2.2. Selection and Configuration of Surface Electromyography (sEMG) Equipment

Various sensors have been developed for controlling automotive steering systems. Joysticks have facilitated the rotation of steering wheels, and in some instances, joysticks were designed to increase safety by replacing steering wheels that may collide with the driver in a frontal collision [28,29,46,47]. Some steer-by-wire joysticks employed electrical sensors connected to computers to perform turning maneuvers without a steering column. However, regulations applicable to countries such as Sweden and Japan prohibited the unauthorized modification of production vehicles with alternative steering controls, including steer-by-wire devices [28,48]. In order to install a joystick for drivers with disabilities, it was possible to request permission from the government of Japan, provided that the electrical sensors of the joystick controlled the steering column with an electric motor and the steering wheel remained usable [28]. Since the proposed sEMG interface was developed in Japan, the electrodes of the interface were designed to control an electric motor that rotates the steering column (Figure 1).

Aside from sEMG electrodes, other sensors have been proposed to control steering with the bodily movements of the driver. Strain gauges, motion sensors, rotary encoders and gyros have converted gestures into steering commands [25,26,27]. Motion sensors had limited mounting locations in the vehicle cabin, with some consumer grade sensors positioned at least 50 cm from a detectable gesture [26]. Gyros that were mounted on the limbs of drivers, on the other hand, could be used in the confines of a vehicle cabin, but gyro signals were subject to drift, in addition to interference resulting from vehicle vibrations [25,26,49]. Strain gauges mounted on the hands of drivers were unaffected by vehicle vibrations or drift when detecting hand gestures [25]. Thorough testing has not been performed, however, apart from driving an automobile along a straight path.

There is a challenge that is common to some existing steering sensor configurations, namely, the accommodation of drivers with disabilities. If amputation or other health conditions preclude the use of some limbs, strain gauges and rotary encoders require mechanical adapters to measure the movement of remaining unaffected limbs [25,27]. Likewise, joysticks designed for hands may need to be adapted to the prosthetic terminal devices of amputees [50,51]. However, the use of prosthetics is not generally accepted by amputees, since only 30% of Korean amputees were satisfied with prostheses, and 56% of amputees in Australia wear prosthetics “once in a while” or “never” [11,12]. On the other hand, electroencephalography (EEG) sensors allow brain signals to control steering without limb movement, but empirical studies revealed that about 20% of users are unable to sufficiently operate brain control interfaces [52,53,54,55]. Steering control without limb movement may be enabled by eye gaze tracking, but the “Midas touch problem” has to be addressed so that algorithms can distinguish gazes that convey commands from gazes that merely obtain visual information [56,57]. The current sEMG interface avoids this problem by accepting myoelectric signals rather than eye gaze input.

Since sEMG electrodes could be individually mounted in selected locations, the proposed steering assistance interface could be readily adjusted to measure residual muscles from amputated limbs as in the case of sEMG-controlled prostheses [30,58]. If an amputee prefers to grip the handle with an unaffected limb, it would be possible to measure sEMG input signals from the affected limb, provided that the myoelectric activity of residual muscles could be detected with electrodes [59]. The interface could also be used by drivers, such as patients with stroke-induced hemiplegia, with one paralyzed arm and one unaffected arm that provides sEMG signals [9,10]. Even though spinner knobs could be mounted to steering wheels to enable one-handed operation in place of the proposed interface, some drivers with disabilities choose not to use spinner knobs [60]. Furthermore, it seems that further research needs to be conducted to determine whether or not the use of spinner knobs poses a risk of shoulder muscle injury, as observed when steering wheels are directly rotated with one hand [4].

Interfaces with sEMG sensors have their own set of challenges. Inaccurate measurement could result from motion artifacts due to the movement of electrode wires and relative motion between electrodes and skin surfaces [61]. Inaccuracy could also result from electromagnetic interference originating from body tissue and environmental sources, such as power lines and electronic devices [61]. However, signal filtering, bipolar electrode configurations, and non-polarized electrodes could mitigate electromagnetic interference [30,61]. Wireless sEMG signal transmission could also reduce noise and prevent driver movements from being impeded by electrode wires [62,63].

Recently developed sEMG sensors for research and commercial applications utilize wireless transmission [64,65]. Unlike conventional wet electrodes that require conductive gel at the skin–electrode interface, emerging sensor technology can detect myoelectric activity without gel, and in the case of capacitive electrodes, without skin contact [64,66,67]. However, unlike some emerging sEMG sensors, disposable silver–silver chloride (Ag/AgCl) wet electrodes are relatively affordable and readily available [67]. Thus, wet electrodes were used in the current study to facilitate experimental replicability and the iteration of electrode mounting configurations (Figure 5) [30]. If the mounting configuration of the conventional electrodes corresponded to path following accuracy that was comparable to a game steering wheel, the configuration would be finalized for conversion to an armband that uses dry electrodes with comparable measurement accuracy [67].

Steering assistance interface adapted to driving simulator.

Electrode placement for the driving simulator trials was performed in accordance with the recommendations of SENIAM (Surface EMG for the Non-Invasive Assessment of Muscles) [68]. The ground electrode was mounted on the wrist (Figure 5), while a set of two bipolar electrodes was placed on the belly of the biceps brachii with the elbow flexed to approximately 90°, since prior testing has shown that the muscle belly is more active at 90° of flexion than other mounting locations on the biceps brachii [69]. As the biceps brachii contracts isometrically, the hand grips a clamp that substitutes for the original handle depicted in Figure 2. Controls from the original interface design, such as a left/right turn toggle switch and photoelectric sensor, were not incorporated into the clamp, since simulator trials only included right turns that required the clamp to be held continuously (Section 2.4); isometric contractions only initiated and terminated right turns.

Relocating the ground electrode from the wrist to a location above the elbow, such as the lateral side of the upper arm, could accommodate some amputees [70]. Whereas the buttons of the interface could be pushed with an intact arm or prosthetic limb, transradial amputees with residual muscles above the wrist could choose to provide steering input with sEMG signals from the residual biceps brachii, although residual forearm muscles that commonly control prosthetic hands may be more preferable to the user [58]. On the other hand, transhumeral amputees could typically provide sEMG signals with residual muscles above the elbow, including biceps brachii, that are capable of isometric contraction [71,72]. Both types of amputations account for a significant portion of amputees that could control the interface with affected limbs [11,12].

Similar to the majority of studies on sEMG-controlled prosthetics, nondisabled drivers in the current study performed isometric contractions that could represent sEMG signals from the affected limbs of transhumeral amputees [73,74,75,76,77]. Consequently, the results of this study are relevant to the operation of the sEMG interface with intact or affected limbs. Regardless of the presence or absence of health conditions, anatomical and physiological differences across drivers could affect the measurement of sEMG signals [61]. In order to detect sEMG amplitudes that vary across a range of drivers, the sEMG input threshold of the steering assistance interface would have to be calibrated for each driver (Section 2.1) [78,79,80]. Hence, the calibration process for driving simulator trials is described in the next section.