RESNA 27th International Annual Confence
Design of a Light-Activated Switch with Improved Specificity for Persons with Spastic Cerebral Palsy
A light-activated switch for operating switch-controlled computer software or communication devices has been developed. A light emitting diode is attached to the user. An array of photo-sensors is placed about six inches away. A switch is activated when the light beam is aimed at the photo-sensors. The light emitted by the transmitter is modulated at a frequency of 1 KHz and the receiver is tuned to that specific frequency. The system is therefore insensitive to other light sources or ambient light changes. The activation of the switch requires a small but controlled movement of the user such as a slight turn of the head. Unintentional motions are less likely to trigger the switch. The system has been used by an 8-year-old child with spastic cerebral palsy and has demonstrated a satisfactory performance. Thus, the light-activated switch provides improved specificity over contact or motion-sensing switches.
Switch technology, computer access and use, spastic cerebral palsy, assistive technology (AT)
Switch-controlled scanning remains the fundamental method of accessing computers and augmentative and alternative communication (AAC) devices for persons with cerebral palsy or other neuromuscular disorders (1,2). Sensitivity of the switch is an important consideration to cope with limitations in movement and strength (3,4). Specificity is another important consideration in the case of spastic cerebral palsy. False activation of the switch due to unintentional motions often presents a problem. Harwin and Jackson (5) used hidden Markov models to classify head movements into 'yes', 'no' and spurious gestures, and showed a 74% success rate. However, such system would require complex computer algorithms. In this study, we address this problem by designing a light-activated switch system that provides an improved performance in specificity as well as sensitivity.
The objective of this study is to develop a light-activated switch difficult to be triggered by unintentional motions. The system consists of a pair of light transmitter and receiver. The switch is activated by aiming a light beam from the transmitter onto a target connected to a receiver. The light is modulated at a specific frequency such that the system is insensitive to light sources other than the transmitter.
The block diagram of the system is shown in Figure 1A. The transmitter consists of an oscillator at 1 KHz, which drives a light emitting diode (LED). The receiver senses the light with an array of phototransistors. The signal from the photo-transistors is amplified, filtered by a band-pass filter tuned to 1 KHz, and amplitude-demodulated. When the resulting signal exceeds a threshold, a one-shot timer is triggered to activate a relay. The closure of the relay provides a switch signal to the computer or other AT devices. The electronic circuit diagrams
The present system is designed for a maximum distance of 8-10 inches between the transmitter and the receiver. This operational distance is chosen for the ease of triggering the switch. The distance can be easily extended by increasing the current to the LED. It is also possible to replace the regular LED by a laser LED if longer range and non-dispersing light are preferred. The system is powered by a 9-volt battery. A 9-volt voltage regulator is also included to receive an alternative external power source such as a wall-mount AC-DC adapter or a 12-volt wheelchair battery.
Prototypes of this system were tested by an 8-year-old child with spastic cerebral palsy. The design was improved through feedbacks from the child with the help of the child's parents and an AAC expert.
After a few development cycles, the final version of the prototype is shown in Figure 2. The electronics and a 9-volt battery are contained in a box measured 5”x 2.5” x 2”. The LED and the photo-sensor array are connected to the box via two leads. For training purpose this prototype also gives an indication when the switch is activated. The indication can be delivered by either a LED indicator or a buzzer. Material cost of this prototype is under $30.
Figure 3A shows the schematic of an effective arrangement. The LED is taped to the user's forehead. The photo-sensor array is mounted on the left side of the face about 6 inches away. A slight left turn of the head directs the light beam onto the target and triggers the switch. The operation requires a head turn in a specific direction for a specific angle. It is possible to achieve this movement consistently under normal conditions. However, during a spastic episode the chance of unintentional trigger of the switch is significantly reduced. Figure 3B shows different configurations of the sensor array. The number of phototransistors and their positions define the geometry of the target area that senses the light.
The light-activated switch provides a simple and inexpensive solution to reducing the chance of unintentional switch triggers for persons with spastic cerebral palsy. The transmitter-receiver distance can be adjusted for an appropriate tradeoff between sensitivity and specificity. When the distance is increased, sensitivity decreases because of more stringent requirement on head movement to trigger the switch. But specificity increases because of the reduced probability for unintentional triggering. The configuration of the sensor array provides another possibility for optimizing the system performance. For future research, a study will be designed to include more human subjects. The sensitivity and specificity will be quantified. The data will be used to determine the optimal transmitter-receiver distance and configuration of the sensor array.
This project was supported by the URI Partnership in Physiological Measurements and Computing and a grant from Rhode Island Department of Mental Health, Retardation and Hospitals.
Ying Sun, Professor,
Biomedical Engineering Program,
Dept. of Electrical & Computer Engineering,
University of Rhode Island,
4 East Alumni Avenue,
Kingston, RI 02881,
Office Phone (401) 874-2515,
Fax (401) 782-6422,