Gravity Balanced Systems

Through the addition of springs or compliant elements, mechanical systems can be designed such that the gravity's effect is removed from their components. This idea can be applied to the design of a wide range of industrial machinery and household appliances. Our laboratory has used the underlying scientific principles in the design of leg and arm orthoses for rehabilitation applications. 


Featured Projects

We design an active gravity balanced planar mechanism, where auxiliary parallelograms are used to physically locate the center of mass of the mechanism. A sliding carriage positions a counterweight directly above the center of mass of the mechanism in order to make the system gravity balanced.  We fabricated a three-link serial chain prototype to demonstrate the effectiveness of the approach.

gravity balance
a gravity balanced 3-link series chain augmented with parallelograms

We present gravity balancing designs for two and three links planar chains using non-zero free length springs. These springs are cheaper, easily available and can be useful when complete weight balancing is not required. These designs are further optimized for spring connection points and parameters of the spring such as free length and stiffness to achieve greater balancing.

gravity balance, non zero free length spring orthosis
Joint torque for a three link design: without spring, nominal, and optimal (from left to right)

A sit-to-stand assist device can serve the needs of people suffering from muscle weakness due to age or disabilities that make sit-to-stand a difficult functional task. We design a passive gravity-balancing assist device for sit-to-stand motion. First, this passive device uses a hybrid method to identify the center-of-mass of the system using auxiliary parallelograms. Next, appropriate springs are connected to the device to make the total potential energy of the system due to the gravity and the springs constant during standing up. A prototype with the underlying principle was fabricated to test the feasibility of the proposed design.

gravity balance sit to stand
The sit-to-stand assist device with the subject in sit and stand positions

We design a gravity balanced device to assist persons with hemiparesis to walk. The systems can be completely passive, and thus do not require any actuators. Also, the system can be adjusted to accommodate the variability in geometry and inertia of the lower limbs.

Gait rehabilitation, gravity balancing, inverse dynamics, passive orthosis, rehabilitation robotics
Parameters and coordinates of the gravity-balancing mechanism

Leg-raising and walking experiments were performed on five healthy subjects and a stroke patient. Electromyographic (EMG) data of the key muscles, involved in the motion of the leg, were collected and analyzed. The results showed the decrease of EMG activity from the rectus femoris and hamstring muscles during static hip and knee flexion. The average torque at the hip joint also decreased by 61.3% during the whole leg-raising task. In the walking experiment, there was a positive impact on the range of movement at the hip and knee joints, especially for the stroke patient: the range of movement increased by 45% at the hip joint and by 85% at the knee joint.

Gait rehabilitation, gravity balancing, inverse dynamics, passive orthosis, rehabilitation robotics, EMG
IEMG percentages averaged across the five subjects for stepping task

An upper-arm wearable exoskeleton has been designed for the assistance and functional training of humans. One of the goals of this design is to provide passive assistance to a user by gravity balancing, while keeping the transmitted forces to the shoulder joints at a minimum. We perform the numerical optimization of the system for gravity balancing and minimization of transmitted forces, as well as the effect of parameter variation on joint moments and joint forces.

arm exoskeleton, gravity balancing, assistive device, portable systems, rehabilitative device
Kinematic chain of the arm with zero configuration and axes of rotation of the arm, and moment plots at the shoulder joint: (a) unbalanced system: (b) balanced system for the full range; and (c) balanced system for a smaller range of the arm motion