Characterization of Diesel Injectors using Piezoresistive Sensors

Erwin Peiner Department of Electrical Engineering, Information

Technology, Physics TU Braunschweig, University of Technology

Braunschweig, Germany e.peiner@tu-bs.de

Lutz Doering Department 5.1 Surface Metrology

Physikalisch-Technische Bundesanstalt (PTB) Braunschweig, Germany

Abstract—A slender piezoresistive sensor was described which can measure the extreme 3D surface topography of microscale mechanical parts. Non-destructive quality testing of, e.g., high-aspect-ratio micro holes is insufficient since suitable sensors are not available. As a consequence more than 50 million fuel injectors are shipped every year but the spray holes which determine through their geometry both efficiency and exhaust emission of combustion engines are not fully tested. Recently, we described a piezoresistive cantilever as the only sensor so far for measuring form and roughness directly inside injector nozzle holes. Based on our initial design sensors are now available from a pilot production using a silicon wafer scale process. They show much improved performance (vertical/lateral resolution, speed, noise, light sensitivity) enabling us for the first time to perform a comprehensive study with the actual generations of state-of-the art injector nozzles as well as prototype holes.

I. INTRODUCTION Injector spray holes of ever decreasing diameter determine

the efficiency of direct injection (DI) combustion engines through their geometry. In Fig. 1 average diameters and numbers of spray holes are given for recent/future generations of diesel DI nozzles [1]. Optimizing inlet chamfer radius from zero to tens of microns and roughness from > 1 µm to few hundreds of nanometers by abrasive machining while leaving the diameter almost unchanged can save fuel consumption by 7 % and reduce NOx and soot emission by 12 % and 40 %, respectively [2]. Nevertheless, so far industry relies on spot testing using destructive methods to control the surface quality of spray holes fabricated using electro discharge machining (EDM) (Fig. 2). To enable in-process control of micro hole fabrication nondestructive testing (NDT) methods are needed which have to offer

• a detection limit better than 10 % of the manufacturing tolerance of 0.1 µm, i.e., a measurement accuracy of 10 nm and

• an evaluation of a nozzle completed within less than minute, i.e., a measurement duration of tens of seconds or less.

Figure 1. Diameters and numbers of spray holes of DI nozzle spray holes [1].

Figure 2. Scanning electron micrographs (SEM) of cross-sectioned micro holes in steel revealing different surface quality.

This work was supported in part by the German Federal Ministry of Education and Research (BMBF) in the framework of the collaborative project “Prüfung und Bewertung geometrischer Merkmale in der Mikrosystemtechnik (µgeoMess)” within the funding cluster “Mess- und Prüftechnik für den Test von Mikrosystemen” (MSTPrüf) under no. 16SV1944.

978-1-4244-8168-2/10/$26.00 ©2010 IEEE 2083 IEEE SENSORS 2010 Conference

Recently, NDT methods based on X-ray computer tomography (XRCT), white light interferometry (WLI), and low-force tactile probing have been investigated to measure the form of deep micro holes [3-9]. Tactile measurement of such high-quality surfaces recommended a contact radius of 2 µm and a maximum spacing of sampled data points of 0.5 µm according to the ISO standards. Fast scanning ability at 1 mm/s was required corresponding to a detection bandwidth of 2 kHz. Furthermore, low replacement cost of broken or worn-out sensors is mandatory to production control at high-throughput, i.e., there was a need for batch-fabricated sensors.

Contour and roughness measurements of diesel injector spray holes were for the first time demonstrated using a high-aspect-ratio piezoresistive silicon cantilever sensor which was able to access 1.1 mm-deep spray holes and to trace the surface via a tip non-destructively ([10], Fig. 3). To achieve the necessary resolution of < 5 nm the piezoresistive read-out had to fulfill challenging specifications with respect to noise (< 1 µV/V), sensitivity (> 0.2 µV/nm), light sensitivity and temperature drift which depended on a proper design. However, the full potential of NDT using piezoresistive cantilever sensors could only be demonstrated by systematic investigations of the spray hole surface topography of various state-of-the-art diesel nozzles which was addressed in the present study.

Figure 3. Piezoresistive sensor accessing a fuel injector nozzle spray hole

II. EXPERIMENTAL Slender silicon cantilevers from a pilot wafer-scale process

were used having a length l = 1.25 mm, a width w = 30 µm, and a thickness h = 25 µm (Fig. 4, upper). The cantilever comprised a tip and a piezoresistive strain gauge at its free (lower left) and fixed end (lower right), respectively. Contact of the tip with the sample surface was recorded via an integrated piezoresistive strain gauge (full Wheatstone bridge) which was connected to an instrumentation amplifier (HBM, ML 10B). A time-dependent deflection δ(t) imposed to the tip during surface scanning generated an output signal V(t) which vice versa represented the surface profile:

( )BBB

BB

VtV

ww

lhK

ww

llllt )(

23131)( 22

Δ×⎟⎟⎠

⎞⎜⎜⎝

⎛⎥⎦

⎤⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛−−−=δ . (1)

with the piezoresistive gauge factor K and the strain gauge supply voltage VB. At its fixed end where the strain gauge was accommodated the cantilever was widened to wB = 200 µm over a length lB = 250 µm.

Figure 4. Slender piezoresistive cantilever sensor.

The specs of sensor prototypes (CiS GmbH, Erfurt, Germany) are given in Table II: Sensor responsivity amounted to 4 nm/ µV/V while offset drift, light susceptibility, and noise were in the range of few µV/V which correspond to a vertical resolution of better than 10 nm according to (1). Tip height, cone angle, and radius of 15 - 25 µm, ~ 40° and 0.05 - 1 µm, respectively, were found which limit vertical range and lateral resolution. Regarding the expected roughness of Ra = 0.1 - 1 µm we could expect that the requirements of characterizing the finish of high-quality surfaces of micro bores can be fulfilled according to the ISO standard. Large values of resonance frequency, over-range capability and wear resistance were specified being prerequisite to high-speed scanning operation. Finally, small temperature dependences of the bridge resistance and the offset voltage were reported within a wide temperature range alleviating the need for stable ambient conditions or elaborate compensation methods.

For systematic measurements with injector nozzle spray holes a home-made setup was employed based on a piezo positioning stage featuring a travel range of 800 µm at sub-nanometer resolution (P-628.2CD with digital piezo controller PI 710, Physik Instrumente, Germany). Blind navigation was necessary during accessing the hole which was possible without the risk of crashing the probe: when the cantilever tip hit the bore wall or an obstacle within the hole a characteristic signal response was detected indicating the actual probe position and deflection condition [11]. After the starting position for a scan was reached the tip was brought into contact with the hole wall. During the subsequent surface

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profiling temperature fluctuations even under normal laboratory conditions could be neglected due to the short typical durations of few seconds necessary for scanning along the axis of a 1-mm-deep hole.

TABLE I. MEMS SENSOR SPECS

Parameter Unit Value

Operation voltage (max.), VB V_DC 1.5

Bridge resistance, @ 30°C kΩ 2.13 ± 0.02

Bridge offset, @ 30°C mV/V -1.5 ± 2

Responsivity nm/µV/V 4.0 ± 0.3 Offset stability, @ 30 h, 30°C µV/V < ± 2 Light susceptibility, @ cold light, 0.1 mW/cm2 µV/V 0.33/2.3a

Noise, @ 1.6 kHz, amplifier 0.4 µV/V µV/V 0.6/0.7a

Tip height µm 15 - 25

Cone angle ° 40 ± 2

Tip radius µm 0.05 - 1

Resonance frequency kHz 46.4

Fracture limit, @ axial/lateral/vertical mm 0.17/0.52/0.47

Tip wear per scan length, @ 0.1 mN nm/m < 0.1

Operation temperature °C -20 … +80

Temperature coefficient %/10 K 3.1 ± 0.1

Temperature coefficient offset µV/V/K -2 ± 10

a. Contact to bridge: Al/p+ silicon

III. RESULTS Prior to surface topography measurements the noise

behavior of sensor prototypes used in the present study was investigated. Figure 5 shows the frequency dependence of the noise spectral density of a full-bridged piezoresistive strain gauge connected to an instrumentation amplifier. We found low values of the 1/f noise corner frequency of 10 Hz and the Hooge parameter of 1.8 × 10-6 [12]. The integrated read-out noise in a bandwidth from 1 mHz to 1.6 kHz was less than 1 µV/V corresponding to a vertical resolution of less than 4 nm according to (1). The basic requirements for a systematic investigation of the spray hole surface topography of state-of-the-art diesel nozzles were fulfilled.

In Fig. 6 the surface contour of a standard diesel nozzle spray hole is displayed as obtained by axial scanning using the piezoresistive sensor at various speed. Its form is characterized by long-range waviness and short-range roughness features [13]. The waviness profile was separated from the roughness by long-pass filtering of the primary profile using a cut-off wavelength λc. The roughness profile is then obtained as the difference between the primary profile and the waviness profile. A software package provided by the PTB was used (RPTB-WEB 0.11, [14]). Primary and waviness profiles calculated using λc = 0.25 mm are superimposed in Fig. 6.

The features of waviness and roughness profiles agree very well in both position and height. For quantitative comparison

values of the arithmetical mean roughness Ra and the average peak-to-valley roughness Rz were determined from the roughness profile according to ISO 4287. Average values of 418 nm and 2.7 µm with maximum deviations of less than 17 nm and 0.5 µm, respectively, were obtained for Ra and Rz indicating the small uncertainty of the method.

Figure 5. Noise curve of a piezoresistive cantilever sensor (2.65 kΩ).

Figure 6. Typical spray hole surface profiles measured at different scanning speed.

Sampling rate and contact force are the parameters which have in combination with the scanning speed an influence on the accuracy of tactile roughness measurements. Both parameters must be selected high enough to ensure that scanning resolves all surface features. According to the ISO standards a data point density of 2/µm and a probing force of 5 mN were favorable settings for roughness measurements using conventional stylus profilometers having a tip radius of 2 µm. To check if these values can be transferred to the

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piezoresistive cantilever sensor we performed scans at various probing forces and sampling distances on standard roughness artifacts (Ra ∼ 600 nm, Rz ∼ 3.7 µm) [15]. A scan length of 0.9 µm was selected which was within the depth range of state-of-the-art spray holes. We found that the uncertainty of Ra and Rz was around 10 nm and 0.2 µm, respectively, at data point densities of at least 2/µm and probing forces of at least 0.25 mN. These results were taken into account for all following roughness measurements with micro holes. The much lower contact force with respect to standard stylus measurements can be assigned to the smaller tip radius of the micro machined piezoresistive cantilever sensor.

In a comprehensive set of measurements spray hole roughness and waviness were determined for various state-of-the-art valve controlled orifice (VCO) and micro sac injector nozzles showing a clear improvement with decreasing diameter (Fig. 7). However, at the same time we observed, related to the hole manufacturing process, drastically impaired uncertainties from nozzle to nozzle, from hole to hole, and within one and the same hole. In the latter case a significant increase of roughness was observed von a hole close to its inlet end. Since the holes were fabricated starting from the outlet to the inlet end this indicated a deterioration of the erosion process with time. Surface quality control appears to become more and more important for the manufacturing process when the structure sizes proceeds to decrease.

Figure 7. Roughness of state-of-the art commercial fuel injector nozzle spray holes.

To check potential of the sensor in course of further reduction of the hole size we investigated the characterization of smaller holes, e. g. for next-generation nozzle designs beyond the current fuel emission standards. In Figure 8 the results of in-hole measurements with test samples are collected. In the upper part top-view micrographs of the holes fabricated at different EDM parameters are displayed. The outer left and right holes correspond to the upper and lower SEM graphs, respectively, in Fig. 2. The data points are related to one sample, respectively, and all but one represent the average and standard deviation of 15 scans. We found that the sensor can detect the differences in the surface quality of

the samples according to the expectation from the conventional destructive analysis.

We investigated NDT of micro holes using the novel non-destructive technique in comparison with stylus probe measurements with sectioned samples, i. e. the conventional destructive method. A good correlation was found between both methods for both Ra and Rz but the differences in surface finish of the investigated holes were more clearly seen using the piezoresistive cantilever sensor. We conclude that the tip size, angle and radius of the MEMS sensor were better matched to the micro hole form and roughness compared with standard stylus instruments.

Figure 8. Micro bores in steel plates (0.7 mm): Top-view optical micrographs (upper) and roughness determined by NDT using piezoresitive

cantilever sensors.

IV. DISCUSSION In Table II the most important features of state-of-the-art

probes for NDT inside deep micro bores were compared with the characteristics of the piezoresistive cantilever sensor described in this study. White-light-interferometric (WLI) [4] and optical-tactile [5] detection principles were selected as examples for high-performance fiber probes which were tested with a micro bore standard artifact and an injector nozzle spray hole, respectively. For all sensors it was demonstrated that they can access holes of depth and diameter of ~ 1 mm and 100 - 160 µm, respectively.

Scanning ability at high speed was reported for the non-contact WLI probe [4]. However, roughness evaluation was not demonstrated so far which may be attributed to the susceptibility of WLI to skewing [16].

In the case of the optical-tactile probe the large contact radius is another obstacle to roughness measurements. A radius of 2 µm which is recommended for high quality surfaces according to ISO 3274 currently appears not to be feasible for this type of sensor. Scanning in continuous contact causes stick-slip which is a major problem of optical-tactile fiber sensors especially when a small probing sphere is used [17]. Finally, high replacement cost must be expected for both the WLI and the optical-tactile sensor due single-part manufacturing.

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Systematic NDT experiments with diesel spray holes of a variety of VCO and micro sac injector nozzles were only be demonstrated so far using the piezoresistive silicon MEMS cantilever sensor which was described in this study. It was shown that roughness parameters can be accurately determined according to the requirements of the ISO standards. Instead of a glued sphere wet etched silicon tips of submicron tip radii were used which are monolithically integrated with the cantilever in a standard batch fabrication process. Its small mass and high stiffness indicated by the large resonance frequency enables scanning in a range of 1 µm/s to 1 mm/s without interfering stick-slip or inertia effects. Sensors tested were from a pilot 4” wafer batch process offering the potential of replacement purchase of broken or worn out sensors at low cost.

TABLE II. PERFORMANCE OF DIFFERENT SENSORS FOR NDT OF THE FORM OF DEEP MICRO HOLES.

Sensor Bosch, WhitePoint

Werth Fiber Probe (WFP)

MEMS cantilever

Reference [4] [5] this work

Principle white-light- interferometry optical-tactile piezoresistive-

tactile

Artifact

PTB micro-hole standard: Ø = 150 µm, d = 0.8 mma

injector nozzle: Ø = 160 µm, d = 0.9 mma

injector nozzles: Ø = 195-114 µm, d = 1.1 - 0.7 mma

test plates: Ø = 100 µm, d = 0.7 mma

Roughness measurem.

susceptible to skewing [16] not demonstrated Ra, Rz

Contact radius non contact 37 µm < 1 µm, fulfills

ISO standard

Scanning/ data rate

1000 rpm, 20 kHz

susceptible to stick-slip [17]

1 mm/s 19.2 kHz

Expected replacment cost

high high low, 4“-wafer fabrication

a. d: depth of bore

V. CONCLUSIONS The feasibility of non-destructive tactile surface

profilometry inside deep micro holes was investigated for the first time in a comprehensive study with diesel injector nozzles. The limits of the NDT method, sensor geometry and performance were evaluated with respect to competing sensor concepts. Its potential for a systematic improvement of micro bores was demonstrated with bores of 100-200 µm in diameter fabricated using electro discharge machining. The results were validated in comparison with the standard destructive roughness measurement method.

ACKNOWLEDGMENT The authors are grateful to the technical assistance by

Doris Rümmler, Daniel Günther, Julian Kähler, Stefan Kahmann, and Sisi Wu.

REFERENCES [1] D. Jung, W. L. Wang, A. Knafl, T. J. Jacobs, S. J. Hu, and D. N.

Assanis, “Experimental investigation of abrasive flow machining effects on injector nozzle geometries, engine performance, and emissions in a DI Diesel engine”, Intern. J. Automotive Technol., vol. 9, pp. 9-15, 2008.

[2] G. Cusanelli, “Micro holes feasibility with regard to diesel and gasoline direct-injection needs,” Engine Expo 2009, Stuttgart, June, 16-18, 2006, http://www.engine-expo.com/forum_2009/pdfs/day1/8_dr_guiseppe_cusanelli.pdf

[3] D. Lazaro, S. Legoupil, G. Blokkeel, and B. Jeanne, “Metrology of steel micro-nozzles using X-ray microtomography”, DIR 2007 - International Symposium on Digital industrial Radiology and Computed Tomography, June 25-27, 2007, Lyon, France, http://www.ndt.net/article/dir2007/papers/13.pdf

[4] R. Kochendörfer, “White Point Interferometrisches Messsystem für Form und Lagemessungen“,

[5] http://www.visquanet.de/portals/visqua/story_docs/vortraege_2009/090617_adaptools_hausmesse/090617_03_kochendoerfer_bosch.pdf

[6] C.-C. Kao and A. J. Shih, “Form measurements of micro-holes”, Meas. Sci. Technol., vol. 18, pp. 3603–3611, 2007.

[7] B. Muralikrishnan, J. Stone, and J. Stoup, “Diameter and form measurement of a micro-hole in a fuel injector nozzle with the NIST fiber probe”, Proceedings of the Annual Meeting of the American Society for Precision Engineering, Monterey, CA, October 6-8, 2009, http://www.nist.gov/customcf/get_pdf.cfm?pub_id=902815

[8] M. B. Bauza, S. C. Woody, B. A. Woody, S. T. Smith, “Surface Profilometry of high aspect ratio features”, Wear, 2010, in press, doi:10.1016/j.wear.2010.03.028

[9] J. D. Claverley and R. K. Leach, “A vibrating micro-scale CMM probe for measuring high aspect ratio structures”, Microsyst. Technol., vol. 16, pp. 1507–1512, 2010.

[10] Y. Mita, J.-B. Pourciel, M. Kubota, A. Higo, S. Ma, S. Morishita, M. Sugiyama, and T. Masuzawa, “An Active Swing Probing Method for High Aspect Ratio Deep Hole Profiler”, MicroMechanics Europe 2009, September 20-22, Toulouse, France, D12, 2009.

[11] E. Peiner, M. Balke, and L. Doering, “Slender Tactile Sensor for Contour and Roughness Measurements Within Deep and Narrow Holes”, IEEE Sensors J., vol. 8, no. 12, pp. 1960-1967, December 2008.

[12] E. Peiner and L. Doering, “MEMS cantilever sensor for non-destructive metrology within high-aspect-ratio micro holes”, Microsyst. Technol., vol. 16, pp. 1259-1268, 2010.

[13] A. A. Barlian, Woo-Tae Park, Joseph R. Mallon, Jr., Ali J. Rastegar, and Beth L. Pruitt, “Review: Semiconductor Piezoresistance for Microsystems”, Proc. IEEE, vol. 97, pp. 513-552, March 2009.

[14] E. Peiner, M. Balke, and L. Doering, "Form Measurement Inside Fuel Injector Nozzle Spray Holes", Microelectron. J., vol. 86, pp. 984-986, 2009.

[15] http://www.ptb.de/en/org/5/51/517/rptb_web/wizard/greeting.php [16] E. Peiner, L. Doering, and A. Stranz, "Surface finish improvement of

deep micro bores monitored using an active MEMS cantilever probe", Intern. Conf. Industrial Technol. (IEEE-ICIT 2010), Viña del Mar - Valparaíso, Chile, 14 -17 March 2010.

[17] T. V. Vorburger, H.-G. Rhee, T. B. Renegar, J.-F. Song, and A. Zheng, “Comparison of optical and stylus methods for measurement of surface texture”, Int. J. Adv. Manuf. Technol., vol. 33, pp. 110–118, 2007.

[18] F. Marinello, E. Savio, S. Carmignato, and L. De Chiffre, “Calibration artefact for the microscale with high aspect ratio: The fiber gauge”, CIRP Annals - Manufacturing Technology, vol. 57, pp. 497–500, 2008.

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