Introduction to Torque Curves

Introduction

Founding members of Laevo have been working on exoskeletons since 2009 and Laevo was founded in 2013. Those were the days people looked at you with frowning faces when you said you were working on an exoskeleton. Nowadays, most people have at least heard the word ‘exoskeleton’ and leading workplace safety experts all know what exoskeletons are. Over the years, a lot of new exoskeleton companies popped into existence, spreading the word about what exoskeletons can offer, which is great!

Currently, people can choose exoskeletons from several brands and several types. However, Laevo sees that the understanding of potential buyers about the differences between exoskeletons can be improved. Not because they do not select a Laevo exoskeleton, but because they pick an exoskeleton that is not suited for their situation. Laevo has seen people lose interest in exoskeletons because their first experience with exoskeletons wasn’t good, while the right exoskeleton might have been great for them.

There are many aspects that differentiate exoskeletons. As a first step, this page will focus on the support specification and aims to inform potential buyers about the often unseen differences in support between exoskeletons.

It is broken down into the following entry-level chapters:

1.     Support Definition

2.     Support Unit

3.     Torque Curve

4.     Conclusion

If you want to know how much power your car has, you can check the amount of horsepower. As exoskeletons are a relatively new product category, no standardized specification is adopted yet among exoskeleton manufacturers to describe one of the most important specifications of the exoskeleton: its support.

Laevo sees the tendency of exoskeletons manufacturers to express the support in kilogram (kg) as this is a unit that potential buyers recognize. But Laevo also sees that this ‘support’ definition is different for many exoskeletons manufacturers. These are a few definitions Laevo has encountered for trunk-supporting exoskeletons like the Laevo:

• The amount of force (converted to weight) applied on the user's torso by the exoskeleton.

• The reduction of weight going through the arms of the user when lifting something.

• A theoretical calculation based on the reduction of internal spinal forces and how this translates to how much support the user experiences.

• Finding the mass difference where the EMG measured muscle activity is about equal between lifting a certain mass without an exoskeleton and a higher mass with an exoskeleton.

These definitions are all fine, but having different definitions, results in different values. An exoskeleton specifying 25kg of support for one definition, might only specify 10kg of support in another definition. Potential buyers with limited knowledge about exoskeletons can't make a comparison about support with all of these different definitions used by manufacturers in their marketing materials. And the worst thing is because the used definition often is not even specified by manufacturers, most potential buyers do not even know they are comparing apples to oranges.

Like some exoskeleton manufacturers are already doing, we should all move to express support as Torque in newton-meter. Torque is a better specification to generally specify exoskeleton support, as it is independent of sizing and how forces are applied to the user.

All engineers know torque and its unit newton-meter, but potential exoskeleton buyers generally do not. But Laevo thinks it is time for these to leave engineer-territory and become a consumer-level specification for exoskeletons.

Understanding the example torque calculation below is not even necessary for potential exoskeleton buyers, it is about exoskeleton manufacturers using the same method and unit so potential buyers can compare exoskeletons in a helpful way.

Example torque calculation

For those unfamiliar with newton (N): newton is the unit of force. Force and weight can be easily converted into each other. In short, on earth, 1kg equals about 10N (9,81N to be more precise).

A torque is like a force, but around a pivot point. Because all exoskeletons exert a force around a (human) pivot point such as the hip, shoulder, or knee, torque is ideal to express the amount of support of exoskeletons. The unit of torque is newton-meter (Nm). Torque is calculated using a force in newton and a torque arm in meter. Multiplied, these create a torque in newton-meter. Therefore, the formula is:

Torque (Nm) = Force (N) x Torque Arm (m)

The torque arm is the distance perpendicular to the force’s working line to the pivot point. For trunk-supporting exoskeletons, Laevo suggests using the users’ hip joint as the pivot point. You can read more about why Laevo suggests using the hip joint in the next Learning Center article, referenced at the bottom of this article.

Below an example of torque calculation is shown. This user receives 50N of support force (so about 5kg) on his chest by an imaginary trunk-supporting exoskeleton, relieving the back muscles in this stooped position. In this example, the perpendicular distance of this 50N force’s working line to the hip joint is 0,4m. Multiplying 50N and 0,4m gives a torque of 20Nm.

SUPPORT FORCE AMOUNT IS SUPPORT FORCE LOCATION DEPENDENT!

Different exoskeletons use different locations on the body for exerting support. Some push on the lower chest, some more on the front of the shoulders, and some even push on the belly area. All of these locations have a different distance to the hip joint, and therefore a different torque arm, and therefore a different support force on the human body, while all potentially having the same amount of torque around the hip.

The same is true for different size settings within the same exoskeleton: because the torque arm changes when adjusting the size, each size setting has a different support force! Would it be honest to specify: “Up to 25kg of support!” in the exoskeleton marketing if the exoskeleton manufacturer knows this ‘up to’ support force value can only be achieved with the smallest user? Laevo does not think so.

Comparing support force on the body interfaces between different exoskeletons, therefore, is comparing apples to oranges. Comparing torques around the hip joint is apple to apples. Torque is independent of exoskeleton support force location and therefore a much better general definition of support.

But we need to go one step further. Laevo thinks it is impossible to express the support of an exoskeleton as a single number, as the amount of support of an exoskeleton is never constant throughout the supporting motion of the exoskeleton.

Introducing: the ‘Torque Curve’. Torque curves are not an invention of Laevo: scientists and engineers use these regularly to describe a system's behavior. To our excitement, exoskeleton researchers are requesting torque curves more and more to understand and model exoskeleton behavior. In short:

Below is a series of pictures showing different bending postures which correspond to specific bending angles indicated by B (beta) at the bottom of each picture. In general, picking up objects from lower positions require bigger exoskeleton bending angles, especially when the user bends the knees when lifting.

Two example Torque Curves are shown below. On the vertical axis, we can read the supporting torque of the exoskeleton, and on the horizontal axis, we can read the bending angle of the exoskeleton. The red and green lines represent example torque curves of two different exoskeletons: so how much support torque the user receives at a given bending angle in each of these exoskeletons.

The red torque curve is straight and reaches its peak torque of 60Nm at the end of its bending range at 120 degrees. The green curve reaches its peak torque of 60Nm after 40 degrees of bending, and after 40 degrees the support torque decreases back to 40Nm at a 120-degree bending angle. These are examples of torque curves, but differences like these are common in commercially available exoskeletons!

Both exoskeletons feature a peak torque of 60Nm and 120 degrees of bending range, so without these torque curves, their support specifications would be the same. But obviously, their support is very different!

Example: A potential user does not make deep bends but works on a waist-height factory line and lifts heavy objects from the line all day. His task would require an exoskeleton with high support at small bending angles, which corresponds to the green torque curve. Based on specifications as they exist in the current market, this user is unable to make the right choice between red and green because all exoskeleton manufacturers only mention peak support values, which are the same in this example. The user will find out after buying the red exoskeleton that it supports very poorly at the small bending angle and might reject all exoskeletons in the future because there is no specification that explains another exoskeleton will do better. This is a loss for all exoskeleton manufacturers. A torque curve support specification could have helped to prevent this situation.

Congratulations if you made it through this information! Thanks a lot for reading! If something is unclear, or have suggestions to improve this page, do not hesitate to contact us.

Laevo is certain that having a torque curve as a standard specification for exoskeletons is a step in the right direction. We have seen too many people make the wrong choice of exoskeleton because of bad or just the absence of information about support by the manufacturer, which costs them a lot of time and money. In the worst case, people will lose interest in exoskeletons because their first experience with exoskeletons wasn’t good, while the right exoskeleton might have been great for them.

We hope we sparked your interest in Torque Curves on this page. Are you interested to learn how Laevo is able to measure torque curves? Or how torque curves can alter your understanding of exoskeletons? Please continue to read our more advanced-level page ‘Learning from Torque Curves’ by clicking the button below.