## 1 Introduction  
Damage to the central nervous system that result in paralysis of the upper limbs. For instance, high level spinal cord injury and certain types of stroke, or double-amputation of the upper limbs, can lead to major difficulties in daily living of a considerable number of people due to loss of function, control, and independence [1]. Although caregivers can provide assistance to overcome some of these barriers, personal care is costly and often available only for a limited period of time. Individuals with severe disabilities desire more independence in their daily activities for tasks such as controlling a computer or driving a wheelchair [1]. To improve the quality of life for this group of people, researchers have developed many assistive technologies (ATs) that allow users to utilize their remaining abilities, such as control of the diaphragm, head, eyes, tongue, voice, and brain signals [2; 3; 4; 5; 6; 7; 8]. The tongue is well suited to control an AT because it is composed of strong and dexterous muscle fibers that do not easily fatigue as long as they are not required to apply force [9]. Also, the tongue is directly connected to the brain through cranial nerves, which are not affected by a spinal cord injury [10]. Furthermore, tongue-operated ATs protect the users' privacy because the tongue movements can be completely hidden from sight [11]. Several tongue-operated ATs have been developed to utilize the power of the tongue, such as the tongue-touch keypad (TTK), optical tongue sensing retainer, joystick-based tongue-point, tongue mouse using pressure sensors, inductive tongue control system (ITCS), and the Tongue Drive System (TDS) [12; 13; 14; 15; 16; 17]. Every one of these tongue-operated ATs has its pros and cons in terms of functionality, usability, and acceptability among its potential users. For example, TTK, tongue-point, and tongue mouse require physical force from the tongue, which may cause fatigue on the tongue, and ITCS and TDS require users to receive a tongue piercing or attach a tracer on their tongue, which may cause temporary discomfort [18]. Nonetheless, researchers are still searching for better solutions to utilize the capabilities of the tongue while improving the user's comfort. The basis for the TDS operation is monitoring changes in magnetic field, generated by a small magnetic tracer attached near the tip of the tongue, using an array of magnetic sensors distributed inside or around the oral space, and processing the resulting data to track the voluntary tongue position in real time [19]. The magnetic field generated by the tracer is not affected by the human body, and despite other sources of magnetic interference, such as the earth's magnetic field, offers the necessary accuracy to track the tongue position. We have developed signal processing algorithms that improve the signal to noise ratio (SNR) and recognize user-defined patterns of magnetic sensor signals that correspond to the tongue commands [20; 21; 22; 23]. A key advantage of the TDS over the other tongue-operated ATs [13; 14; 15; 16; 17] is its ability to detect the tongue position in the 3-D intraoral space, which allows users to define a virtually unlimited number of commands by moving their tongues to various desired locations, as long as they can remember and repeat those tongue gestures. In addition, the TDS does not need any steering force or pressure applied by the tongue, which can result in tongue fatigue in several other tongue-operated ATs [13; 15; 16; 18]. The TDS was first developed in the form of a headset, referred to as the external TDS (eTDS), with a pair of poles locating the magnetic sensors near the cheeks to measure the tracer magnetic field with sufficient SNR. The eTDS headset has been evaluated and its performance has been compared with a popular AT, called Sip'n'Puff, through two rounds of clinical trials in two rehabilitation centers [24; 25; 26].