The IVT’s transmission ratio includes a zero point that can be approached from either a positive side or a negative side. In other words, a constant input, like an electric motor turning the same direction at the same speed, can be converted to an output that’s turning faster, turning slower, turning the opposite direction, or not turning at all (in this ‘geared neutral’ mode, infinite input revolutions are required to cause one output revolution, hence the name infinitely variable transmission).
The concept is not brand new. What SRI has done is made its Inception Drive an order of magnitude smaller and lighter than existing IVTs. This means that the Inception Drive could be integrated into robotic applications in a way that was physically impossible before.
The reason why a transmission like this is important for robotics is primarily because of the efficiency that it enables, as SRI’s principal research engineer - robotics and medical systems, Alexander Kernbaum explained: “It’s not uncommon for a robotic transmission to be less than 50% efficient. When the output speed changes frequently, it is impossible to optimise a motor and fixed-ratio transmission system for size, performance, and efficiency at the same time.”
The fixed ratio of the transmission causes substantial losses within the motor. For instance, a high-quality electric motor may be 90% efficient when operating under high velocity and low torque, but in robotic applications, the system must instead be designed to achieve the needed peak torques and speeds in the smallest package possible, resulting in a system that rarely operates near its peak efficiency.
A variable-ratio transmission can help align the motor velocity with its peak efficiency or peak power, but their size, weight, and complexity have historically precluded their use in most robotic and industrial applications.
“This is where SRI’s ultra-compact IVT can play a key role,” Kernbaum continued. “Its small size, simplicity, and reversible output enable several new applications for variable-speed transmissions. It is small enough to replace fixed-ratio transmissions in robots, where we believe it can cut the energy consumption of many robotic platforms in half, doubling battery life for mobile platforms.”
The Inception Drive is made up of two pulleys that interact with a V-belt, but not in a conventional way. Rather, the outer pulley is fixed and doesn’t turn at all, while the inner pulley, which is nested inside the drive, is mounted off-centre to the input shaft.
As the input shaft causes the inner pulley to wobble in a circular motion, it continuously lifts the V-belt off the outer pulley on its outward wobble, and putts the V-belt back down again when the wobble is inward. If the effective diameter of the inner and outer pulleys is the same, the V-belt will get transferred back and forth between the two pulleys. But if the effective diameter of the inner and outer pulleys is different, the V-belt will get pulled in one direction or the other as it tries to keep the two pulleys connected. Since the outer pulley doesn’t rotate, the force exerted by the V-belt makes the inner pulley rotate instead, this is the output of the transmission.
Each pulley splits into ‘pulley pairs’, with the V-belt running between them. To alter the transmission ratio, one pulley pair moves farther apart, causing the V-belt to drop deeper into the groove between the pulley halves, while the other pulley pair moves closer together, causing the V-belt to ride up higher in the groove between the pulley halves.
This changes the effective diameter of the pulleys — as far as the V-belt is concerned, the first pulley just got smaller, while the second pulley just got bigger – and this changes the transmission ratio. Making one pulley effectively larger than the other will drive the output in one direction, while switching the ratio will drive the output in the reverse direction, and the ratio can be varied smoothly all the way down to zero, where the pulleys are the same effective diameter and the output doesn’t move at all.
SRI acknowledges that “there is a great deal to be learned about this transmission”. For example, while the gear ratio can’t be changed instantaneously, it is said to be quick enough for most robotic applications. There is also work to do to investigate total system efficiency as well as durability, and SRI hopes the next generation of prototypes will help to more fully quantify the characteristics of the drive.