MTech Project :

“Development and Analysis of a Soft Humanoid Robot Made up of Flexible fluidic Actuators and Compliant Links”

 

By Bhushan Darekar MTech Mechatronics College of Engineering Pune

Under the guidance of Dr. S. S. Ohol

The robot consists of soft compliant links and flexible fluidic actuators that make up the soft arm of the robot designed for payload 500 gram. An arm consists of three Pneumatic Network Actuator(PNA) gripper, a metacarpal joint(palm), a variable stiffness forearm, an elbow joint actuator, a twist actuator and finally a shoulder movement mimicking mechanism. The PNA grippers act as fingers, the elbow actuator acts as the bending joint while the twist actuator induces the twisting action in the arm, all actuators are pneumatically controlled by onboard embedded electronics system

Spanda is an intrinsically safe soft humanoid robot made up of flexible actuators and compliant links, capable of working in close proximity with humans.

    Spanda (A Soft Humanoid Robot) is the first generation of soft-humanoid robots capable to be used in human assisting functions such as handling objects of different sizes and shapes, as an Assistive Companion, a Navigator, a Walk-through Guide, a child-friendly cuddly robot, etc.

Spanda is inspired from Baymax, the soft robotic superhero from the Marvel comic series of the same name and the movie ‘Big Hero 6’. Robots have always been perceived by people as a Heavy-metal structure, Which can kinetically cause harm to humans if there are any moveable links. Soft-robotics is a sub-branch of robotics that deals with breaking this traditional perspective by making the robot more compliant in nature and inspired from living beings. Robots which are inherently soft and cushy for interaction would prove to be much preferable in our daily assistance at Homes, Offices, Hospitals and Industries, thus Impacting the society.

Intrinsic safety is the important aspect that makes Spanda a unique robot. Soft robots like Spanda are specially designed for compatibility to work alongside humans without causing any potential harm. Its soft robotic humanoid arm is biologically inspired to mimic the human arm. The main objective of soft robotics is not to make a super precise machine but to make a machine compatible to work in any real-life scenario effectively and with considerable safety just like a biological entity. Conventional robots use high precision systems which eventually multiply the cost and thus makes this technology difficult to reach out to masses. On the other hand, Spanda is a first of its kind soft-humanoid robot which is very cost effective and has the capability to make a paradigm shift in the field of robotics

The field name Soft Robotics itself suggesting the materials used in it has to be soft. Soft Material selection is a vital step in the building of flexible actuators and compliant links.

                                                                                          PDMS(Polydimethylsiloxane)                                      Flex Foam

The soft materials are different types of elastomers and foams and they have a shore A hardness measure. Elastomers and Polyurethane foams are widely used in consumer recreation, Marine and Transportation.

In the Ashby Chart showing these materials have low strength and low density in comparison with metals. For our project work, we needed prototyping material which is Room Temperature vulcanised, so we found out PDMS and Foam materials which are RTV and they are having different shore hardness measure.

           Selected three different types of transparent PDMS material ZA13, HT33, HT 45 for this material 13,33 and 45 is shore A measure, similarly we selected Flexible Pu foam flex foam 3, Flex Foam 4, Flex foam 6 and Flex foam 10 in this material 3,4,6 and 10 are densities of material in pound per cubic feet. Conducted uniaxial tension test in a material testing laboratory of ASTM standard D412 test and obtained the material stress-strain graphs were obtained, and these material properties were eventually used to define the materials for the simulation software ANSYS, ABACUS and to decide the Hyper-elastic Mathematical models of the Materials.

                                                                                      Ashby Chart Strength vs Density                           Uniaxial Test D412

  1. B) PDMS Samples (HT 33, HT 45, ZA 13):

                                                                                                 PDMS Test Sample a)Before Test   b)After Test

  1. B) Flexible PU foam Samples:                             

                                                                  Flex Foam Test Sample a)Flex foam                                            b)Flex foam

Testing of Hand gripping on variety of shape object

                                                                                                  Gripping of different shape objects at pressure 2 bar

Different shape objects can be grasped by the compliant gripper with a three fingers orientation of the hand. All objects are handled at the constant pressure of 2.5 bar.            

  

                                                      Time laps of robot working video to handle delicate object with dexterity at working pressure 2.00 bar

 

Conclusion

The objective of design and development of a soft humanoid robot is to perform safe operations in a proximity with humans and this is achieved by partially making robot arm and its Actuators from soft materials. Design and analysis has been done for each major part for assured performance.

  1. PDMS(Polydimethylsiloxane) Elastomers of shore hardness 33A and 45A and Flexible PU foam of shore 00 to 20 Soft materials as per soft humanoid arm requirement has been selected.
  2. Flexible fluidic actuators with end connectors were built by using PDMS materials having average tensile strength 2.05 MPa and 1.50 MPa.
  3. 3D printed molds are made from PLA (Polylactic Acid) material. These molds were used to produce the components such as a finger, forearm, elbow actuator etc.
  4. Pneumatic Network Actuator (PNA) were built by casting of PDMS material. In testing we found 3 bar pressures is the burst pressure for PNA and at 2.5 bar pressure PNA is producing average 5 N force which is sufficient to grip the object of 500 gm
  5. Elbow actuator were built by using 3D printed molds and PDMS casting process. Maximum angle achieved in unloaded condition by elbow actuator is 72 degree . By using test setup, It is found that actuation time for elbow actuator is 4.64 seconds at 2.5 bar and it is producing force of 20 Newton.
  6. Twist Actuator of Left and Right helix were built separately by using 3D printed molds and PDMS casting process. Maximum Twist produced by twist actuator is 52 degree in unloaded condition at pressure 3 bar with average actuation time of 1.7 seconds. The torque produced by twist actuator is varying between range of 2.62 Kgcm at 1bar pressure to 6.88 Kgcm at 3bar pressure.
  7. Robot is designed for payload of 500 gm and in testing it is handling maximum weight of 338 gm.
  8. Arm has angular velocity of 0.3946 rad/sec at 2.5 bar pressure while producing the twist up to 45 degrees and also arm has maximum velocity of 155 mm/sec while elbow is actuated at pressure of 2.5 bar.

Hence this soft humanoid robot arm having operating parameters as per below

 a)Lifting capacity – 338 gm

b)Shoulder movement – 140 degree rotation in x and y direction

c)Elbow movement – 70 degree bending

d)Each finger movement – 85 degree curling

e)Total reach of arm from shoulder joint – 650 mm