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Special Section: Micromanufacturing

# An Experimental Investigation of the Feasibility of “Self-Sensing” Shape Memory Alloy Based Actuators

[+] Author and Article Information
K. Malukhin, K. F. Ehmann

Department of Mechanical Engineering, Northwestern University, Evanston, IL 60208-3111

J. Manuf. Sci. Eng 130(3), 031109 (May 12, 2008) (10 pages) doi:10.1115/1.2917294 History: Received November 08, 2006; Revised March 07, 2008; Published May 12, 2008

## Abstract

Shape memory alloys (SMAs) change their crystallographic structure and shape during heating/cooling and, as a consequence, their electrical resistance also changes. This allows the determination of the location of a SMA-based structure in space without separate sensors by suitably measuring this change. In this paper, this “self-sensing” concept is explored in SMA wire-type actuators. Step responses, expressed in terms of resistance (voltage drop) across the wire, and the corresponding displacement changes during heating/cooling, were measured. It was shown that the relationship between the displacement and the voltage drop can be approximated by a linear regression with a correlation coefficient close to 1. System identification has shown that SMA wire actuator performance can be best approximated by first or by second order system response depending on the thermal insulation condition of the actuator. The resolution and the sensitivity of the self-sensing method were evaluated based on experimental data and it was shown that their minimal values were less than $1.7μm$ and $0.7μm$, respectively, thus supporting the feasibility of the “self-sensing concept.” Both values exponentially increase with the increase in the range of the measured displacements whose magnitudes vary under different working conditions.

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## Figures

Figure 1

Principle of integral sensing

Figure 2

DSC thermogram of the SMA wire

Figure 3

Schematic of the experimental setup

Figure 4

Electrical schematic

Figure 5

Controlled recovery schematics

Figure 6

Controlled strain recovery of the nonprestrained SMA wire: (a) SMA wire displacement, (b) SMA wire amplified reference voltage, and (c) SMA wire temperature

Figure 7

Controlled strain recovery of the initially prestrained (2.77% strain) thermally insulated SMA wire: (a) SMA wire displacement, (b) SMA wire amplified reference voltage, (c) SMA wire temperature, (d) zoom of (a), and (e) zoom of (b) and zoom of (c)

Figure 8

(a) Correlation coefficients between temperature and displacement RTD, temperature and voltage RTV, and voltage and displacement RVD and (b) homoscedasticity test

Figure 9

Cumulative correlation coefficients (a) as a function of prestrain and (b) as a function of temperature

Figure 10

Displacements versus amplified voltage of the SMA-wire-based actuator for different voltage steps (shown in Fig. 7): (a) time range=330,…,399s, displacement range 131μm and (b) time range=413,…,553s, displacement range=200μm

Figure 11

Cumulative sensitivity (a) and resolution (b) versus displacement ranges

Figure 12

Input and output data for the SMA wire step response with no thermal insulation: (a) output displacement and (b) input voltage

Figure 13

Input and output data for the SMA wire step response with thermal insulation: (a) output displacement and (b) input voltage

Figure 14

Frequency response data for the SMA wire step response with no thermal insulation: (a) amplitude and (b) phase

Figure 15

Frequency response data for the SMA wire step response with thermal insulation: (a) amplitude and (b) phase

Figure 16

Measured and simulated output of the thermally noninsulated SMA wire: (a) Exp.27 and (b) Exp.32

Figure 17

Measured and simulated output of the thermally insulated SMA wire: (a) Exp.45 and (b) Exp.41

Figure 18

Amplitudes versus averaged temperatures (a) from the frequency response of the thermally noninsulated SMA wire and (b) from the frequency response of the thermally insulated SMA wire

Figure 19

SMA wire heat balance diagram

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