Design considerations for pattern recognition systems were signal quality, data segmentation, number of features, processing time and classi cation accuracy. The proposed control archi- tecture utilised four dierent input control signals and each signal had a prede ned number of channels. Although the proposed control architecture utilised four dierent types of signals namely, sEMG, inertial, positional (encoder) and force signals, it was the sEMG signal that was utilised for pattern recognition. This was mainly because the sEMG signals are directly linked to the detection of the user's intent and their impact on the control system determines the success
of the design. The other three signals were used to aid reliability of the control architecture through the establishment of feedback signals and control interlocks in the knowledge base.
During experiments carried out using the protocol explained in section 3, the study revealed that there are three active sEMG channels namely the Tibialis Anterior, Medialis Gastrocnemius and Lateralis Gastrocnemius muscles. The experiments for gait analysis also revealed that the position of the limb could be determined in space. Also, the protocol revealed that the gait can easily be monitored by identifying core events of the gait such as the heel-strike, toe-o and the foot at. The goal of the study is to develop an optimised system which utilises sEMG sensors, FSRs, encoders and IMU sensors integrated together to reliably control a prosthetic ankle. The integration of sEMG signals, IMU sensors and force sensors will improve the reliability of the proposed system. The toe-o, double support, single support and heel-strike events could be monitored using force sensitive resistors placed within the prosthetic leg. For a detailed block diagram of the proposed architecture refer to Appendix C and Appendix D.
Design factors and considerations were rst highlighted in this section followed by the de- velopment of analog front ends for the inertial measurement sensor, the myoelectric sensor and the force sensitive resistors. Lastly, a comprehensive presentation of the architecture was carried out. In some cases, the method used to achieve the design is explained with reference to section 3 and supported by ndings in sections 4, 5 and 6.
The design and manufacture of medical devices are governed by several standards. The Inter- national Electrotechnical Commission (IEC 60601-1) standards for the design and development of electronic medical equipment and the Application of Risk Management to Medical Devices (ISO 14971) were considered. All safety features and assumptions included in the design were based on IEC60601-1 and ISO 14971. The major consideration was such that the device could conform to ISO 13485 Quality Management System for managing medical devices and equip- ment. Furthermore, the other design considerations were signal input range, bandwidth, noise interference, sampling rate, communication protocols, power consumption robustness and adapt- ability. According to [98], the notable requirements for the sEMG signal acquisition circuitry are isolation, high gain, high input impedance, low input bias current, wide input common mode range, low noise, low input oset voltage and good common mode rejection ratio (CMRR). Some of the design requirements are:
Isolation: The characteristics of the circuit to provide reliable isolation in the unfortunate event of the ampli ers sending a current to the human body within the electrodes should be clear and well-de ned [80]. The isolation should be able to cater for analog power supply, digital power supply and digital data bit stream [285].
Data processing: The reliability of an electronic device is mainly aected by the processing capability of the main processor [286]. Even after sucessfully establishing the feature extrac- tion mechanism and classi ers [107], the need to develop compatible data processors was emphasised. The possible processors include digital signal processors, eld programmable gate arrays, microprocessors and microcontrollers. Furthermore, there is the need for on- board processing capability to reduce post-processing requirements. This includes the use of adaptive lters which could adaptively adjust the lter constants in real time.
Analog-to-digital converters: The digital bit-stream conversion from an analog signal is mainly dependent on the sampling rate. For the conversion to occur in real-time, anti- aliasing lters are required and as such there is a need to select analog front-ends that
have such an in-built capability [96]. This is mainly because aliasing is a result of a signal whose highest frequency component is greater than one half of the sampling frequency and this higher frequency component is not distinguishable from lower frequencies. The sEMG signals have a useful frequency range of 15 Hz-500 Hz and the recommended sampling frequency is 1000 Hz or greater.
High gain: The demand for high gain is necessitated by the low amplitude signals available at the amputated limb. The gain will enable the suppression of internal noise of the ampli ers since high gain ampli ers are sensitive to small changes in input signal [287]. Low and stable oset voltage: An unstable and unreasonably high oset voltage can lead
to input signal saturation, thereby reducing the reliability of the circuit. There is a need to employ an external nulling potentiometer to adjust the oset voltage to zero [288], [289]. High input impedance: Most signal distortions are as a result of low input impedance, hence
the need for high input impedance on the ampli ers to minimise noise due to electrical loading [288].
Low input bias current: The dierential input values are very low and as such a low input bias current is a requirement and should usually be less than 10 pA [290]. This is also necessary for human safety and as a junction current for the transistors [285]
Wide input common-mode range: The input common mode must be large enough to avoid distorting the input signal. The rail-to-rail voltage is the expected range for common mode range [291].
Common Mode Rejection Ration (CMRR): The advantage of using instrumentation am- pli ers is that they have a high CMRR, that is they have a high capability of rejecting common signals that appear in phase and at both inputs, thereby increasing the dierential signal [289].
Low internal noise: The presence of thermal and icker noise within ampli ers usually results in the generation of internal noise [292].