In martial arts, there is a saying that 1 minute of practice is worth 1000 minutes of theory. But how would someone go about making a Do It Yourself (DIY) powered exoskeleton? The solution is to start by concentrating all efforts on a single joint. The knee and the elbow are the two easiest joints to connect to and actuate. Different building materials can be used but the Lego NXT sets remain the ultimate hobbyist robotics kit that can be used and re-used over and over again.
Each Lego Knee Exoskeleton (video above) uses a Lego NXT control brick, 3 motors and reduction gear assemblies, connecting beams and 3.6m of double sided velcro. No pieces were glued, cut or damaged, so every single block can be re-used for later projects. You can buy pre-owned NXT sets for about $200 dollars each from the internet. Lego is inherently flexible and conforms easily to the contours of the human body. In general, Lego constructions have great difficulty transferring torque, but this limitation was bypassed by connected two motors against a third.
This simple and functional exoskeleton design was inspired by the AlterG Bionic Leg. The Bionic Leg is a very small device compared to most wearable robots in development and all components are integrated into a single mechanism. This design eliminates the usual back power pack and waist harness, limiting the power of the device but making it significantly simpler and more compact. The 3 Lego NXT motors combined provide enough torque to equal about 1% of the torque generated by the human knee, but their activation can still be easily felt.
The human machine interface was simplified to a button switch for each leg. As the user is prepared to take a step, the button is pushed which activates all 3 lego motors to assist in knee flexion and then quickly reverse direction to assist in knee extension. For the remaining time, the motors are turned off and the NXT motors and reduction gears provide minimum resistance. This setup reveals why so many powered exoskeletons use foot plates with pressure sensors or EMG sensors that detect nerve impulses to the leg muscles: any automation in the wearable device is better than having the user manually trigger the movement assistance.
Without more power, the Lego Knee Exoskeleton is a proof of concept rather than a useful device. Nevertheless, building simple DIY exoskeletons can provide great insight into wearable robotics beyond reading research papers and news articles.
For more information on the Bionic Leg visit the AlterG main website.