Ingenium 2018
A Wearable Sensing System to Estimate Lower Limb State for Drop Foot Correction Levi Burner and Dr. Nitin Sharma
Neuromuscular Control and Robotics Laboratory, Department of Mechanical Engineering and Materials Science, Swanson School of Engineering, University of Pittsburgh, Pittsburgh, PA, USA
Abstract A common side effect of stroke is drop foot. This causes a patient’s foot to drop or drag onto the floor during the swing phase while walking. Drop foot can be corrected by using functional electrical stimulation (FES). This technology often uses a contact sensor to detect gait phases. The contact sensor is not suitable for high fidelity control of FES because it provides a binary feedback (on/off). For accurate control of FES, full limb state information such as joint angles and angular velocities is highly preferred. Therefore, a wearable sensing system that is capable of real time estimation of limb angles was developed and demonstrated. The system uses inertial measurement units to determine limb angles. The system is untethered and wirelessly communicates limb state information to a recording platform. A preliminary test showed the system’s capability to estimate limb angles in real-time. Future studies will focus on programming advanced estimation algorithms for the system. The limb state data provided by the system will be used to reconstruct a user’s gait in real-time and validated against a ground truth such as measurements from a motion capture system. Once validated the wearable sensing system can be potentially integrated with a multi-channel FES system to correct drop foot or used as a wearable sensing system to study gait abnormalities. Keywords: Inertial Measurement Units, Gait Analysis, Functional Electrical Stimulation, Limb Angle Estimation
1. Introduction Over 800,000 strokes are reported annually. A common side effect of stroke is drop foot, which causes a foot to drag or slap on the floor during walking (swing phase). The condition is due to the inability to control ankle muscles that produce dorsiflexion. It affects a patient’s ability to balance and walk at a steady pace. The loss of control also increases the risk of fall from tripping [1].
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Current solutions include using single channel functional electrical stimulation (FES) and a ground contact sensor. FES artificially activates muscles via application of external electrical currents. When the ground contact sensor detects the foot has lifted off the ground, the FES system activates and stimulates the peroneal nerve (to elicit dorsiflexion) so that the foot rises and clears the ground. Another sensing solution is to use electromyography to predict gait posture of a subject and apply stimulation to the peroneal nerve at an appropriate time [2]. Recently, another technique has been investigated that uses inertial measurement units (IMU) to measure the gait cycle of a patient and apply corrective FES. In theory, the IMU will allow more accurate gait cycle estimation than the simple contact sensor. Modulated stimulation of the peroneal nerve can be achieved with more comprehensive data. [3] The goal of this research is to develop a wearable, realtime, foot drop correction system that can be attached to an individual. The system uses IMUs attached to the thigh, shank, and foot for predicting limb angles during gait. A commercial stimulator can be used to apply FES. The motivation for a multiple IMU system is so that comprehensive sensory information on lower limb angles can be obtained and thus the ability to provide multi-channel FES. Compared to existing single channel drop-foot systems, this will facilitate high fidelity, multichannel control of multiple muscles that govern gait. Figure 1 illustrates the parts of a typical closed loop control system that the study focused on.
2. Methods Three primary subsystems were designed. The IMU data collection system (IMU Mux), the Control Software, and the FES system. In Figure 2 the overall system architecture is illustrated. The low-cost Invensense MPU9250 was used. It supports sampling of the gyroscope and accelerometer at 1 kHz and is available on a high-quality board from Sparkfun. Testing verified that a single 2 MHz Serial Peripheral
Undergraduate Research at the Swanson School of Engineering