This mechanical prosthetic leg can even move its toes

Watch a video of "Mechanical Ascension":

The little brother put on a mechanical prosthetic leg, walked up the stairs calmly, and even stepped across. Going downstairs is also perfectly fine:

In addition to the flexibility visible to the naked eye, its battery life is also excellent -it can take 15,460 steps on a single charge. In other words, if the wearer does not go out and run wild, it can be used for about 10 days on a single charge. (People wearing prosthetics walk an average of 1,500 steps a day). When a person walks on flat ground, the "passive mode" is activated, and they can walk without even charging.

Now, the powered prosthetic is featured on the cover of the latest issue of Science Robotics. The reason why it is so flexible is that the bionic knee joint, ankle joint, and toe of this powered prosthesis can move. The second is that it is much lighter than its predecessors, otherwise... it would be difficult to even lift a leg. So, how is this prosthetic limb made that can be filled with 3 layers of " lightweight + flexible + strong battery life " BUFF at the same time?

Prosthetic leg that generates electricity

The research team behind it is from the University of Utah in the United States. They mainly start with three important movement joints of the legs: Knee joints, ankle joints, and toe joints.

During walking in normal people, the torque of the knee joint (the product of force and moment arm) varies widely, for example, the torque of the knee joint when it is extended can be up to 4 times that when the leg is bent. In fact, powered prosthetics are not a new concept, but the power devices they carry are often cumbersome. In the previous power system, the shaft at the knee joint was relatively long. In order to alleviate the contradiction between the weight and volume of the equipment and the joint torque, the researchers came up with a new idea: On the sagittal plane (Sagittal Plain) of the kinematic joints, the principles of biomechanics are analyzed, and the elements are transferred to the mechanical system. The sagittal plane mentioned here is the anatomical plane that divides the human body or organs or tissues into left and right parts.

Based on this idea, they designed a unique torque-sensitive driver  (Torque Sensitive Joint) at the knee joint.

The drive is a passive variable transmission that not only can change the torque ratio continuously and rapidly, but also requires a relatively large reduction in motor torque and current. As a result, a small motor weighing only 170 grams can provide sufficient torque at the knee joint with a shorter shaft length. However, in addition to the torque, the movement speed of the knee joint is also an important factor affecting the bionic effect. Fortunately, the researchers found that the knee's motion speed and torque don't peak at the same time, so this small motor can provide enough speed.

Therefore, under the blessing of this torque-sensitive driver, the structure of the knee joint becomes more compact and lighter, and the bionic effect is also very nice~ In addition to the knee joints, there are also many "hard cores" on the bionic ankle joints and toe joints.

The researchers found that the torque of the toe joint and the ankle joint is almost proportional, and the speed of the two joints is equivalent and the direction is basically opposite. That is, while the toes expend power, the ankle generates power.

So here comes the point:

A single driver can power both the ankle and toe joints and requires fewer mechanical and electrical components than two separate drivers. (lightened again by the way) This device, which powers both the ankle and the toes, is called an underactuated system, meaning that it has less input than it needs to control.
The actual measurement results show that if only the ankle is powered, the power required for each step is 14.4J, and the overall efficiency is 43.8%. With the under-actuated design, the power required for each step drops to 8.2J, and the overall efficiency reaches 76.8%.

It can be seen that this can also save power step by step. The researchers calculated that with the current battery (2,400 mAh) of the prosthesis, it can take 15,460 steps in standard mode on a single charge. And this is still talking about the standard mode of using electricity. In passive mode, not only does not use electricity, but it can also be reversely charged :

The system can convert mechanical energy into electrical energy and can charge the battery with an average of 2.0±0.6J per step. In passive mode, the knee joint acts primarily as a damper, and the ankle joint acts primarily as a spring. Although this is not as natural as the standard mode, it is more than enough to walk on flat ground.

In addition, there is a little innovation:

In this mode, the knee and ankle prostheses move mechanically, much like they would under microprocessor control, but none of the other existing electric prostheses do. So far, three above-knee amputees have tried the cool prosthetic. The results show that although their walking style is unique, they can eventually walk independently, which is not significantly different from normal people.

The researchers believe that this prosthesis is suitable for a wide range of people: amputee patients with a height between 160 and 191 cm and a net weight of 59 to 91 kg can wear it normally. Of course, they also said that this prosthesis still has some room for improvement. For example, the wearer cannot control the ankle and toe joints independently, and the ratio between the torques two is fixed and cannot be changed according to the user's needs or preferences. In the future, they will carry out more practice to improve the practical effect of the prosthesis.

In fact, there may be more amputee people in the world than everyone thinks. According to reports, about 280,000 people in my country have their feet amputated each year due to diabetes. The light and the easy-to-use prosthesis are undoubted of great significance to this huge group.

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