Soft Robotics · Pneumatic Gripper Control
Grasping Profile Control
of a Soft Pneumatic
Robotic Gripper
for Delicate Gripping
Abstract
Soft pneumatic grippers (SPGs) excel at handling delicate, irregularly shaped, and soft objects -- outperforming conventional rigid grippers. However, controlling the grasping profile of an SPG with pneumatic pressure is challenging because the relationship between air pressure and finger volume is nonlinear and the internal volume cannot be directly measured.
This work presents a synergy pressure control system combining a regulator valve, solenoid valve, and DC motor, paired with a PID controller and a novel Unknown Input Observer (UIO) that estimates SPG volume in real time. Simulation and experimental results confirm the system can precisely track desired grasping profiles -- enabling delicate handling of objects as fragile as tofu.
Key Contributions
Hardware Design
The Soft Pneumatic Gripper
Figure 1
Step-pressure vs. delicate-profile grasping. Conventional step input causes abrupt grip force; the proposed system follows a smooth grasping profile.
Figure 2
System overview: DC motor drives a pneumatic cylinder connected to the SPG via a regulator valve and solenoid valve. Encoder and pressure sensors provide feedback.
Figure 3
SPG fabrication: silicone mold (left) and finished two-finger soft gripper (right). The fingers are cast from Dragon Skin 30 silicone with embedded air channels.
Hardware Specifications
| Gripper material | Dragon Skin 30 silicone |
| Actuation | Pneumatic (air pressure) |
| Pressure range | -0.2 to +0.2 Bar |
| Volume range | ~15-32 mL per finger |
| Motor | DC motor + encoder |
| Valve type | Regulator + solenoid |
| Controller | MATLAB Simulink (real-time) |
| Sensor | Pressure sensor (no volume sensor) |
Control System
PID + Unknown Input Observer
Figure 4
Free body diagrams: (left) pneumatic cylinder model with piston force balance; (right) SPG finger model relating air pressure to bending volume.
Figure 5
Control architecture block diagram. The PID controller tracks the desired SPG volume; the Unknown Input Observer estimates the actual volume from pressure and encoder readings.
How the Control Loop Works
Set Target Volume
Operator defines a desired grasping profile -- e.g., slowly inflate to 28 mL over 3 seconds.
Measure Pressure
Pressure sensor reads the current air pressure P inside the pneumatic cylinder in real time.
Estimate Volume (UIO)
The Unknown Input Observer uses the pressure reading and motor encoder to estimate the SPG volume without a direct volume sensor.
PID Correction
The PID controller compares estimated volume to the target and adjusts the motor/valve to correct the pressure -- closing the loop.
Experimental Results
Volume Estimation & Tracking
Fig. 11 -- Volume Tracking Response: The SPG volume (red) accurately follows the reference profile (black dashed) as it switches between 15 mL and 25 mL. The air pressure (blue) adjusts accordingly.
- Reference (mL)
- Actual Volume (mL)
- Air Pressure (Bar)
Figure 9
SPG volume estimation using the Unknown Input Observer. The estimated volume (red) closely tracks the actual volume (blue dashed) despite the nonlinear pressure-volume relationship.
Figure 10
SPG volume response for different PID gain parameters (Kp = 1, 3, 5, 7 with Ki = 0.01). Kp = 3 provides the best trade-off between speed and overshoot.
Figure 11
SPG volume tracking response. The gripper successfully tracks a step reference profile between 15 mL and 25 mL, with inset photos showing the physical gripper states.
Publication
MDPI Robotics · Open Access
Grasping Profile Control of a Soft Pneumatic Robotic Gripper for Delicate Gripping
Gridsada Phanomchoeng, Patchanok Pitchayawetwongsa, Nattaphat Boonchumanee, Saravut Lin, and Ratchatin Chancharoen.
"Soft pneumatic grippers have garnered significant attention owing to their remarkable flexibility, safety, and adaptability. [...] This breakthrough empowers precise and delicate gripping actions, enabling the handling of delicate objects such as tofu."
-- Abstract, Robotics 2023, 12(4), 107