Introduction
When you are new to occupational exoskeletons, you will inevitably come across different categories and definitions of what kind of exoskeletons exist.
Categories like “lift-exoskeletons” or “overhead exoskeletons” describe the kind of activity and work supported. Categories like “active” or “passive” exoskeletons and “soft” or “rigid” exoskeletons represent what technology is used to build them.
We have seen that the categorization by technology can be a source of confusion, which makes it difficult for interested users to decide what kind of exoskeletons might be right for them. This article will discuss the difference between passive and active exoskeletons and highlight why passive systems can be an excellent choice for many applications.
What are the differences between active and passive exoskeletons?
Active exoskeletons have integrated powered actuators that support the user with the required force and torque. They are, therefore, also referred to as powered exoskeletons. Electric motors are usually used as actuators, but you can also find pneumatic or hydraulic systems. Passive systems, on the other hand, lack these ’active’ actuators. They instead rely on springs and their frame to support their users typically by providing gravity compensation.
What are the advantages of passive exoskeletons?
The main advantage is that, by not integrating active components like the motors, power supply, control computers, etc., passive exoskeletons are typically much smaller, lighter, less constraining, easier to use, require less maintenance, don’t need recharging of the batteries, and are less expensive.
A test at DPD, an international parcel delivery service, has recently demonstrated the importance of a lightweight design. During the test, DPD tested active and passive lift support exoskeletons Article in German.
Their results and feedback identified the high weight of the active exoskeleton as a significant problem that lowered the acceptance by the workers:
"The response for the active exoskeletons was mixed. In particular, the extra weight was perceived as uncomfortable over a longer period of time. In addition, for lighter packages, the exoskeleton was perceived as more of a burden. As a result, it was also not used as often as planned."
The passive exoskeletons, on the other hand, were well received explicitly because of their reduced weight and better flexibility:
“The reduced weight makes the passive exoskeletons more comfortable to wear. You can compare it to a winter jacket. They are more flexible, lighter, and more scalable. The feedback from the employees is always positive, and the wearing time is massively increased.”
And while the advantage of a 1kg exoskeleton over an 8kg exoskeleton is relatively intuitive, on the other hand, it is often argued that active systems with their powerful motors can, by default, provide more support and therefore justify the increase in size, mass, complexity, and costs.
This is a common misconception recently highlighted by two scientific publications that evaluated the support level of a passive, soft exoskeleton and a relatively similar active, soft exoskeleton design. The first study "Link" showed that a passive textile exoskeleton reduces the peak back muscle activity by 14% to 21% when lifting 6kg.
The second study "Link" tested an active textile exoskeleton and reported peak back muscle load reductions between 16% and 18% when lifting a similar load of 6kg.
In this comparison, the passive system offered equal support during a similar task while being significantly smaller and lighter, not requiring a battery, and costing less.
Figures visualizing the peak back muscle activity reduction when lifting 6 kg while the abdominal muscles do not need to work more.
The second study also highlights the disadvantage of the increased weight of the active exoskeleton. The active exoskeleton in the study had a mass of 2.7 kg. Around 2.5 kg of it (the actuation unit) is located on the upper back of the user. This weight pushes the upper body downwards during lifting, directly counteracting the exoskeleton’s support. Or in other words, the exoskeleton provides support when you lift loads, but it also makes you lift an additional 2.5 kg each time.
Another common misconception is that only active systems provide real support, while passive systems only redistribute the load. This argument is typically pushed by manufacturers of active exoskeletons but cannot be justified by biomechanical principles, nor is it supported by scientific evidence. The reason is that adequately designed passive exoskeletons simply counteract gravity and do not require the user to invest additional energy. The first study mentioned above actually measures abdominal muscle activity and lifting kinematics. The results showed that the passive exoskeleton provides back support while the stomach muscle activity was not increased, and the lifting form was not negatively affected.
In the end, you simply do not need a powered actuator for gravity compensation. A mechanical spring, well integrated into a passive exoskeleton, can provide a similar level of support.
Summary
These examples highlight some of the advantages passive exoskeletons can have over their active counterparts while addressing some of the common misconceptions about passive exoskeletons. This does not mean passive exoskeletons are always superior, and there are applications, especially in the medical field, where active exoskeletons can be the better choice. But the examples given here and our experiences in the field have led to the conviction that a majority of occupational exoskeleton users will profit the most from a passive exoskeleton.
Ultimately, the best way to assess if an exoskeleton is suitable for you is to test it for a while in the intended environment. We invite you to contact us if you have any questions on the topic.