxidation at room temperature and hence difficult to use as a substrate for microfabrication. Sensor fabrication on stainless steel substrates (Cheng et al., 2008) and 17-4PH steel substrates (Boedecker et al., 2011) has been reported, but fabrication on a reactive steel substrate like this is not yet state-of-the-art.
Two primary steps are involved in creating the sensor-integrated workpiece as
mentioned above (Dumstorff et al., 2016).
Development of the sensor inlay by designing a sensor layout and
microfabrication of sensors on steel substrates (wafers) using thin-film
technology. The sensor inlay is obtained from the fabricated wafers and it
consists of a sensor layer with several sensors on it. Embedding the sensor inlay on the actual workpiece that is made of the
same steel as the sensor inlay. The workpiece is pre-machined with a
cylindrical shape and a groove of 10 mm
The grinding process in which the sensor-integrated workpiece was used is a
cylindrical grinding process. In the process, the rotating grinding wheel
plunges into the surface of the workpiece up to a pre-defined depth, the
so-called total depth of cut
Schematic diagram of the cylindrical grinding process.
The sensor layer in this work consists of resistive structures in the shape of a meander made of metal films that operates with the same principle as that of a strain gauge. The design of the sensor layer on one sensor inlay has several meanders in various sizes arranged in a specific pattern. A total of 16 sensor inlay layouts were created using the AutoCAD software.
A typical sensor layout, with four resistive structures, on one of the steel inlays. Dimensions are in millimeters.
Each individual sensor layout has about four to eight meanders. It should be noted that the strain distribution across the surface of the workpiece during most machining processes is non-uniform, and the microstructure of the material may also introduce variations in the locations of strain fields (Lang and Chou, 1998). This is why meanders of different sizes have been used in the design. Also, more than one meander is used per design in order to facilitate more than one sensor measurement simultaneously during grinding. Each meander is separately connected to a quarter-bridge Wheatstone bridge network externally which consists of the meander as the variable resistor on one arm and three fixed resistors on the other arms, thereby completing the bridge. This has been further explained in Sect. 2.5.
In this work the agglomerated strain over the sensor area is being measured.
Hence, in the design most of the meanders are oriented parallel to the
cutting direction during machining. The layout is implemented in a photomask
used for forming the sensor design on the steel wafers using
photolithography. In addition to these 16 designs for the sensor inlays, some
designs for test structures were also included. The test structures are
square beams with a cross section of 2
The steel wafers for fabricating sensors were cut out in the form of circular
discs of 150 mm in diameter from a sheet of the steel by water-jet cutting.
The wafers are very prone to oxidation and for use in microfabrication a long
process of lapping, polishing, and smoothing has to be followed. To polish
the wafers, chemical–mechanical polishing (CMP) was used. The discs were
laminated on a big circular blank and polished on the PM5 polishing machine
from Logitech. Both sides of the wafer were lapped gradually with an
alumina grit of size 40
To select the material of the meanders (sensor layer), it is important that
the temperature coefficient of resistance of the material is low and that the
gauge factor is high. Also, due to the harsh environment in manufacturing
processes, the material should be stable at higher temperatures and higher
strain rates. Metal films are the most suitable for this, and here aluminum
(Al) was selected as the sensor material. A major issue in successfully
fabricating sensors on robust substrates like steel is the selection of the
isolation layer. The sensors have to be well isolated for their proper
functioning. But in this case the isolation layer has to provide adequate
electrical and chemical insulation to the sensor, not only from the top to
protect it from the outside environment, but also from the bottom to avoid
connection to the conducting steel substrate. So the sensor element should be
sandwiched between two isolation layers. Also, the isolation layer must have
good adhesion to steel and should have high thermal and mechanical stability
to withstand the continuous mechanical and thermal loading on the sensors
when they are used in the machining process (Cheng et al., 2008). From the
literature it was found that aluminum oxide (Al
After polishing and cleaning the wafers, they are taken to a clean room for
microfabrication. The first step is to prepare the surface for deposition of
the bottom isolation layer. Though Al
The meanders have to be covered with a top layer of Al
Steps for fabricating the sensor inlay:
After depositing the top Al
SEM micrographs of
After fabrication, the wafer is diced and sensor inlays as shown in Fig. 6 are obtained from it.
The sensor inlay cut-out from the wafer after fabrication (50
The contact pads of each sensor in the sensor layer are wire bonded to an external circuit board (PCB) using ultrasound and a micro-tip wire bonder. This establishes a contact from the meander to the PCB as shown in Fig. 7. The wire bonds are then coated with epoxy glue and cured for 12 h. This provides mechanical stability. Each meander has three lead wires which are connected to a three-wire quarter bridge configuration of the Wheatstone bridge circuit. The meander is the variable resistor in the circuit and three other resistors (adjustable trim potentiometers) are used externally to complete the Wheatstone network. In unstrained conditions, the bridge is balanced by adjusting the trim potentiometers so that the output voltage is zero. The circuit is powered with a DC excitation voltage of 5 V using an external battery-operated power circuit and the output voltage at each grinding stage is acquired with a NI 6009 data acquisition device (DAQ) from National Instruments. The wire bonded PCB, where the sensor is attached, is covered with a heat-resistant and mechanically stable material.
Contact pads of the meander wires bonded to a PCB.
The strain response and the temperature response of the resistive meanders were calibrated for characterizing the sensors by conducting two separate calibration experiments.
To calibrate the temperature response of the meander, the sensor inlay was
placed on a hot plate and its change in resistance to every 5
Temperature response showing the change in resistance with respect to change in temperature for a resistive sensor in a sensor inlay.
For the strain response, the change in resistance due to mechanical strain on
the sensor inlay was recorded. A test steel beam
(2
From the stress, the strain
Strain response of the resistive sensors showing the ratio of change in resistance to initial resistance as a result of applied strain.
During experiments, the output voltage of the bridge circuit due to changes
in the resistance of the sensor (meander) is acquired as described in
Sect. 2.5 at each grinding step over a period of time and later converted to
strain. To calculate the measured strain, a standard technique for
quarter-bridge circuits is used. The strain (
The cylindrical grinding process was designed so as to have a maximum
temperature gradient of 1–2
For measurement, the fabricated sensor inlays are to be embedded in an
AISI4140 steel workpiece. Depending on the manufacturing process, the shape
of the workpiece may vary, but the sensor inlay can be embedded in the same
way for every suitable workpiece. In this work, the experiments were done in
a cylindrical grinding process. Hence, a workpiece, as shown in Fig. 10a, was
used and a groove of 12
Two experiments were conducted on a Studer S41 grinding machine which is
equipped with an integrated three-component force measurement system with
piezoelectric sensors mounted on the machine tool. The system is capable of
measuring forces ranging from
Process parameters of the grinding process.
The sensor inlay is 2 mm thick. The sensor inlay was positioned at about
180
The grinding parameters were selected to promote larger mechanical load and
low heat. The grinding wheel used has a good cutting ability and generates
high surface roughness. It has a relatively coarse grain size and vitrified
bond. The grinding wheel is dressed using a dressing procedure. Dressing
reduces wear and tear of the wheel which is inevitable due to the abrasive
nature of the process and is conducted before each grinding step in the
presented work. The dressing procedure was designed to generate a rough
grinding wheel surface with increased cutting properties. These measures
further ensured a higher mechanical load and a relatively lower thermal load
on the sensor inlay and on the workpiece. The grain structure and the surface
hardness of the sensor inlay material and the workpiece material, both
AISI4140, were made similar by quenching and tempering the workpiece material
with the help of a heat treatment process that was specifically designed for
this purpose. The resulting surface hardness was 203 HV1 with a standard
deviation of
During each grinding step, the strain measured by the sensor layer over the
grinding time was recorded. Along with this, the piezoelectric force
measurement system of the grinding machine was used to record the tangential
and normal forces on the workpiece as external material loads. A strain
measurement during the second grinding step over the grinding time, when the
tool was 900
The maximum value of the strain recorded during every grinding step can be considered the true strain experienced by the sensor inlay. The maximum strain values at every grinding stage measured simultaneously by two sensors during both experiments are shown in Fig. 12a and b.
The strain induced by the process into the sensor inlay material is actually
measured by the change in resistance of the integrated sensors on the
workpiece. During grinding, the surface of the workpiece experiences two
major forces, normal force and tangential force. To strain the sensors, the
resultant force (
Comparing both the figures, it can be proved that the sensor measurements are repeatable and consistent because the range of measured strain and the external forces coincide in both sets of experiments. The outlier at grinding step seven in Fig. 12b was probably caused by an error during the experimental procedure and can be neglected. There is a slight increase in the maximum strain measured in the last grinding step when the tool is closest to the sensor layer as compared to the measured strain value during the first grinding step, but this increase is not as significant as was expected. A possible reason could be the wound effect of sensor embedding, i.e., the glue surrounding the sensor inlay which could have prevented the sensor inlay from expanding to its full potential under the induced strain. But to prove this, further investigation is required and numerical models have to be built in the future.
The results prove that in situ measurement of process quantities like strain is possible with the use of a sensor-integrated workpiece developed as described in this work. A problem arises in fabricating the sensors on the steel wafers due to their non-planarity. In the future, laser lithography will be used as an alternative solution. Also, to separate the effect of temperature from strain, the sensor layer will be fabricated from two different metals and the influence of embedding the sensor inlay in the workpiece by glue will be further investigated.
The goal of the measurements is to record the internal loads during the
grinding process and later to generate a transfer function of internal loads.
The process forces remain within a narrow range during every grinding step.
They alone do not facilitate characterization of the material modifications
in the workpiece. This shows that it is not sufficient to measure the
external loads alone, but imperative to know the internal material loads and
depth profiles to characterize the material modifications and their depth
profiles independently of the process as is intended in the so-called process
signatures. This can only be done using integrated sensors. With the
presented method, it is possible to obtain the internal loads as strain.
However, the measurement technique needs to be validated and the results
presented are an important contribution to that. Also, the sensor integration
to the workpiece needs to be improved for future investigations to obtain the
real strain on the workpiece in general and not only on the sensor inlay. So,
as mentioned before, the material transition effect should be minimized. The
experimental results presented here can be considered to be an important step
towards the development of the sensor integration for in situ measurement
technology and transferring it to a real machining process. Future work needs
to concentrate on complete integration of the sensor in the actual workpiece.
Hence, adapting the material properties of the workpiece to make it similar
to the sensor inlay material is an important measure to realize a nearly
homogenous internal load distribution which the sensor layer can measure. In
this way, one can ensure that the sensor layer is able to measure actual
strain in the workpiece and not just an equivalent strain on its surface. The
measurement of the process forces does not indicate that there is a complete
continuum between the workpiece and the sensor inlay due to the increase in
forces when the grinding wheel passes the sensor inlay. This should be
improvised as it is probably an effect of the change in the contact width
Experimental data containing the maximum bridge voltage acquired by the data acquisition system in real time during measurement by both sensors simultaneously, the converted strain values and the maximum external forces (normal and tangential) for both experiments have been made available in Excel files (see Supplement).
The authors declare that they have no conflict of interest.
The authors thank the Deutsche Forschungsgemeinschaft (DFG) for funding the present work in subprojects C04 and F01 within the transregional Collaborative Research Center SFB/TRR 136. Edited by: Andreas Schütze Reviewed by: two anonymous referees