HARDWARE & SOFTWARE
Christopher Obiozor
Armed Force's Exoskeleton is controlled via a hand-held remote, or a phone App. Both the remote and the App wirelessly communicate to the Arm's on-board processors. The Exoskeleton Arm is controlled via an Arduino Uno. But, it was designed to easily integrate EMG signal processing, to actively predict the wearer's intended motion via EMG sensors placed on the wearers arm. The Exoskeleton Arm was designed to be powered through a 120 V wall socket, and utilizes a DC converter. The motors in this design are the Lx-16a Servo motor, to power the Cam, and the NEMA-34 and NEMA-24 stepper motors, with their associated stepper drivers.
DIGITAL REMOTE
The Power Arm has been designed for single handed operation using a remote held in the hand of the arm opposite that of the power arm. The digital remote was implemented with an analog joystick and touch sensor for input, using an Arduino Nano as a processor, a 433 MHZ transmitter for wireless communication, and two 3.7V LiPo batteries wired in series - charged micro-USB LiPo chargers.
MICRO-CONTROLLER & MOTOR CONTROLLER
Armed Forces' Exoskeleton is designed to need no more than the computing power of an Arduino compatible ARM based microprocessor with flash memory, processing speed, and functionality not exceeding that of the Arduino UNO. The closed-loop control system, stepper driver, and servo driver are all easily handled by existing libraries which can also modulate motor acceleration and speed. Nonetheless, for the sake of keeping the door open to user controlled motion via EMG, all aspects of the project are simultaneously designed for easy substitution or addition of a Raspberry Pi 4 Model B+, which has a Unix environment allowing the use of python libraries for the real time signal processing needed to interpret EMG signal. It is possible to integrate the Arduino and Pi together via the Serial Peripheral interface to take advantage of the Arduino’s analog sensing capabilities and the Pi’s processing capabilities.
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POWER REQUIREMENTS
The Arduino Uno operates at a voltage of 5V DC and has a nominal current draw of 50mA under load, and a Raspberry Pi should be supplied with 5V DC at a nominal current of 3A. In addition to this, the handheld remote utilizes an Arduino Nano, with an operating voltage 5V at a nominal current of 19mA.
From characteristics considering power and weight, a NEMA-34 motor was selected for the shoulder joint. A NEMA-34 motor with the largest possible max torque, 13 N-m, was chosen in order to maximize the load lifting capabilities of the exoskeleton system. While a vendor search demonstrated that many smaller motors have attached gearboxes in order to further augment their torque, NEMA-34 motors are rather limited in this respect, thus the motors’ output torques must be modulated by the cable pulleys they are attached to. Based on the routing configuration of the bowden cable, a safe operating torque of about 8 N-m and a pulley radius of about 1 inch will allow a fully extended arm (~2 ft long), parallel to the arm, holding the target load of about 30 lbs.
Again approximating the human forearm as a foot long, the torque needed to actuate the elbow joint when carrying 30 lbs of load would equal about 40 N-m, which would correspond to approximately 118 N of force transmitted through an actuation cable mounted 9 inches from the pivot point. With a pulley radius of about 1 inch, a 3 N-m operating torque would be required. A NEMA-23 motor with 4 Nm of peak torque was selected for this purpose.
Both these motors need to be driven using high voltage digital stepper motor drivers like the Y-Series brands from Stepper Online. The NEMA-24, operating at a max amperage of 4.24A, a would require a driver in the neighborhood of the Stepper Online DM556Y, rated for 1.7-5.6A at 20V-50V DC. Similarly, the NEMA-34, operating at a max amperage of 6A, would require a driver like that of the Stepper Online DM860Y, rated for 2.4-7.2A at 36V-110V DC. Thus, an AC to DC power supply able to supply more than 10.24A and an additional 3.5A to account for both the motor power draw and the micro-controller. Power supplies at this amperage often have an operating voltage of 36v to 110V DC, thus ~1000W-1400W is a reasonable expectation of the Power Arm power draw.
REMOTE
A graphical user interface was designed for operation with the Power Arm. This GUI can take parameters of max movement speed and parameter precision. Displayed in real time are the arm joint angles and power output. Above is the GUI made for use with the digital remote. For increased accessibility, a version of the GUI was adapted for mobile devices and serves to replace the digital remote.