Torque Curve Measurement Service

NEW: Now including Vertical Compression Force measurement!

Introduction

Many years ago, Laevo developed a measurement device to measure exoskeleton torque curves. The device was initially developed for research and development of Laevo exoskeletons. For example, measuring torque curves at set intervals during a life cycle test gives great insight into the performance that can be expected from an exoskeleton over time and where mechanical improvements can be made. The device is still used regularly for research and development, but also for quality control of new production batches of parts and assemblies.

Over the years, the existence of this device became known in our network, and researchers started to reach out about the possibilities of comparing different exoskeletons using torque curves. In Laevo’s opinion torque curves are the best and most informative specification for exoskeletons when it comes to objectively describing exoskeleton support behavior, so Laevo was not shy to provide external access to this device to potentially strengthen our standpoint. In the meantime, many papers have been written using measurements done on this setup as a basis.

Laevo has 3 free-to-read Learning Center pages about what Torque Curves are and their use for exoskeletons. Visit the first one using the button below:

Laevo is now opening the doors even more, offering an exoskeleton torque measurement service for whoever is interested!

Description of the torque measurement setup

The device consists of two main parts: (1) a calibrated Testometric M250 universal testing machine and (2) a bending mannequin that stoops forward around the hip joint. These two are connected by a cable and pulley system to convert the linear vertical movement of the testing machine to a rotational stooping motion of the mannequin. The testing machine measures the force through this cable throughout the stooping motion which describes the effort the machine must make the mannequin stoop forward.

This force measurement can be converted to torque based on the dimensions of the pulley system inside the mannequin. Subtracting the measurements of the empty mannequin and the mannequin wearing an exoskeleton provides the pure torque the exoskeleton applied to the mannequin, even considering the gravitational effects of the mass of the exoskeleton parts mounted on the torso. Bending down as well as coming back up are recorded in one measurement, as this complete cycle unlocks further exoskeleton specification data.

now including Vertical Compression Force sensor!

The device has received many upgrades over time, improving the accuracy of the device and its compatibility with other exoskeletons. The last major upgrade is the introduction of an additional load cell inside the torso of the mannequin that measures how much force the exoskeleton is pulling down on the torso of the mannequin, throughout the entire stooping motion. We write 'vertical’, but what we mean is parallel to the mannequin spine.

Using this sensor Laevo can provide additional objective data showing the differences between exoskeletons. Laevo believes it is good to minimize the vertical compression force the exoskeleton applies on the torso to optimize the reduction of spine compression force, which is what back-supporting exoskeletons should be doing. Read our Learning Center page to learn how heavy lifting creates high spine compression and what it can do to the spine. A new white paper will also be online in our Learning Center soon, showcasing measurements using different exoskeletons.

The results

As a client of this service you can of course opt to take the raw data and process it yourself, but Laevo can also make your life a bit easier using a Laevo-written, and many times verified, script that can automatically output multiple things:

• Torque-angle graphs
  As mentioned, the script converts the force-deflection output from the Testometric to torque-angle data based on the pulley geometry inside the mannequin. Plotting this data creates the torque-angle graph, also known as the torque curve. This graph already provides the most important information:

• The amount of torque
  Describes the amount of support the exoskeleton can provide during the mannequin its entire stooping motion.

• The shape of the curve
  This is one of the biggest differentiators when it comes to specifying exoskeleton support. Using the torque curve, you can see what torque the exoskeleton delivers at every bending angle. Many exoskeletons feature a linear torque curve, meaning that the peak torque is at the deepest bend and builds up gradually. However, other exoskeletons have a different shapes. Laevo for example, places the peak torque at around 40 degrees of bending using its Smartjoint technology.

• The difference in torque between bending down and coming back up
  You will notice that almost every exoskeleton has a different torque curve when comparing stooping down to coming back up. Sometimes vastly different. More about this in the next bullets. Torque curves are the best tool to visualize this difference.

• Stored energy and returned energy
  The script can automatically detect when the mannequin is in its deepest bend and calculates the energy stored in the exoskeleton while stooping forward, and separately the energy that is given back to the user when it comes back up. These amounts say something about the amount of work the exoskeleton does for the user. In other words, energy the user does not have to put in himself anymore because the exoskeleton is supporting his work. It might therefore be an even better indicator of the amount of support of an exoskeleton than a torque curve is.

• Hysteresis
  The subtraction of the total stored and returned energy is the energy lost in the exoskeleton such as friction due to rubbing of parts. The ratio between this lost energy and the total energy stored is called the hysteresis property, which is an excellent performance indicator for passive exoskeleton efficiency.

• NEW: Vertical force-angle graphs
  Very similar in appearance to torque curves, we are now able to provide measurements of the vertical force applied by the exoskeleton to the torso of the mannequin over the entire stooping cycle.

Please read our white paper about this device and have a look at the example measurements we made in the Learning Center or use the button below:

Interested?

Are you interested or do you have questions? Just contact us using our contact form. We are ready to help!