The ostrich foot has excellent travelling performance on sand and plays a vital role in efficient locomotion. The tendon-bone assembly characteristics of an ostrich foot were studied by gross anatomy, and the 3D model of ostrich foot was reconstructed and analyzed using reverse engineering techniques. Further, the bionic mechanical foot, suitable for locomotion on loose sand, was designed based on the structural characteristics of ostrich foot and its rigid-flexible coupling mechanism of tendon-bone synergies. The travelling performance on sand of the bionic mechanical foot was tested on a test platform by using Simi Motion. After analyzing the angle changes of the ankle joint and the metatarsophalangeal (MTP) joint, the displacement changes of the knee joint, the ankle joint, the MTP joint, and each phalanx along the
Bionic walking robot has been widely applied in many fields due to its strong adaptability in complex environments, such as desert, mud, and ruins [
The hindlimb of the African ostrich (
Each phalanx touches the ground at the same time and leaves the ground in order when an ostrich walk on loose sand, which improves the travelling performance and trafficability on sand [
Ostrich (Struthio camelus Linnaeus, 1758) foot was purchased from an ostrich farm in Heilongjiang province (Figure
Ostrich foot specimen and 3D model. (a) Ostrich foot specimen. (b) Dissected ostrich foot. (c) 3D model of ostrich foot. (d) 3D simplified model of ostrich foot.
The ostrich foot specimen was scanned using computed tomography (CT; Philips MX 8000 IDT 16 scanner, 220 kV, 220 mAs, and 1.25 mm slice thickness), and the three-dimensional (3D) model of the ostrich foot was reconstructed by Mimics and Geomagic Studio (Figure
The bionic mechanical foot has a similar structure and function compared with the real ostrich foot. (1) In terms of structure, the bionic mechanical foot also has a two-toed structure, and an imitated ligament structure was added between the two toes. Moreover, in order to imitate the function of the flexor tendon and extensor tendon of the ostrich foot, the energy storage and shock absorption mechanism and the flexible passive control mechanism were designed. (2) In terms of function, the bionic mechanical foot can imitate the movement postures of the ostrich foot. During locomotion, its two toes can open and close, the phalanges of the third toe can leave the ground in order, and the toenails can thrust against the ground.
The structure of the bionic mechanical foot (Figure
Bionic mechanical foot structure: (1) support plate, (2) stud, (3) flexion spring, (4) flexion frame, (5) the first phalanx of the auxiliary toe, (6) imitated ligament structure, (7) the second phalanx of the auxiliary toe, (8) the fourth phalanx of the main toe, (9) toenail structure, (10) the third phalanx of the main toe, (11) extension spring, (12) the second phalanx of the main toe, (13) the first phalanx of the main toe, (14) pulley, (15) wire rope, and (16) lever.
The left end of the lever was connected with the energy storage and shock absorption mechanism and the linear servoelectric cylinder, and the right end was connected with the flexible passive control system. When the linear electric cylinder moved downward, the flexion spring in the energy storage and shock absorption mechanism was compressed to store energy and shock absorption. The extension spring in the flexible passive control mechanism was stretched to store energy, and the steel wire rope drove the imitated ligament structure to move to the left. Then, the auxiliary toe and the main toe separated to realize the opening of the two toes. After the lever moved to contact with the baffle, it continued to move upward, and the mechanical foot lifted. Finally, when linear electric cylinder moved upward, the flexion spring in the energy storage and shock absorption mechanism and the extension spring in flexible passive control mechanism released energy simultaneously, and the wire rope moved to the right to achieve the closure of two toes. When the lever moved upward, it touched the main structure and drove the whole foot to move. The running speed and running time of the linear servoelectric cylinder were adjusted by the controller.
The actuator of the bionic mechanical foot was a linear servoelectric cylinder with a maximum load of 350 kg. The linear servoelectric cylinder was composed of a servomotor, electric cylinder, and connecting parts. The rated speed of the servomotor (ECMA C20604RS) was 3000 r/min and the rated torque was 1.27 N m.
In order to reduce the weight of the bionic mechanical foot, all parts were made of aluminum alloy while ensuring its rigidity. Because most of the parts on the mechanical foot were not standard parts, the processing method adopted was mainly linear cutting. The diameter of the wire rope adopted by the flexible passive control mechanism was 3 mm, the minimum breaking tension was 5.07 kN, and the maximum bearing capacity was 517.3 kg. The bionic mechanical foot was equipped with a variety of elastic elements; according to the function and type of elastic element, the flexion spring was used to replace the flexor tendon of ostrich foot, and the extension spring was used to replace the extensor tendon of ostrich foot. The bionic mechanical foot after assembly is shown in Figure
Bionic mechanical foot after fabrication and assembly.
Specifications of the bionic mechanical foot.
Name | Value |
---|---|
The main toe (mm) | 320 |
The auxiliary toe (mm) | 165 |
Imitated ligament structure (mm) | 120 |
Weight (kg) | 8.2 |
This section mainly introduces the process of the travelling performance test of the bionic mechanical foot on the test platform. The test was carried out on the solid ground and the loose sand at low speed, medium speed, and high speed, respectively, and the kinematics data of the bionic mechanical foot under six working conditions were processed by Simi Motion. Furthermore, the plantar pressure changes under six working conditions were measured by the thin-film pressure sensor. Combined with the kinematic analysis results of the bionic mechanical foot, the excellent travelling performance on sand of the bionic mechanical foot was verified.
The bionic mechanical foot test platform was an important equipment for the travelling performance test of the bionic mechanical foot on sand and was mainly composed of a frame and a hanger. The overall dimensions of the frame were
Test platform and camera position distribution diagram.
In order to test the performance of the bionic mechanical foot, we designed a mechanical leg that imitated the structure of the ostrich hindlimb (Figure
Mechanical leg for testing the bionic mechanical foot: (1) T-plate, (2) motor transmission shaft, (3) imitated femur structure, (4) bionic mechanical foot, (5) actuator of the foot, and (6) imitated tibia structure.
In order to clearly record the trajectory of each marker, white reflective balls were pasted on the corresponding marker before the travelling performance test of the bionic mechanical foot on sand, and a total of 8 markers were selected (Figure
Position of the bionic mechanical foot markers.
The speed of the bionic mechanical foot was controlled by the main motor on the leg and linear servoelectric cylinder on the foot. The main motor controlled the forward speed of the foot, while the linear servoelectric cylinder controlled the lifting and falling speed of the foot. These two actuators cooperated to control the locomotion of the bionic mechanical foot. We set the low, medium, and high speeds to test the kinematics of the bionic mechanical foot on the loose sand and solid ground by changing the parameters of the main motor and linear servoelectric cylinder (Table
Speed parameters of the actuator on the bionic mechanical foot.
Name | Low speed | Medium speed | High speed |
---|---|---|---|
Main motor frequency | 6.00 | 8.00 | 10.00 |
Linear servoelectric cylinder frequency | 38000 | 42000 | 46000 |
Linear servoelectric cylinder pulse | 46500 | 46500 | 46500 |
Before the test, the space calibration was carried out on the loose sand and the solid ground, respectively, by using the calibration frame. The sand was leveled before each test, and the parallelism of sand was measured with the infrared tester to ensure that the angle displayed did not exceed 0.1°. A hardwood board was laid flat on sand as the solid ground, and the solid ground was consistent with the height of the loose sand. The size of the hardwood board was
The plantar pressure acquisition of the bionic mechanical foot was carried out by the piezoresistive flexible film pressure sensor (D2027). The plantar pressure was calculated according to the weight of the bionic mechanical foot and mechanical leg, as well as effective area of the foot. Finally, a thin-film pressure sensor with a measuring range of 15-25 kg was selected. The outer diameter of the sensor was 27 mm, and the diameter of the sensing area was 20 mm. According to the structure of the bionic mechanical foot and data collection requirements, we attached three thin-film pressure sensors to the bottom of the third phalanx of the main toe, the second phalanx of the auxiliary toe, and the toenail.
The test was also carried out under six working conditions; the movement speed of the bionic mechanical foot was consistent with that in the movement parameter collection (Figure
The travelling process of the bionic mechanical foot. (a) Travelling process on solid ground. (b) Travelling process on loose sand.
The movement video of the bionic mechanical foot was processed by Simi Motion, and the kinematic data of each marker was extracted. A total of 12 videos were processed in this test, with 6 for the solid ground and 6 for the loose sand. The average speed and average foot spacing of the bionic mechanical foot under six working conditions are shown in Table
Average speed and average foot spacing of the bionic mechanical foot.
Working conditions | Average speed (mm/s) | Average foot spacing (mm) |
---|---|---|
S-L | 27.22 | 506.0 |
S-M | 42.60 | 554.5 |
S-H | 64.00 | 639.5 |
L-L | 28.31 | 505.0 |
L-M | 46.92 | 564.0 |
L-H | 60.43 | 591.0 |
On the same ground, the average speed and average foot spacing of the bionic mechanical foot gradually increased with increase of the running speed. When the main motor and linear servoelectric cylinder were set to operate at medium and low speeds, the average speed of the bionic mechanical foot on the loose sand was higher than that on the solid ground. When the main motor and linear servoelectric cylinder were set to operate at high speed, the average speed of the bionic mechanical foot on the solid ground was higher than that on the loose sand, and the average foot spacing was also significantly larger. It is difficult to see the performance difference between the bionic mechanical foot on the solid ground and loose sand from the average speed, so it is necessary to further analyze the kinematics performance of the bionic mechanical foot.
The angle changes curve of the ankle joint and the MTP joint of the bionic mechanical foot is shown in Figure
The angle changes curve of the ankle joint and the MTP joint. (a–f) The angle changes of each joint under six working conditions.
The displacement curve of the ankle joint, the MTP joint of the bionic mechanical foot, and the knee joint along the
The displacement curve of the knee joint, ankle joint, and MTP joint along the
S-L
L-L
S-M
L-M
S-H
L-H
In order to study the movement rule of the phalanges of the bionic mechanical foot, the second phalanx of the auxiliary toe, the second phalanx of the main toe, the third phalanx of the main toe, and the toenail were selected as the research objects. The displacement curve of the phalanges of the bionic mechanical foot along the
The displacement curve of the phalanges along the
S-L
L-L
S-M
L-M
S-H
L-H
When the bionic mechanical foot moved at medium and low speeds, its movement speed on the loose sand was higher than that on the solid ground, while the situation was just the opposite when moving at high speed. Through the analyses of the angle changes of the ankle joint and MTP joint, as well as the displacement changes of the knee joint, ankle joint, MTP joint, and phalanges along the
The curve of the plantar pressure of the bionic mechanical foot under six working conditions is shown in Figure
The plantar pressure change curve of the bionic mechanical foot. (a–f) The plantar pressure changes under six working conditions.
S-L
L-L
S-M
L-M
S-H
L-H
When the bionic mechanical foot moved on the solid ground, the middle part of its pelma had no direct contact with ground, and the traction was mainly provided by the front end of the main toe and the auxiliary toe. When the bionic mechanical foot moved at medium and low speeds on the loose sand, its movement posture was similar to the real ostrich foot. When it touched the ground, each phalanx contacted the ground at the same time, and when it left the ground, each phalanx left the ground in order. Moreover, the plantar pressure of the bionic mechanical foot on the solid ground was higher than that on the loose sand, and the contact time between the bionic mechanical foot and the ground was longer than that on the loose sand. Through the analyses of the change of plantar pressure, the bionic mechanical foot designed in this paper is more suitable for medium-speed and low-speed locomotion on the loose sand; it is helpful to reduce the sinkage and improve the trafficability on the loose sand.
The footprints of the bionic mechanical foot on the loose sand at different speeds were shown in Figure
The footprints of the bionic mechanical foot on the loose sand. (a) Footprints at low speed. (b) Footprints at medium speed. (c) Footprints at high speed.
In this paper, a bionic mechanical foot suitable for locomotion on loose sand was designed based on the structural characteristics of the ostrich foot and its rigid-flexible coupling mechanism of the tendon-bone synergies. Like the real ostrich foot, the mechanical foot can open and close its two toe structure when moving, with the phalanges of the main toe successively off the ground, and the toenail thrust against the ground. The travelling performance of the bionic mechanical foot on the loose sand was tested on a test platform by using Simi Motion.
The speed of the bionic mechanical foot under six working conditions (S-L, S-M, S-H, L-L, L-M, and L-H) was, respectively, 27.22 mm/s, 42.60 mm/s, 64.00 mm/s, 28.31 mm/s, 46.92 mm/s, and 60.43 mm/s. When the bionic mechanical foot moved at medium and low speeds, its movement speed on the loose sand was higher than that on the solid ground, while the situation was just the opposite when moving at high speed. By analyzing the angle changes of the ankle joint and MTP joint and the displacement changes of the knee joint, ankle joint, MTP joint, and each phalanx along the
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that there is no conflict of interest regarding the publication of this paper.
We would like to thank the National Natural Science Foundation of China (Nos. 51675221 and 91748211), the Science and Technology Development Planning Project of Jilin Province of China (No. 20180101077JC), and the Science and Technology Research Project in the 13th Five-Year Period of Education Department of Jilin Province (No. JJKH20190134KJ).