e., the knee joint axis [9,13,14,24]. In other words, the projections of the upper and lower leg into the joint plane, to which the joint axis is normal, confine this angle; see Figure 1. However, we shall note that considering the knee as a hinge joint is an approximation. Although flexion/extension is the major degree of freedom, a biological joint, such as the knee, is not perfectly constrained to rotation around one axis. This is often addressed by additionally considering abduction/adduction and internal/external rotation, which leads to a three-dimensional knee joint angle, as in [10,14,25]. However, abduction/adduction and internal/external rotation angles
Since White and Voltmer [1] excited surface acoustic waves (SAWs) by utilizing interdigital transducers (IDTs) deposited on a piezoelectric crystal surface, SAW technology has developed rapidly and is widely used in the telecommunication and other areas.
Because the energy of SAWs is conserved near the surface of the piezoelectric substrate, a surface perturbation will lead to significant changes in surface acoustic wave properties such as propagation velocity, phase, attenuation and wave form. This characteristic can be used to develop acoustic sensors with good performance. Wohltjen and Dessy [2] first reported a chemical sensor for organic gas detection by coating a sensitive film on the surface of a SAW device. Since then a variety of SAW gas sensors have been developed for gas sensing [3�C5]. The key unit of most of the reported sensors is a SAW oscillator (shown in Figure 1), which consists of a periphery circuit, a SAW detector and a sensitive film deposited on the detector surface.
The sensitive film can strongly absorb a certain kind of gasses and almost dose not absorb other gasses; therefore we can obtain the content of the gas by measuring the change in oscillation frequency. The emphasis of this kind of sensor is to coat a sensitive film with high Brefeldin_A selectivity and high adsorption capacity. Such a SAW sensor or sensor matrix can only perceive one or several kind of gases; thus they are applicable to measure the content of some special gases [6�C9]. In many areas, such as environmental monitoring, food security, explosive detection, there is a strong need for sensors which have a wide detecting range to monitor volatile organic or semi-volatile organic compounds (VOCs or SVOCs) [10�C12].
To satisfy the demand, another kind of SAW sensor [13�C17] was reported, in which a gas separation apparatus such as a gas chromatography (GC) column is set in front of the detector to separate and identify the tested multi-component gas. A multi-component gas can be separated and identified by their characteristic retention time in the GC column; thus SAW-GC sensors are available to detect VOCs and SVOCs in a wide range.Figure 1.Schematic of SAW gas sensors.