Soft Gripper Design and Fabrication for harvesting tomatoes (Team Project)


This thesis presents the design and fabrication of a soft gripper for harvesting tomatoes.  In this study, we designed and developed a soft pneumatic robotic gripper which can grasp a tomato of diameter 70 – 100 mm. and weighing between 180 – 300 g.  The robotic gripper model consists of three fingers which made of soft silicone elastomeric material (food grade silicone rubber SF820). The gripper inflating and deflating the fingers by compressed air to grasp and release the tomato.   The design of soft gripper was analyzed through Finite Element Method (FEM) which allowed us to model the behavior of the soft pneumatic finger and observed the effects of various parameters such as pressure, displacement and contact force. Due to the lack information of the material properties that required to use in the FEM model, the similarity properties were used instead (silicone rubber Sylgard 186).  According to the simulation result, the operating pressure was in the range of 0 – 95 kPa.  The final design layout of the gripper was developed into a prototype and tested.  First, we experimentally tested the displacement and the bending angle of each finger to compare the experimental results with the simulation results. Also, the relationship between input pressure (0 – 95 kPa) and behavior of the finger were observed. The results showed that the behavior of the finger from the experiment and the simulation have the same trends. However, the errors from the process was occurred due to the fabrication process and the material properties. The second experiment was a grasping test which we experimentally tested with spherical ball and tomatoes. A force sensor was attached to the fingers in order to measure the maximum reaction forces. The force-pressure relationships presented the maximum forces which were occurred at the tips of the fingers— 0.6512 N(finger1), 0.5054 N(finger2) and 0.8156 N(finger3). The measured force was used to estimate the minimum force that required to hold a tomato. Furthermore, the results also showed that the contact forces are less than the tomatoes Bioyeild (2.57 N).  The Finally, we tested the harvesting process at the real tomato tree. The gripper was mounted onto a one DOF robot arm, force sensors and the cutter mechanism for cutting the branch and also the basket for collecting the tomato.

Keywords: soft Gripper / harvesting tomatoes / soft pneumatic finger / FEM model/ Input pressure / Forces


Candidate:  Tawan Thintawornkul,  Supakrit Kumnon,  Asiwan Kultongkham
Advisor:  Asst. Prof. Dr. Teeranoot Chanthasopeephan
Department of Mechanical Engineering, KMUTT


A. Kultongkham, S. Kumnon, T. Thintawornkul, and T. Chanthasopeephan*. “Design and fabrication of a soft gripper for harvesting tomatoes”. The 9th TSME International Conference on Mechanical Engineering Hosted by Mechanical Engineering Department, Thammasat School of Engineering, Thammasat University. 11-14 December 2018. Thavorn Palm Beach Resort, Phuket, Thailand. [December 2018] *Corresponding Author:

Giant Tomato

Giant Tomato is one of valued vegetable in consumer market. In the near future, automation agriculture .Its quality depend on  is bruised easily while harvested, transported to market destination. One method to reduce bruising cost is to harvest it with proper force. Those issues are inspiration of this project.

Novel Soft Gripper Design

Our soft gripper consist of 3 fingers, two at both sides are to hold tomato in gripper, and another one at bottom side is to hold and carry tomato weight. The gripper actuated by applying air pressure along fingers inner chamber making fingers bend.

The design is optimised geometry parameter for achieving suitable grasp scenario, which nearly tip section must sensitive with pressure, to achieve more gripping force at tip. The FEA technique is selected for simulating the single finger bending behavior under pressure actuation. The simulation result is analysed with geometry parameter, and redesigned to fit in tomato shape.

Soft finger is made of silicone elastomer, and fabricated by using casting technique with 3d-printed mold.

Single soft finger is tested for its bending characteristics (i.e. bending angle, displace of finger in each point along the finger) to make sure the finger could fit with tomato, and the operation pressure ranged from this simulation is used to select proper electronic pressure regulator.

Soft gripper is made of three soft finger joint together using silicone elastomer, assembled in 3d-printed assembled mold.

Grasp Tomato test

Soft gripper have mount into single degree-of-freedom robot arm, and pneumatic control system. Harvesting test results shows that gripper could achieve proper gripping force (the force is under tomato bruise force, bio-yield force, 2.5N). This could one achievement for reducing tomato bruise in harvesting process.

Soft gripper pneumatic control system and Robot arm Extension

The pneumatic control system and one degree-of-freedom arm are our own developed. System consist of pneumatic control system, robot arm, stem cutting system, and feedback-force control system.

Pneumatic Control System: Pressure source pass through air filter then enter electronic pressure regulator, which is regulate down pressure into operational range. The 3/2 way normally-closed solenoid valve is installed to control air direction from soft gripper (in and out). The gripper actuated using single-line pneumatic air.

Grasping Force Limit Control System: The force sensors have attached at each finger tips. The measured force was used to estimate the minimum force that required to hold a tomato, and used to control soft gripper grasping force by our practical force limit control algorithm.

One DOF Arm: The arm moves along vertical axis. The movement is from stepper-motor. It transfer rotational movement through power-screw, which is convert rotational movement to translational movement. Two end-stop switch are installed in both bottom and top of axis. It use to initiate position and prevent motor move out of axis distance.

Stem Cutting System:  Stem cutting system consist of scissor and one DOF translational axis. Scissor is are consisted of very sharp razor blade, and actuated by using high torque RC-servo motors. Scissor module mount on one DOF translational axis which actuated by small stepper motor through power screw.

Embeded Controller Set: All of 3 subsystems above are controlled from this embeded controller set. The soft gripper control system powered by microcontroller (Arduino Due), which it inteprete commands from computer through USB serial protocol. The commands are available for user to tell machine harvest a tomato automatically, or allow user operate each module manually.