- Email: [email protected]
A simple technique for automated performance testing of piezoelectric micro-motors by transient motion analysis
Available online at www.sciencedirect.com
Sensors and Actuators A 144 (2008) 130–134
A simple technique for automated performance testing of piezoelectric micro-motors by transient motion analysis P.J. Rayner a , S.A. Wilson a,∗ , R.W. Whatmore a , M.G. Cain b a
Materials Department, School of Applied Sciences, Cranfield University, Cranfield, Bedfordshire MK43 0AL, United Kingdom b National Physical Laboratory, Teddington, Middlesex TW11 0LW, United Kingdom Received 24 July 2007; received in revised form 1 November 2007; accepted 3 December 2007 Available online 15 December 2007
Abstract An automated method of testing a 5 mm diameter flextensional ultrasonic motor has been assembled using off-the-shelf components, which derives performance measurements from the transient start up and stopping behaviour of the motor using a non-contact position encoder. The method gives full performance characteristics in less than a second and it is scaleable for use with other sizes and other types of motor. Furthermore, it provides a relatively simple technique for non-contact measurements of position, angle or velocity that can be rapidly cycled as part of a development programme. The method has shown sufficient sensitivity, using basic components, to measure torque in the range 10–200 N m and speed in the range from 0 to over 2000 rpm. The torque and speed ranges are only dependant on the sensitivity of encoder instrumentation, inertial load on the motor and data acquisition. This system is believed to be a significant improvement over previously reported work that has detailed friction brake or manual ‘one-off’ systems and the test format has greater applicability for micro-motors that are used for their positioning capabilities. Crown Copyright © 2007 Published by Elsevier B.V. All rights reserved. Keywords: PZT; Actuator; Flextensional; Ultrasonic; Amplifier
1. Introduction Piezoelectric ultrasonic motor technology [1] is becoming more widespread in commercial and industrial applications and it is now acknowledged that such devices have advantages over their electromagnetic counterparts in a variety of situations. The motors typically consist of a piezoelectric vibrator operating in one or more vibration modes at an ultrasonic frequency which can couple to a rotor or slider that is pressed into frictional contact. Small repetitive movements of the order of 0.1–2 m, at frequencies between 20 kHz to 1 MHz, drive the rotor via frictional coupling. The piezoelectric vibrator is driven at resonance by an AC sine or square signal, giving macroscopic behaviour that is broadly similar to that of a DC motor. However, ultrasonic motors have key features of higher torque, lower speed and rapid speed response. Braking torque with no power applied, can be compared to using a gear box of 10–50x with an equivalent sized DC motor, but with the advantage of <1 ms response to full speed.
∗
Corresponding author. Tel.: +44 1234 750111; fax: +44 1234 751346. E-mail address: [email protected] (S.A. Wilson).
The authors have developed a miniature planar motor (see schematic in Fig. 1), which uses a flextensional diaphragm to rectify the piezoelectric vibrations and couple them mechanically to a rotor by means of inclined rigid fins. The ‘Flexmotor’ can be scaled in size between 2 and 25 mm in diameter, and it is constructed simply from pressed metal parts and either a disc or square of PZT [2–5]. The simplicity of the concept, which involves no 3D coil windings, a low part count and low drive voltages at small sizes, provides significant benefits when using the motors in micro-scale applications (5 mm diameter or less). In contrast electromagnetic motors often require an additional gearbox at this scale. The performance of such micro-scale piezoelectric motors can be characterised primarily in terms of their torque, speed and efficiency. In order to be meaningful these parameters must, however, be measured concurrently and an effective test system is therefore essential for design, development, device optimisation and assessment of reliability. At the micro-scale conventional methods of testing rotary motors, such as pulley brakes, are not easy to implement, mainly due to size constraints and also to the difficulty of measuring very small dynamic forces. This paper describes an automated test system for a 5 mm diameter variant of the Flexmotor which was created as a development
0924-4247/$ – see front matter. Crown Copyright © 2007 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2007.12.002
P.J. Rayner et al. / Sensors and Actuators A 144 (2008) 130–134
131
Fig. 1. Flexmotor–ultrasonic micro-motor.
aid. Basic functions such as torque and speed can be evaluated and the system has the advantage that higher level functions such as frequency operating point tests and reliability are easily programmable and quick to perform. A practical motor test system has to measure the speed of the motor at a given torque loading. For micro-motors in particular torque measurement is the primary obstacle; requiring measurements typically in the range 10–200 N m with a resolution of 1 N m, which equates to a linear force resolution of 0.4 mN at the outer radius of a 5 mm diameter motor. There are two main strategies that can be used to measure motor performance. These are: • Steady state tests (‘pulley brake’ or dynamometer) o Applying a measured braking torque and measuring the speed. • Transient tests o Obtain the acceleration of the motor with an known inertial load during starting and stopping. o Calculate the torque delivered and its change with repeated cycling. 2. Overview and equipment Steady state systems [6,7] require specialist force sensors and equipment to measure the torque applied. Such systems have inherent difficulties in scaling down to smaller sizes; for example, in applying a braking torque via a fine wire or via non-linear viscous coupling to a fluid (air or liquid). These systems are best for ‘prime mover’ or constant speed motor uses, although during testing significant time can be required to adjust the braking force applied over a motor’s full range. In comparison, the transient system is highly scalable, requiring only a non-contact position or speed sensor and a known inertial mass. Most micro-motors will eventually be used as positioning servos and this method allows for rapid testing (<1 s) in this operation mode. Whilst other transient systems have used a vibrometer [8] to measure rotor velocity, we have automated the method with software, using standard industrial light gates and a simple encoder disc to generate sufficient positional information for accurate performance estimation. Fig. 2 shows the detail of the etched encoder disc (20 mm diameter, 0.1 mm thick CuBe 2%) with 100 slots and the probes of a Keyence optical light gate FS-M1H. The polar moment of
Fig. 2. Schematic of encoder disc, optical probes and motor.
inertia of the rotating mass is 1.086 × 10−8 kg m2 with 98.2% of this due to the encoder disc. A diagram of the system layout is given in Fig. 3 showing the PC, data acquisition hardware. A mechanical clamp is used for mounting the motors and applying a preload force of 0–1.5 N pressing the rotor disc into frictional contact with the piezoelectric stator. Preload is measured by a force sensor. The data acquisition system includes an NI5401 function generator and amplifier to provide a triggered drive signal of between 1 and 60 Vpp at 0–1 MHz, an NI5102 15 MHz scope card for measuring input voltage and current and a PCI6023E DAQ card for sampling force sensors and the position encoder.
Fig. 3. Automated test system hardware layout.
132
P.J. Rayner et al. / Sensors and Actuators A 144 (2008) 130–134
2.1. Transient motor theory Ultrasonic motor performance in running approximates to that of DC motors with torque reducing linearly as speed increases. This gives the characteristic ‘capacitor charging’, (exponential) variation [9] in the rotor velocity. Whilst this is a simplified model without higher order effects [10] it provides reasonable accuracy for the essential performance parameters of Ωmax , Tstall , ηmax , Thold which are respectively the no-load speed, stall torque, maximum efficiency and the holding torque. The starting behaviour of the motor is described by: T (1) Ω = Ωmax 1 − Tstall which gives the rotor velocity as: t Ω = Ωmax 1 − exp − τstart
Fig. 4. Data processing schematic.
(2) 3. Motor test results
where: τstart
JΩmax = Tstall
(3)
During the motor stopping period, power is removed and the equation of motion is a constant deceleration due to friction alone: ˙ = −Tbrake = −μFpreload r Ω J J
(4)
where Fpreload and r are the preload force and contact radius. The holding torque (dynamic friction) can be estimated by using the time for the motor to stop (τ stop ): Thold =
JΩ0 τstop
(5)
The 5 mm Flexmotor is driven at 20 Vpp using a 270 or a 380 kHz sine (or square) wave, typically with a 0.4–0.8 N preload and giving a maximum no-load speed of up to 5000 rpm. The inertial load provided by the encoder disc was selected to increase the start up time constant of the motor to 10–60 ms (equivalent to 100–200 encoder slots in rotation during start up) starting with an assumption that motor stall torque would fall approximately in the range of 10–200 N m. Sampling speeds are set at 3 MHz for electrical signals and 50 kHz for the encoder and force sensors. As shown in Fig. 4, the captured data is divided into start and stop regimes and the rotor velocity data is derived from the position encoder time series by a numerical differentiation algorithm which linearly interpolates the digital position information. The rotor velocity is then fitted to the theoretical exponential equation (or an alternative polynomial fit), and then the fitted data is again differentiated to give rotor acceleration. The rotor angular acceleration is directly related to the motor torque, thereby allowing a full performance plot. Electrical power consumption has also been added to create an efficiency plot concurrently.
A sample of a test on a 5 mm Flexmotor driven at resonance at 383 kHz by a 16 Vpp square wave is given in Fig. 5 and the corresponding performance plot in Fig. 6. The system demonstrates that this motor has peak efficiency in this test of 5%, with a stall torque of 0.13 mN m and a no-load speed of around 480 rad/s. The stopping transient (occurring after power off at 0.24 s) gives a braking torque of 0.26 mN m defined by the gradient of the stopping deceleration. Time constants are 32 ms starting and 20 ms stopping with the encoder to give around 0.56 and 0.35 ms for motor only, respectively. The motor efficiency is predicted to be increased by design adjustments, friction material changes and by improvement of mechanical rotor/stator axial alignment in the test rig. A realistic prediction of the system efficiency of a motor of this type and size is at least 30%. 4. Discussion The system produces good quality estimates of the motor performance with the key advantage of high speed data acquisition. Use of a standard digital encoder provides a trade-off between the position resolution that is necessary to give sufficient definition during the transient movements and the no-load speed detection. The system was originally designed for 200 rad/s and hence some quantisation in velocity can be seen in Fig. 5. Using a 50 kHz sample rate there is an upper limit on encoder slot events of around 10 kHz before quantisation noise develops significantly. Faster data capture speed would remove this effect for the high speed range or alternatively, increasing the rotor inertia will lower the position resolution (and hence slot count) requirement. However, the high speed rotor data equates to the no-load speed (or low torque) condition in starting. In practice, this is of lesser interest than accurately defining the early start up to a speed of Ωmax (e − 1)/e. The main sources of error in the test system lie in the accuracy of calculating the rotor inertia from its dimensions, accuracy of curve fitting and accommodating low level torque variation
P.J. Rayner et al. / Sensors and Actuators A 144 (2008) 130–134
133
Fig. 7. Reliability tests over 50,000 start–stop cycles.
with position. The value determined for inertia is almost wholly dependant on the precise measurement of the encoder radius. An error of 100 m in radius, for example, would contribute to 2.5% systematic error in polar inertia. Encoder dimensions and density were cross-checked here with the actual rotor mass. 4.1. Higher level tests: reliability and parameter sweeps
Fig. 5. Sample test results of a 5 mm Flexmotor: (a) time series and (b) polynomial fit to motor velocity.
The modular software allows the test routine to be called repeatedly by a higher level program, taking the key values of each test: Ωmax , Tstall , ηmax , Tmax , Thold , τ stop and the preload imposed and logging these values over time. An example is shown in Fig. 7 of a reliability test for a different 5 mm micromotor, showing no-load speed and stall torque during ‘bedding in’ of the motor followed by long term steady behaviour. The long term behaviour also demonstrates the repeatability of performance prediction measurements when the motors variations are analysed: ±10 rad/s (∼7%) in no-load speed and ±5 N m (∼8%) in stall torque. Similarly the input parameters such as drive voltage, frequency and preload pressure can be adjusted, whilst the test procedure is quickly repeated, giving parameter maps and optimisation of drive conditions for a motor design. When compared to electromagnetic motors, ultrasonic motors have an extra important parameter which is the resonant frequency of the drive. This can easily show variability from part to part. An automated frequency-sweep test allows for individual motors and motor components to be compared for performance to give information for part aging and drive control schemes. 5. Conclusion
Fig. 6. Performance plot for 5 mm Flexmotor during starting.
An automated test system for micro-motors, which uses a transient testing strategy, has been presented and demonstrated though performance characterisation of 5 mm ultrasonic motors with torque outputs of 10–200 N m. The methods used can be flexibly applied to ultrasonic and DC motors and scaled to smaller micro-motors with relative simplicity, using readily available analogue or digital position/velocity detection methods. Whilst steady state methods may provide superior measurement accuracy over transient tests, the method used here provides greater flexibility and much faster acquisition time. Higher level functions such as reliability and operating parameter searches can realistically be performed owing to the fast test time.
134
P.J. Rayner et al. / Sensors and Actuators A 144 (2008) 130–134
References [1] K. Uchino, Piezoelectric Actuators and Ultrasonic Motors, Kluwer Academic, Boston MA, 1997. [2] P.J. Rayner, R.W. Whatmore, Piezoelectric ultrasonic motor using flextensional amplification of a disc radial mode with elastic fin drive, in: Proceedings of the 9th US–Japan Seminar on Dielectric and Piezoelectric Ceramics, Okinawa, Japan, November 2–5th, 1999, pp. 151–154. [3] J.T. Leinvuo, S.A. Wilson, R.W. Whatmore, Flextensional piezoelectric motor using the contour mode of a square piezoelectric plate, IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control UFFC-51 (2004) 929–936. [4] J.T. Leinvuo, S.A. Wilson, R.W. Whatmore, M.G. Cain, A New flextensional piezoelectric ultrasonic motor—design, fabrication and characterisation, Sensors and Actuators A – Physical 133 (January (1)) (2007) 141–151. [5] J.T. Leinvuo, S.A. Wilson, R.W. Whatmore, M.G. Cain, Flextensional ultrasonic piezoelectric micro-motor, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control UFFC-53 (December (12)) (2006) 2357–2366. [6] P. Wurmsdobler, R. Duffait, T. Messaoudi, W. Brenner, A torque testing stage for rotating micro-electro-mechanical devices, in: Proceedings of the 9th Int. Symposium on Transport Phenomena and Dynamics of Rotating Machinery (ISROMAC-9) Honolulu, Hawaii, USA, 10th–14th February, 2002. [7] H. Ota, T. Ohara, Y. Karata, S. Nakasima, M. Takeda, Novel micro torque measurement method for micro-devices, Journal of Micromechanics and Micro-engineering JMM-11 (2001) 595–602. [8] J. Friend, K. Nakamura, S. Ueha, A Piezoelectric micro-motor using in-plane shearing of PZT elements, IEEE/ASME Transactions on Mechatronics TM-9 (3) (2004) 467–473. [9] S. Ueha, Y. Tomikawa, Ultrasonic Motors: Theory and Applications, Clarendon Press, Oxford, 1993. [10] T. Takano, Y. Tomikawa, M. Aoyagi, C. Kusakabe, Transient response characteristics of a same-phase drive-type ultrasonic motor, Japanese Journal of Physics JJP-33 (1994) 5370–5373.
Biographies Philip J. Rayner has over 7 years experience in micro-technology and piezoelectrics, focusing on product development in fibre optic telecommunications, spectroscopy and MEMS. His current specialities are in automation and robotics, smart materials and sensors. Philip completed his PhD in micro-engineering and
nanotechnology at Cranfield University in 2000 having previously studied engineering (ME) at Cambridge University. He is a consultant and MD with Tecray Ltd. in Peterborough, UK. Stephen A. Wilson completed his PhD studies at Cranfield University in 1998 conducting original research into the processing of piezoelectric ceramicpolymer composite materials. His thesis introduced a valuable new method for composite assembly termed ‘electric-field structuring’ which has been patented world-wide with uses in assembly of novel highly engineered composite sandwich panels with integrated functional elements. As Senior Research Fellow in the School of Applied Sciences, he has worked on materials development to optimise the performance of advanced sensor arrays for real-time 3D ultrasonic imaging (heart monitor). His work on advanced materials processing techniques has contributed to a new integrated fabrication route for thick film piezoelectric micro-actuators, which uses ultra-precision machining of commercial bulk ferroelectric ceramics in combination with standard micro-fabrication techniques. Roger W. Whatmore graduated with his PhD from Cambridge University in 1977 and spent nearly 20 years working with the GEC Marconi (formerly Plessey) research laboratories at Caswell in the UK on the development and exploitation of ferroelectric materials in a wide range of electronic devices, particularly sensors and actuators, for which work he was awarded GEC’s Nelson Gold Medal in 1993. From October 1994 until January 2006 he held the Royal Academy of Engineering Chair in Engineering Nanotechnology at Cranfield University, where he lead a group developing the use of ferroelectrics in micro-systems and nanotechnology. He is currently Chief Executive Officer at the Tyndall National Institute in Cork, Ireland. He has published over 200 papers and 30 patents in the field. He is a Fellow of the Royal Academy of Engineering and a Fellow of the Institute of Materials, Minerals and Mining who in 2003 awarded him the Griffith Medal for Excellence in Materials Science. Markys G. Cain graduated with his PhD from Warwick University in 1990 and spent the next 2 years in the Materials Department of the University of California, Santa Barbara studying thin film epitaxial science. Subsequent research in ceramic composite materials technology in the UK utilised many of the principles learnt at Santa Barbara in the deployment of new interfacial fibre coatings for advanced high temperature ceramic matrix composites for gas turbine applications. Research with Oxford Instruments led to the development of an SEM based instrumented indentation system and he joined NPL in 1997 to lead the Functional Materials Research group. Research activity includes the development of measurement methods to elucidate materials behaviour in ferroelectric and piezoelectric ceramics and thin film materials. He has published over 80 papers and 2 patents in the field. He is a member of the Institute of Physics.
Sensors and Actuators A 144 (2008) 130–134
A simple technique for automated performance testing of piezoelectric micro-motors by transient motion analysis P.J. Rayner a , S.A. Wilson a,∗ , R.W. Whatmore a , M.G. Cain b a
Materials Department, School of Applied Sciences, Cranfield University, Cranfield, Bedfordshire MK43 0AL, United Kingdom b National Physical Laboratory, Teddington, Middlesex TW11 0LW, United Kingdom Received 24 July 2007; received in revised form 1 November 2007; accepted 3 December 2007 Available online 15 December 2007
Abstract An automated method of testing a 5 mm diameter flextensional ultrasonic motor has been assembled using off-the-shelf components, which derives performance measurements from the transient start up and stopping behaviour of the motor using a non-contact position encoder. The method gives full performance characteristics in less than a second and it is scaleable for use with other sizes and other types of motor. Furthermore, it provides a relatively simple technique for non-contact measurements of position, angle or velocity that can be rapidly cycled as part of a development programme. The method has shown sufficient sensitivity, using basic components, to measure torque in the range 10–200 N m and speed in the range from 0 to over 2000 rpm. The torque and speed ranges are only dependant on the sensitivity of encoder instrumentation, inertial load on the motor and data acquisition. This system is believed to be a significant improvement over previously reported work that has detailed friction brake or manual ‘one-off’ systems and the test format has greater applicability for micro-motors that are used for their positioning capabilities. Crown Copyright © 2007 Published by Elsevier B.V. All rights reserved. Keywords: PZT; Actuator; Flextensional; Ultrasonic; Amplifier
1. Introduction Piezoelectric ultrasonic motor technology [1] is becoming more widespread in commercial and industrial applications and it is now acknowledged that such devices have advantages over their electromagnetic counterparts in a variety of situations. The motors typically consist of a piezoelectric vibrator operating in one or more vibration modes at an ultrasonic frequency which can couple to a rotor or slider that is pressed into frictional contact. Small repetitive movements of the order of 0.1–2 m, at frequencies between 20 kHz to 1 MHz, drive the rotor via frictional coupling. The piezoelectric vibrator is driven at resonance by an AC sine or square signal, giving macroscopic behaviour that is broadly similar to that of a DC motor. However, ultrasonic motors have key features of higher torque, lower speed and rapid speed response. Braking torque with no power applied, can be compared to using a gear box of 10–50x with an equivalent sized DC motor, but with the advantage of <1 ms response to full speed.
∗
Corresponding author. Tel.: +44 1234 750111; fax: +44 1234 751346. E-mail address: [email protected] (S.A. Wilson).
The authors have developed a miniature planar motor (see schematic in Fig. 1), which uses a flextensional diaphragm to rectify the piezoelectric vibrations and couple them mechanically to a rotor by means of inclined rigid fins. The ‘Flexmotor’ can be scaled in size between 2 and 25 mm in diameter, and it is constructed simply from pressed metal parts and either a disc or square of PZT [2–5]. The simplicity of the concept, which involves no 3D coil windings, a low part count and low drive voltages at small sizes, provides significant benefits when using the motors in micro-scale applications (5 mm diameter or less). In contrast electromagnetic motors often require an additional gearbox at this scale. The performance of such micro-scale piezoelectric motors can be characterised primarily in terms of their torque, speed and efficiency. In order to be meaningful these parameters must, however, be measured concurrently and an effective test system is therefore essential for design, development, device optimisation and assessment of reliability. At the micro-scale conventional methods of testing rotary motors, such as pulley brakes, are not easy to implement, mainly due to size constraints and also to the difficulty of measuring very small dynamic forces. This paper describes an automated test system for a 5 mm diameter variant of the Flexmotor which was created as a development
0924-4247/$ – see front matter. Crown Copyright © 2007 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2007.12.002
P.J. Rayner et al. / Sensors and Actuators A 144 (2008) 130–134
131
Fig. 1. Flexmotor–ultrasonic micro-motor.
aid. Basic functions such as torque and speed can be evaluated and the system has the advantage that higher level functions such as frequency operating point tests and reliability are easily programmable and quick to perform. A practical motor test system has to measure the speed of the motor at a given torque loading. For micro-motors in particular torque measurement is the primary obstacle; requiring measurements typically in the range 10–200 N m with a resolution of 1 N m, which equates to a linear force resolution of 0.4 mN at the outer radius of a 5 mm diameter motor. There are two main strategies that can be used to measure motor performance. These are: • Steady state tests (‘pulley brake’ or dynamometer) o Applying a measured braking torque and measuring the speed. • Transient tests o Obtain the acceleration of the motor with an known inertial load during starting and stopping. o Calculate the torque delivered and its change with repeated cycling. 2. Overview and equipment Steady state systems [6,7] require specialist force sensors and equipment to measure the torque applied. Such systems have inherent difficulties in scaling down to smaller sizes; for example, in applying a braking torque via a fine wire or via non-linear viscous coupling to a fluid (air or liquid). These systems are best for ‘prime mover’ or constant speed motor uses, although during testing significant time can be required to adjust the braking force applied over a motor’s full range. In comparison, the transient system is highly scalable, requiring only a non-contact position or speed sensor and a known inertial mass. Most micro-motors will eventually be used as positioning servos and this method allows for rapid testing (<1 s) in this operation mode. Whilst other transient systems have used a vibrometer [8] to measure rotor velocity, we have automated the method with software, using standard industrial light gates and a simple encoder disc to generate sufficient positional information for accurate performance estimation. Fig. 2 shows the detail of the etched encoder disc (20 mm diameter, 0.1 mm thick CuBe 2%) with 100 slots and the probes of a Keyence optical light gate FS-M1H. The polar moment of
Fig. 2. Schematic of encoder disc, optical probes and motor.
inertia of the rotating mass is 1.086 × 10−8 kg m2 with 98.2% of this due to the encoder disc. A diagram of the system layout is given in Fig. 3 showing the PC, data acquisition hardware. A mechanical clamp is used for mounting the motors and applying a preload force of 0–1.5 N pressing the rotor disc into frictional contact with the piezoelectric stator. Preload is measured by a force sensor. The data acquisition system includes an NI5401 function generator and amplifier to provide a triggered drive signal of between 1 and 60 Vpp at 0–1 MHz, an NI5102 15 MHz scope card for measuring input voltage and current and a PCI6023E DAQ card for sampling force sensors and the position encoder.
Fig. 3. Automated test system hardware layout.
132
P.J. Rayner et al. / Sensors and Actuators A 144 (2008) 130–134
2.1. Transient motor theory Ultrasonic motor performance in running approximates to that of DC motors with torque reducing linearly as speed increases. This gives the characteristic ‘capacitor charging’, (exponential) variation [9] in the rotor velocity. Whilst this is a simplified model without higher order effects [10] it provides reasonable accuracy for the essential performance parameters of Ωmax , Tstall , ηmax , Thold which are respectively the no-load speed, stall torque, maximum efficiency and the holding torque. The starting behaviour of the motor is described by: T (1) Ω = Ωmax 1 − Tstall which gives the rotor velocity as: t Ω = Ωmax 1 − exp − τstart
Fig. 4. Data processing schematic.
(2) 3. Motor test results
where: τstart
JΩmax = Tstall
(3)
During the motor stopping period, power is removed and the equation of motion is a constant deceleration due to friction alone: ˙ = −Tbrake = −μFpreload r Ω J J
(4)
where Fpreload and r are the preload force and contact radius. The holding torque (dynamic friction) can be estimated by using the time for the motor to stop (τ stop ): Thold =
JΩ0 τstop
(5)
The 5 mm Flexmotor is driven at 20 Vpp using a 270 or a 380 kHz sine (or square) wave, typically with a 0.4–0.8 N preload and giving a maximum no-load speed of up to 5000 rpm. The inertial load provided by the encoder disc was selected to increase the start up time constant of the motor to 10–60 ms (equivalent to 100–200 encoder slots in rotation during start up) starting with an assumption that motor stall torque would fall approximately in the range of 10–200 N m. Sampling speeds are set at 3 MHz for electrical signals and 50 kHz for the encoder and force sensors. As shown in Fig. 4, the captured data is divided into start and stop regimes and the rotor velocity data is derived from the position encoder time series by a numerical differentiation algorithm which linearly interpolates the digital position information. The rotor velocity is then fitted to the theoretical exponential equation (or an alternative polynomial fit), and then the fitted data is again differentiated to give rotor acceleration. The rotor angular acceleration is directly related to the motor torque, thereby allowing a full performance plot. Electrical power consumption has also been added to create an efficiency plot concurrently.
A sample of a test on a 5 mm Flexmotor driven at resonance at 383 kHz by a 16 Vpp square wave is given in Fig. 5 and the corresponding performance plot in Fig. 6. The system demonstrates that this motor has peak efficiency in this test of 5%, with a stall torque of 0.13 mN m and a no-load speed of around 480 rad/s. The stopping transient (occurring after power off at 0.24 s) gives a braking torque of 0.26 mN m defined by the gradient of the stopping deceleration. Time constants are 32 ms starting and 20 ms stopping with the encoder to give around 0.56 and 0.35 ms for motor only, respectively. The motor efficiency is predicted to be increased by design adjustments, friction material changes and by improvement of mechanical rotor/stator axial alignment in the test rig. A realistic prediction of the system efficiency of a motor of this type and size is at least 30%. 4. Discussion The system produces good quality estimates of the motor performance with the key advantage of high speed data acquisition. Use of a standard digital encoder provides a trade-off between the position resolution that is necessary to give sufficient definition during the transient movements and the no-load speed detection. The system was originally designed for 200 rad/s and hence some quantisation in velocity can be seen in Fig. 5. Using a 50 kHz sample rate there is an upper limit on encoder slot events of around 10 kHz before quantisation noise develops significantly. Faster data capture speed would remove this effect for the high speed range or alternatively, increasing the rotor inertia will lower the position resolution (and hence slot count) requirement. However, the high speed rotor data equates to the no-load speed (or low torque) condition in starting. In practice, this is of lesser interest than accurately defining the early start up to a speed of Ωmax (e − 1)/e. The main sources of error in the test system lie in the accuracy of calculating the rotor inertia from its dimensions, accuracy of curve fitting and accommodating low level torque variation
P.J. Rayner et al. / Sensors and Actuators A 144 (2008) 130–134
133
Fig. 7. Reliability tests over 50,000 start–stop cycles.
with position. The value determined for inertia is almost wholly dependant on the precise measurement of the encoder radius. An error of 100 m in radius, for example, would contribute to 2.5% systematic error in polar inertia. Encoder dimensions and density were cross-checked here with the actual rotor mass. 4.1. Higher level tests: reliability and parameter sweeps
Fig. 5. Sample test results of a 5 mm Flexmotor: (a) time series and (b) polynomial fit to motor velocity.
The modular software allows the test routine to be called repeatedly by a higher level program, taking the key values of each test: Ωmax , Tstall , ηmax , Tmax , Thold , τ stop and the preload imposed and logging these values over time. An example is shown in Fig. 7 of a reliability test for a different 5 mm micromotor, showing no-load speed and stall torque during ‘bedding in’ of the motor followed by long term steady behaviour. The long term behaviour also demonstrates the repeatability of performance prediction measurements when the motors variations are analysed: ±10 rad/s (∼7%) in no-load speed and ±5 N m (∼8%) in stall torque. Similarly the input parameters such as drive voltage, frequency and preload pressure can be adjusted, whilst the test procedure is quickly repeated, giving parameter maps and optimisation of drive conditions for a motor design. When compared to electromagnetic motors, ultrasonic motors have an extra important parameter which is the resonant frequency of the drive. This can easily show variability from part to part. An automated frequency-sweep test allows for individual motors and motor components to be compared for performance to give information for part aging and drive control schemes. 5. Conclusion
Fig. 6. Performance plot for 5 mm Flexmotor during starting.
An automated test system for micro-motors, which uses a transient testing strategy, has been presented and demonstrated though performance characterisation of 5 mm ultrasonic motors with torque outputs of 10–200 N m. The methods used can be flexibly applied to ultrasonic and DC motors and scaled to smaller micro-motors with relative simplicity, using readily available analogue or digital position/velocity detection methods. Whilst steady state methods may provide superior measurement accuracy over transient tests, the method used here provides greater flexibility and much faster acquisition time. Higher level functions such as reliability and operating parameter searches can realistically be performed owing to the fast test time.
134
P.J. Rayner et al. / Sensors and Actuators A 144 (2008) 130–134
References [1] K. Uchino, Piezoelectric Actuators and Ultrasonic Motors, Kluwer Academic, Boston MA, 1997. [2] P.J. Rayner, R.W. Whatmore, Piezoelectric ultrasonic motor using flextensional amplification of a disc radial mode with elastic fin drive, in: Proceedings of the 9th US–Japan Seminar on Dielectric and Piezoelectric Ceramics, Okinawa, Japan, November 2–5th, 1999, pp. 151–154. [3] J.T. Leinvuo, S.A. Wilson, R.W. Whatmore, Flextensional piezoelectric motor using the contour mode of a square piezoelectric plate, IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control UFFC-51 (2004) 929–936. [4] J.T. Leinvuo, S.A. Wilson, R.W. Whatmore, M.G. Cain, A New flextensional piezoelectric ultrasonic motor—design, fabrication and characterisation, Sensors and Actuators A – Physical 133 (January (1)) (2007) 141–151. [5] J.T. Leinvuo, S.A. Wilson, R.W. Whatmore, M.G. Cain, Flextensional ultrasonic piezoelectric micro-motor, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control UFFC-53 (December (12)) (2006) 2357–2366. [6] P. Wurmsdobler, R. Duffait, T. Messaoudi, W. Brenner, A torque testing stage for rotating micro-electro-mechanical devices, in: Proceedings of the 9th Int. Symposium on Transport Phenomena and Dynamics of Rotating Machinery (ISROMAC-9) Honolulu, Hawaii, USA, 10th–14th February, 2002. [7] H. Ota, T. Ohara, Y. Karata, S. Nakasima, M. Takeda, Novel micro torque measurement method for micro-devices, Journal of Micromechanics and Micro-engineering JMM-11 (2001) 595–602. [8] J. Friend, K. Nakamura, S. Ueha, A Piezoelectric micro-motor using in-plane shearing of PZT elements, IEEE/ASME Transactions on Mechatronics TM-9 (3) (2004) 467–473. [9] S. Ueha, Y. Tomikawa, Ultrasonic Motors: Theory and Applications, Clarendon Press, Oxford, 1993. [10] T. Takano, Y. Tomikawa, M. Aoyagi, C. Kusakabe, Transient response characteristics of a same-phase drive-type ultrasonic motor, Japanese Journal of Physics JJP-33 (1994) 5370–5373.
Biographies Philip J. Rayner has over 7 years experience in micro-technology and piezoelectrics, focusing on product development in fibre optic telecommunications, spectroscopy and MEMS. His current specialities are in automation and robotics, smart materials and sensors. Philip completed his PhD in micro-engineering and
nanotechnology at Cranfield University in 2000 having previously studied engineering (ME) at Cambridge University. He is a consultant and MD with Tecray Ltd. in Peterborough, UK. Stephen A. Wilson completed his PhD studies at Cranfield University in 1998 conducting original research into the processing of piezoelectric ceramicpolymer composite materials. His thesis introduced a valuable new method for composite assembly termed ‘electric-field structuring’ which has been patented world-wide with uses in assembly of novel highly engineered composite sandwich panels with integrated functional elements. As Senior Research Fellow in the School of Applied Sciences, he has worked on materials development to optimise the performance of advanced sensor arrays for real-time 3D ultrasonic imaging (heart monitor). His work on advanced materials processing techniques has contributed to a new integrated fabrication route for thick film piezoelectric micro-actuators, which uses ultra-precision machining of commercial bulk ferroelectric ceramics in combination with standard micro-fabrication techniques. Roger W. Whatmore graduated with his PhD from Cambridge University in 1977 and spent nearly 20 years working with the GEC Marconi (formerly Plessey) research laboratories at Caswell in the UK on the development and exploitation of ferroelectric materials in a wide range of electronic devices, particularly sensors and actuators, for which work he was awarded GEC’s Nelson Gold Medal in 1993. From October 1994 until January 2006 he held the Royal Academy of Engineering Chair in Engineering Nanotechnology at Cranfield University, where he lead a group developing the use of ferroelectrics in micro-systems and nanotechnology. He is currently Chief Executive Officer at the Tyndall National Institute in Cork, Ireland. He has published over 200 papers and 30 patents in the field. He is a Fellow of the Royal Academy of Engineering and a Fellow of the Institute of Materials, Minerals and Mining who in 2003 awarded him the Griffith Medal for Excellence in Materials Science. Markys G. Cain graduated with his PhD from Warwick University in 1990 and spent the next 2 years in the Materials Department of the University of California, Santa Barbara studying thin film epitaxial science. Subsequent research in ceramic composite materials technology in the UK utilised many of the principles learnt at Santa Barbara in the deployment of new interfacial fibre coatings for advanced high temperature ceramic matrix composites for gas turbine applications. Research with Oxford Instruments led to the development of an SEM based instrumented indentation system and he joined NPL in 1997 to lead the Functional Materials Research group. Research activity includes the development of measurement methods to elucidate materials behaviour in ferroelectric and piezoelectric ceramics and thin film materials. He has published over 80 papers and 2 patents in the field. He is a member of the Institute of Physics.