The discrimination of postures and movements using a minimal set of uniaxial accelerometers was investigated. Postures and movements were distinguished on the bitsis of the high-pm filtered, rectified and low pass filtered signal of one accelerometer. Postures were discriminated by combining the constant valued signals of the accelerometers, mounted on different segments of the body. One sensor mounted ndiully on the trunc and one mounted radially or tangentially on the upper leg appeared to be suficient to discriminate shnding, sitting and lying. Methods are proposed for the discrimination of different cyclical movements

Bussman, Hans B.J.

Franken, H.M.

Franken, Henry M.

Koelma, Frank

Martens, Wim L.J.

van Lummel, Rob C.

Veltink, Petrus H.

English

University of Twente Research Information

THE FEASIBILITY OF PO!SI'URE AND MOVEMENT DETECTION BY ACCELEROMETRY Peter H. Veltink', Hans B.J. Bussmann2, Frank Koelma', Henry M. Franken', Wim L.J. Martens3 and Rob C. van LummeI4 ' Biomedical Technological Institute, University of Twente, Erasmus University, Department of Rehabilitation Medicine, Rotterdam, the Netherlands PhyVision bv, Gemert, the  Netherlands ' McRoberts bv, the Hague, the NetherIands P.O. Box 217, 7500 AE Enschede, the Netherlands Absrrad - The discrimination of postures and movements using a minimal set of uniaxial accelerometers w u  investigated. Postures and movements were distinguished on the bitsis of the h igh-pm filtered, rectified and low p m  filtered signal of one accelerometer. Postures were discriminated by combining the constant valued signals of the accelerometers, mounted on different segments of the body. One sensor mounted ndiully on the trunc and one mounted radially or tangentially on the upper leg appeared to be suficient to discriminate shnding, sitting and lying. Methods are proposed for the discrimination of different cyclical movements. INTRODUCTION In order to optimize and evaluate the rehabilitation treatment of patients it is important to evaluate their daily life activities in their domestic environment. Such an evaluation requires a portable measurement unit and kinematic sensors which do not influence the activities. In this study, the feasibility of a posture and movement detection scheme using a limited number of uni-axial accelerometers is investigated. DISTINGUISHING POSTURES AND MOVEMENTS USING ACCELEROMETERS A ccelerornerers Small uni-axial accelerometers exist which can be easily mounted on the skin. If mounted on a segment of a human body, they measure a component of the actual acceleration of the segment at the site of the sensor, and a component of the equivalent gravitational acceleration, which depends on the orientation of the segment. I t  is important to use accelerometers which measure also dc-components, like the piezoresistive types. Willemsen et al. proposed methods for the application of these sensors in the analysis of body movement [I]. Distirinuishirr p between no.mres arid movements If the sensor signal is not changing over time, it can be assumed that the body segment is not moving, because a constant acceleration or an exact cancelation of acceleration and gravitational components during movements is not to be expected for prolonged periods of time during normal body movements. This characteristic can be used to discriminate between postures and movements: a movement detector was constructed by applying a threshold to the high-pass filtered, rectified and low-pass filtered signal of one of the accelerometers. Discrirnirmfinn Dostures If no movement is detected, it is assumed that the body is in a posture. By combining the value of several accelerometers mounted on different body segments several postures can be discriminated. The postures standing, sitting and lying can be distinguished by the orientations of only the trunc and one upper leg. Therefore, two accelerometers with their sensitive axes in the sagittal plane are sufficient to discriminate between these postures: one accelerometer radially mounted on the trunc and one radially or tangentially mounted on an upper leg. A tangentially mounted trunc accelerometer can not be used, because lying on a side would not be distinguishable from standing. Combining a radially mounted trunc accelerometer and a tangentially mounted upper leg accelerometer also enables the discrimination of lying on a side, lying on the back and lying face down. If more postures or more aspects of postures have to be distinguished, more sensors might be required. Discrirnirmtin p cvclical movements Directly after a movement has been detected, the cyclical nature of the movement can be investigated. Potentially applicable time domain analysis methods include the detection of repeating peaks in the signals. Combined time and frequency domain methods include locking in onto the cycle frequency by instantaneous frequency analysis [2] ,  tracing 0-7803- 1 377- 1/93 $3.00 0 1993 EEE 1230 variations in cycle frequency while walking at varying speecls. Potential methods for the discrimination of cyclical movements include multi-correlation analysis (simultaneously applied to all accelerometer signals) and combining the average signal values per cycle for all sensors. EXPERIMENTAL METHODS Experiments were performed on five healthy subjects, who performed several activities, including the postures and movements of interest, according to a strict and supervised protocol. Up to 6 uni-axial accelerometers (IC sensorsTM, 5g) were simultaneously.mounted on the trunc, upper legs and lower legs in varying orientations. Footswitches were placed under the feet and a marker switch was operated by the subject to mark the beginning and end of activities. The sensor signals were sampled and stored on a portable measurement system (Vi taportTh4). In. - 20 h v) \PI W O  E v) - ib. 6 ,  I 1 I I !  I I 15 0 t (s) Fipure 1 .  Tvpical perfontiatice of the tnoveiiierir detector. a. Sigrinl of tniigerrrinlly nroutired upper leg nccelerorneter. b. 7he snnie sigiinl afrer high-pnss jilterirtg flrsr order Burretworth filter. & = 0.5 Hz), rectificntioti nrd low-pus filreririg first order Buttern~orrhfilter,& = 0.5 H:), arid the movement detector oufpur (rhreshold 0.5 rri/s=). Figure 2. discritniiintitig I& postures standing. sitting and lyirrg using orie accelcrotneter radially mounted on the upper leg (x -a is )  ntd oiie accelerometer radially mounted on [he I rum (y-axis). RESULTS Postures and movements could be distinguished, using first order Buttenvorth high and low pass filters at cut-off frequencies of 0.5 Hz and a threshold of 0.5 m/s? (figure 1). The discrimination between standing, sitting and lying using one accelerometer on the trunc and one on the upper leg is illustrated in figure 2. The discrimination bemeen various movements has not be analyzed sufficiently yet. Simply considering the average values of the accelerometer signals per cycle already appeared CO discriminate between some cyclical movements. DISCUSSION This study indicates that, in principle, distinction of postures and movements using dc-accelerometers is possible. However, only a limited number of postures and movements were considered, according to a preset protocol. The applicability of this method for activity monitoring of patients suffering from motor control disorders. moving in their home environment, has still to be investigated. Accelerometers seem to be better suited for activity measurements than conventional kinematic sensors like goniometers because they have small sizes and weights, are easily mountable and do not cross joints. REFERENCES [ 1 J A.Th.M. Willemsen, J.A. van Alstt5 and H.B.K. Boom, "Real-time Gait Assessment utilizing a new way of Accelerometry ", J. Eiomechan., vol. 23, pp. 859-863, 1990. [2] W.L.J. Martens, "The Fast Time Frequency Transform (F.T.F.T.): a novel on-line Approach to the Instantaneous Spectrum", Proc. IEEE Eng. i r i  Med. & Biol. Soc., Paris, pp. 2594 - 2595. 1992. 123 1 

The feasibility of posture and movement detection by accelerometry

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Real-time Gait Assessment utilizing a new way of Accelerometry &quot;,

The Fast Time Frequency Transform (F.T.F.T.): a novel on-line Approach to the Instantaneous Spectrum&quot;,

http://purl.utwente.nl/publications/16744

The feasibility of posture and movement detection by accelerometry

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