Jun 24, 2026
Comprehensive characterization protocol for a 2-DOF pneumatic wrist with controllable braking torque
- Hongbo Liu1,
- Hongbo Liu1
- 1Beihua University
- WS
Protocol Citation: Hongbo Liu, Hongbo Liu 2026. Comprehensive characterization protocol for a 2-DOF pneumatic wrist with controllable braking torque. protocols.io https://dx.doi.org/10.17504/protocols.io.81wgbmpo3vpk/v1
License: This is an open access protocol distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited
Protocol status: In development
We are still developing and optimizing this protocol
Created: June 22, 2026
Last Modified: June 24, 2026
Protocol Integer ID: 319533
Keywords: universal benchmark for flexible wrist testing, flexible wrist testing, overall performance evaluation of pneumatic flexible wrist, pneumatic flexible wrist, pneumatic wrist, dof pneumatic wrist, wrist deflection angle, brake airbag force, comparative performance analysis of prototype, test platform, prototype, braking torque
Abstract
To standardize the overall performance evaluation of pneumatic flexible wrists, this paper proposes a complete unified experimental workflow covering multi-dimensional tests: actuator elongation and output force, wrist deflection angles, brake airbag force, braking torque, combined drive-brake load resistance, dynamic response, static pose retention and practical handling performance.
The test platform was assembled and calibrated per design requirements, with dedicated sensors for data acquisition. Uniform wiring, hardware arrangement and test protocols were adopted to guarantee identical test conditions, reliable measurements, favorable operability and repeatability.
This workflow provides a universal benchmark for flexible wrist testing, improves result repeatability and credibility, and supports comparative performance analysis of prototypes with varied structural parameters and control algorithms.
Image Attribution
Figure 1 Experimental setup. (a) Actuator elongation; (b) Axial output force; (c) Wrist pivot angle and direction; (d) Wrist braking torque; (e) Wrist dynamic characteristics.
Guidelines
1.2 Prototype manufacturing process
1.2.1 Material preparation
(1) Constraint rings: Rapidly fabricated via nylon 3D printing, followed by polishing of the inner and outer surfaces to remove burrs and protrusions, thereby preventing damage to the silicone tubes and ensuring assembly accuracy.
(2) Elastic washers: VMQ silicone O-rings are adopted as standard parts, which can be purchased from physical stores or online suppliers.
(3) Components of the brake mechanism: The sleeve, upper and lower end covers, lug plates, plugs, friction plates, and braking ball are all made of aluminum alloy and fabricated by conventional machining according to the design drawings.
(4) Other connecting parts: Bolts, pneumatic quick-connect fittings and air tubes are standard components and can be procured directly.
1.2.2 Actuator fabrication
The actuator assembly adopts a stepwise sealing process, which mainly consists of five steps. (1) First, the silicone tube is press-fitted with the upper and lower end plugs, and a steel wire binding Experimental method is used to complete the sealing of the silicone tube and the end plugs. (2) To ensure overall airtightness, a preliminary pneumatic leak test is conducted after assembling the constraint ring and elastic washers; the formal assembly process proceeds only after confirming the absence of leakage. (3) The lower end cover is designed with a matching installation groove. The lower plug is equipped with lateral threaded holes and is connected to the lower end cover via screws. A central through-threaded hole is reserved in the lower plug for the installation of a pneumatic quick-connect fitting. (4) After passing the airtightness test, the constraint ring and elastic washer are sequentially mounted on the outer surface of the silicone tube. (5) The upper plug, which is press-fitted with the silicone tube, contains a central threaded hole; finally, the upper plug is connected to the upper end cover using screws, completing the assembly of a single pneumatic actuator.
Following the above standardized process, three sets of pneumatic actuators with identical structures and parameters are manufactured in batches.
1.2.3 Brake fabrication
The brake adopts a modular, layered assembly process from the inner components to the outer structure. The main fabrication procedure consists of six steps: (1) The brake airbag is first press-fitted and sealed with the upper and lower end plugs to form an independent sealed pneumatic chamber. A pneumatic quick-connect fitting is installed at the lower plug to ensure smooth and airtight airflow. (2) The integrated airbag assembly is then embedded and fixed inside the sleeve. (3) Two friction plates are mounted on the upper end face of the sleeve and bolted to the upper plug of the braking airbag through countersunk holes at the bottom of the friction plates. (4) A thin-walled braking ball is assembled between the two friction plates, ensuring that the center of the ball is positioned 60 mm from the reference height of the upper and lower end covers. (5) Four groups of stepped shafts uniformly distributed at the equatorial region of the braking ball are interference-fitted with the end bearings of lug plates arranged at a circumferential diameter of 54 mm, forming a cross-hinge rotational mechanism. (6) Finally, identically dimensioned upper and lower symmetric end covers are assembled to complete the system integration. The phase of the lug plates at both ends is adjusted to a 90° stagger before bolting and locking.
After assembly, low-pressure gas is introduced to perform airtightness and rotational flexibility tests, ensuring that the structure is free of leakage and operates without sticking.
1.2.4 Wrist prototype assembly
The three manufactured pneumatic actuators are symmetrically mounted on the upper and lower end covers of the pneumatic cross-hinge brake. All actuators are evenly arranged at circumferential intervals of 120° and fixed by screws to maintain a uniform radial distance from the brake center. This symmetric assembly ensures consistent and coordinated structural deformation during wrist motion.
After assembly, a sealing re-inspection was conducted for all pneumatic interfaces and chamber junctions. Compressed air at 0.30 MPa was introduced and maintained for 30 s to perform a preliminary system-level airtightness test. The prototype was deemed qualified for subsequent performance experiments only after confirming the absence of leakage and pressure loss.
3 Pre-experiment preparation
3.1 The lower end covers of the wrist, actuator, and brake prototypes are fixedly installed on the experimental platform, which thoroughly eliminates test errors caused by base vibration and displacement during the experiment.
3.2 The pneumatic system is built by sequentially connecting an air pump, a pneumatic triple unit, a precision pressure reducing valve, an air pressure sensor, and pneumatic pipelines. The driving air circuit and the braking air circuit are independently arranged without mutual interference, so as to avoid experimental accuracy errors induced by pressure crosstalk.
3.3 According to the target experimental measurement parameters, a force sensor, an angle sensor, and a direction angle measuring instrument are installed at the center of the upper end cover of the wrist, actuator, and brake as required. The laser displacement sensor and the 3D motion capture system adopt a non-contact measurement Experimental method, which only needs to meet the respective specified measurement distances.
3.4 All devices are powered on and started, and the corresponding sensor acquisition software is opened for zero calibration of sensors (including the angle sensor and 3D motion capture system) to eliminate initial offset errors. Digital display sensors (laser displacement sensor and digital push-pull force gauge) are equipped with built-in zero-set buttons. After zeroing, the elongation and output force under different air pressures can be directly recorded.
3.5 System air tightness test is carried out. The rated working air pressure is applied to all actuators and brakes, and the pressure is maintained for 30 seconds. No air leakage or pressure drop occurs in the air circuits and cavity interfaces. Formal experiments can be conducted only after the system air tightness is confirmed to be qualified.
4 Division of Experimental Modules
The driving, braking, kinematic, load-bearing, dynamic response, and static posture-holding performances of the flexible wrist are experimentally evaluated through six sub-experiments: actuator performance experiment, wrist pivot experiment without braking, braking performance experiment, coupled driving–braking load experiment, dynamic characteristic experiment, and practical application experiment.
4.6 Experiments on application performance of the wrist
_Experimental objective_: This parallel drive-brake wrist exhibits drastically varied stiffness and damping under braking. To verify its pose adjustment, retention and load-handling performance, we serially integrate it with a self-developed five-finger manipulator for no-load pose adjustment and loaded grasping-transport tests. Comparisons between depressurized and locked brake states reveal braking improvements in overall stiffness, anti-eccentric load capacity and pose retention accuracy.
_Experimental method_: An integrated prototype comparison method is used. The flexible wrist and five-finger manipulator are serially fixed on a linear sliding table, with actuator and brake air pressure as well as motion sequences uniformly controlled by a pneumatic system. With brake status set as the single variable, two test conditions are arranged: no-load depressurized state and loaded pressurized braking. Multi-directional pose adjustment and object grasping-transport tests are carried out to compare the wrist’s mobility, deformation behavior and pose retention performance.
4.6.1 Pose tests under no-load conditions
(1) Assemble the wrist and five-finger manipulator, mount the unit on a linear sliding table and link it to the pneumatic control system, debug air circuits and control programs for stable operation.
(2) Depressurize the wrist brake to release braking constraints, placing the wrist in a flexible free-driving state.
(3) Adjust the air pressure of the three sets of actuators through the pneumatic control system to drive the wrist to realize two-degree-of-freedom motions, including upward pitching, downward pitching, inward yaw and outward yaw.
(4) Record the wrist’s multi-directional swing motions and neutral posture, analyze manipulator self-weight-induced sagging deflection without braking, and quantify the wrist’s flexible-state stiffness performance.
4.6.2 Yaw motion of the wrist under load
(1) Taking a mineral water bottle as the operation object, control the linear sliding table to drive the five-finger manipulator close to the target and adjust the wrist posture to a state suitable for grasping.
(2) Apply pressure to the pitch brake to improve the stiffness and posture retention capability of the wrist in the pitching direction. Subsequently, control the five fingers of the manipulator to stably grasp the target object.
(3) Maintain the pressurized locking state of the pitch brake and depressurize the yaw brake to release the degree of freedom for wrist yaw motion.
(4) Adjust the air pressure of the yaw actuator to drive the wrist to complete small-amplitude swinging and pose adjustment under load, so as to realize small-range object handling operations.
(5) Observe and record wrist posture under loaded braking, and verify its outstanding stiffness and deformation resistance by comparison with no-load unbraced test results.
Materials
The pneumatic actuator primarily consists of upper and lower connecting end covers, silicone tubes, constraint rings, elastic washers, and plugs. Actuator parameters are presented in Table 1. The brake primarily consists of upper and lower end covers, braking airbags, friction plates, braking balls, sleeves, and lug plates. The wrist features a cylindrical flexible structure, configured in a parallel manner with three circumferentially distributed pneumatic actuator groups and a central pneumatic cross-hinge brake. Most structural parts of the wrist are fabricated from aluminum alloy.
Silicone tube effective deformation length: 25 mm
Initial inner and outer diameters of silicone tube: 20×24 mm
Initial inner and outer diameters of sleeve: 25×30 mm
Sleeve height: 35 mm
Clearance between friction plate and brake ball: 0.5 mm
Brake ball radius: 20 mm
Brake ball center distance to upper 26 lower end covers: 60 mm
Four lug plates distance to brake ball center: 54 mm
Upper and lower end cover thickness: 3.5 mm
Pneumatic cross-hinge brake mass: 460 g
Operating pressure: [0, 0.40] MPa
The experimental system mainly consists of three parts: a pneumatic control system, a sensing and measurement system, and a control system. The main equipment, sensor models, measurement ranges, and accuracies are listed as follows.
2.1 Pneumatic control system
(1) Air compressor (or Air pump): Model 1100W-30L. It is equipped with a 30 L air tank, with a rated discharge pressure of 0.7 MPa and a maximum safe output pressure of 0.8 MPa. Its theoretical air displacement is 120 L/min, which serves as the air source for the entire pneumatic system.
(2) Pneumatic triple unit: Model AC2000-02. The rated flow rate is 500 L/min, and the pressure regulation range is 0.05~0.85 MPa.
(3) Precision pressure reducing valve: Model IR2000-02BG. The pressure regulation range is 0.01~0.8 MPa, with a sensitivity of 0.2% FS and a repeat accuracy of ±0.5% FS.
(4) Electro-pneumatic proportional valve: Model SMC ITV1030-312L. Its pressure regulation range is 0.0005~0.5 MPa, regulation accuracy is ±0.001 MPa, response time is no more than 10 ms, repeat accuracy is ±0.5% FS, and the sensitivity is less than 0.2% FS.
(5) Electromagnetic directional valve: Model SYJ314-5LZD-M5. The rated voltage is DC 24 V, the maximum operating frequency is 10 Hz, and the working pressure range is 0.15~0.7 MPa.
(6) Pressure sensor: Model SMC-PSE560. The measuring range is 0~1.0 MPa with a power supply of DC 12~24 V. The overall accuracy is ±1% FS and the repeat accuracy is ±0.2% FS.
(7) PU tubes: Specifications of 4×2.5 mm and 6×4 mm in outer and inner diameters.
(8) Various two-way and three-way pneumatic fittings.
2.2 Sensing and measurement system
(1) Laser displacement sensor: Model HG-C1200CDK. It has a central distance of 200 mm and a measuring range of 0~160 mm, with an accuracy of 200 μm and linearity of ±0.2% FS.
(2) Digital push–pull force gauge: Model HF-100. Its measuring range is 0~100 N, with an accuracy of ±0.5% FS and a resolution of 0.01 N. It is adopted to acquire the steady-state output force of actuators and braking airbags.
(3) Attitude/angle sensor: Model HWT905-TTL. Measurement ranges are ±180° (X, Z) and ±90° (Y), with accuracies of 0.05° (X, Y) and 1° (Z). Output frequency is 0.1–200 Hz. It supports TTL communication with baud rates from 2400 to 921600 bps and operates at 3.3–5 V. The sensor is used to measure wrist pivot angles under different pneumatic pressures.
(4) Pivoting direction angle measuring instrument. It consists of a dual-ball inclinometer and a digital angle gauge (Model: 82311-200P). It features a measuring range of 0~360° with an accuracy of ±0.1°, and is used to real-timely collect pivoting direction angle data of the wrist under various pneumatic pressure combinations.
(5) 3D Motion Capture System: Model NDI Optotrak Certus. It features a resolution of 0.01 mm and a 3D measurement accuracy of 0.15 mm, supporting up to 512 marker points with a marker sampling frequency as high as 4600 Hz. The system enables multi-channel synchronous signal acquisition with a data transmission error no greater than 0.1%.
2.3 Main Electronic Components of the Controller
(1) Programmable Logic Controller (PLC): Model S7-200CN. It is powered by 24 V DC and supports expansion of digital and analog I/O modules, with compatibility with STEP7-Micro/Win software. Control programs are developed in software and downloaded to the PLC. Based on the programmed logic, the PLC regulates the electric proportional valve and solenoid valve bank to drive the flexible wrist through predefined motions.
(2) Analog Input Module EM231: Model 6ES7 231-0HC22-0XA8. It converts analog signals (voltage or current) into digital values readable by the PLC.
(3) Analog Output Module EM232: Model 6ES7 232-0HD22-0XA0. It enables the PLC to output continuous analog signals (voltage or current) to control proportional valves, solenoid valve banks, and other actuators.
(4) Communication Module: Siemens USB/PPI programming cable (6ES7901-3DB30-0XA0) is used to connect the host computer and the S7-200CN. With STEP 7 Micro/Win software, control programs can be downloaded to the PLC, enabling real-time monitoring and debugging of program execution and I/O status.
(6) DC power supply: Model PS3003. It accepts an input voltage of 220 V, provides adjustable output voltage ranging from 0-30 V and adjustable output current ranging from 0–3 A, with a maximum output power of approximately 90 W.
Before start
3 Pre-experiment preparation
3.1 The lower end covers of the wrist, actuator, and brake prototypes are fixedly installed on the experimental platform, which thoroughly eliminates test errors caused by base vibration and displacement during the experiment.
3.2 The pneumatic system is built by sequentially connecting an air pump, a pneumatic triple unit, a precision pressure reducing valve, an air pressure sensor, and pneumatic pipelines. The driving air circuit and the braking air circuit are independently arranged without mutual interference, so as to avoid experimental accuracy errors induced by pressure crosstalk.
3.3 According to the target experimental measurement parameters, a force sensor, an angle sensor, and a direction angle measuring instrument are installed at the center of the upper end cover of the wrist, actuator, and brake as required. The laser displacement sensor and the 3D motion capture system adopt a non-contact measurement Experimental method, which only needs to meet the respective specified measurement distances.
3.4 All devices are powered on and started, and the corresponding sensor acquisition software is opened for zero calibration of sensors (including the angle sensor and 3D motion capture system) to eliminate initial offset errors. Digital display sensors (laser displacement sensor and digital push-pull force gauge) are equipped with built-in zero-set buttons. After zeroing, the elongation and output force under different air pressures can be directly recorded.
3.5 System air tightness test is carried out. The rated working air pressure is applied to all actuators and brakes, and the pressure is maintained for 30 seconds. No air leakage or pressure drop occurs in the air circuits and cavity interfaces. Formal experiments can be conducted only after the system air tightness is confirmed to be qualified.
Frequency-domain response experiments
Fourier transforms are performed on the time-domain experimental data acquired in this series of tests. Under both non-braking and braking operating conditions, the influence laws of driving air pressure, excitation pulse width and braking air pressure on the natural frequency, amplitude-frequency characteristics and vibration amplitude of the wrist are investigated. The detailed experimental procedures are listed as follows.
(1) Frequency-domain response analysis without braking
Time-domain motion data of the wrist under step excitation and pulse excitations with various pulse widths are collected separately, followed by Fourier transform processing to extract frequency-domain response characteristics. The influence of driving air pressure on the low-frequency natural frequency of the wrist is analyzed under step excitation. The coupling effects of driving air pressure and pulse width on the wrist’s natural frequency and vibration amplitude are also explored to reveal the mechanism of the pulse width threshold affecting the wrist’s inherent dynamic characteristics.
(2) Frequency-domain response analysis with braking
At a fixed drive air pressure of 0.30 MPa, step and 1.5 s pulse excitations are tested under graded brake pressures. Time-domain data for each condition are transformed to frequency domain to compare amplitude-frequency responses and analyze how braking alters system damping, natural frequency and vibration amplitude.
(3) Summary of frequency-domain characteristic laws
All frequency-domain experimental results are summarized to clarify the action mechanisms of driving air pressure, excitation pulse width and braking air pressure, and verify the effectiveness of the drive-brake coordinated scheme in suppressing wrist vibration and improving dynamic stability.
Experiments on application performance of the wrist
Experimental objective: This parallel drive-brake wrist exhibits drastically varied stiffness and damping under braking. To verify its pose adjustment, retention and load-handling performance, we serially integrate it with a self-developed five-finger manipulator for no-load pose adjustment and loaded grasping-transport tests. Comparisons between depressurized and locked brake states reveal braking improvements in overall stiffness, anti-eccentric load capacity and pose retention accuracy.
Experimental method: An integrated prototype comparison method is used. The flexible wrist and five-finger manipulator are serially fixed on a linear sliding table, with actuator and brake air pressure as well as motion sequences uniformly controlled by a pneumatic system. With brake status set as the single variable, two test conditions are arranged: no-load depressurized state and loaded pressurized braking. Multi-directional pose adjustment and object grasping-transport tests are carried out to compare the wrist’s mobility, deformation behavior and pose retention performance.
4.6.1 Pose tests under no-load conditions
(1) Assemble the wrist and five-finger manipulator, mount the unit on a linear sliding table and link it to the pneumatic control system, debug air circuits and control programs for stable operation.
(2) Depressurize the wrist brake to release braking constraints, placing the wrist in a flexible free-driving state.
(3) Adjust the air pressure of the three sets of actuators through the pneumatic control system to drive the wrist to realize two-degree-of-freedom motions, including upward pitching, downward pitching, inward yaw and outward yaw.
(4) Record the wrist’s multi-directional swing motions and neutral posture, analyze manipulator self-weight-induced sagging deflection without braking, and quantify the wrist’s flexible-state stiffness performance.
4.6.2 Yaw motion of the wrist under load
(1) Taking a mineral water bottle as the operation object, control the linear sliding table to drive the five-finger manipulator close to the target and adjust the wrist posture to a state suitable for grasping.
(2) Apply pressure to the pitch brake to improve the stiffness and posture retention capability of the wrist in the pitching direction. Subsequently, control the five fingers of the manipulator to stably grasp the target object.
(3) Maintain the pressurized locking state of the pitch brake and depressurize the yaw brake to release the degree of freedom for wrist yaw motion.
(4) Adjust the air pressure of the yaw actuator to drive the wrist to complete small-amplitude swinging and pose adjustment under load, so as to realize small-range object handling operations.
(5) Observe and record wrist posture under loaded braking, and verify its outstanding stiffness and deformation resistance by comparison with no-load unbraced test results.