Friction Torque Model Research Articles

Sensorless collision detection based on friction model for a robot manipulator

The importance of human-robot collision safety has been increasing recently due to the introduction of collaborative robots. However, conventional collision detection methods usually require additional sensors such as skin sensors, joint torque sensors, and acceleration sensors, which are impractical to implement due to their high cost. To address this problem, in this study we propose a collision detection method using only a manipulator’s encoder without any extra sensors. In the proposed scheme, the external torque due to collision is estimated using a generalized momentum-based observer and a friction torque model in the harmonic drive developed for a robot that is conducting position control. The performance of the proposed collision detection method was evaluated using a 6 DOF industrial manipulator. The experimental results show that collision can be reliably detected without any extra sensors for any type of robot manipulator.

Friction torque in thrust ball bearings lubricated with polymer greases of different thickener content

In this work a series of experimental tests were performed in thrust ball bearings lubricated with polymer greases. The tested greases were formulated with the same base oil but different thickener content. A multi-purpose lithium thickened grease was also tested as reference.The friction torque was measured at constant temperature and load, while varying the rotational speed. The coefficients of friction under boundary and full film lubrication were numerically calculated through the optimization of a rolling bearing friction torque model to the experimental measurements. The results show that the higher the thickener content, the smaller is the friction torque generated by the lubricating greases, phenomenon which was found to be especially important at low speeds and low specific film thickness.

Running Torque of Slow Speed Two-Point and Four-Point Contact Bearings

A decoupled slow speed friction torque model has been developed to predict the running torque of a double-arched angular contact bearing when operating as a four-point, as well as a two-point contact bearing. The load distribution model from Amasorrain et al. (2003) and the kinematics model developed by Leblanc and Nelias (2007) have been combined after ignoring centrifugal and gyroscopic effects, a valid assumption for slow speed operation. Results from the model are compared with previous literature data, as well as with tests done on a specially-developed friction torque rig. The comparison with the literature results was done for two specific cases: (i) when only one out of the two contact lines in the four-point contact bearing is active (effectively creating a two-point contact bearing); and (ii) where both contact lines in the four-point contact bearing are active. Further, the comparison was done with a custom-built friction torque rig with FAG QJ309 bearings, again for two cases: (i) bearings mounted with a specific clearance (two-point contact); and (ii) bearings mounted with larger size balls to obtain interference (four-point contact). All tests were performed at low speeds. The sliding friction, which is an important input to the friction torque model, is carefully measured on ball-on-plate test using the same interface roughness, speed and contact pressure conditions as seen in the QJ309 friction test. The model comparison with experimental results is covered. The comparison is found to be encouraging, with the RMS difference being less than 7% between the model and experimental data for a four point contact.

Research on the Motor Friction Model of Electro-Hydraulic Servo System

In order to improve the performance of the electro-hydraulic servo system, considering the special structure of servo motor, nonlinear model of the servo motor was established in time domain. Several friction models commonly used in project were introduced and applied to the simulation analysis of servo motor with friction torque. The simulation curves show the relationship between driving torque, friction torque, angular velocity and angular displacement. The result confirms the influence of the different friction torque model on the performance of motor, and the LuGre friction model is identified which is fit for the friction torque compensation. This study lays the foundation for the friction torque compensation of electro-hydraulic servo motor.

Friction torque in grease lubricated thrust ball bearings

Thrust ball bearings lubricated with several different greases were tested on a modified Four-Ball Machine, where the Four-Ball arrangement was replaced by a bearing assembly. The friction torque and operating temperatures in a thrust ball bearing were measured during the tests. At the end of each test a grease sample was analyzed through ferrographic techniques in order to quantify and evaluate bearing wear.A rolling bearing friction torque model was used and the coefficient of friction in full film lubrication was determined for each grease, depending on the operating conditions.The experimental results obtained showed that grease formulation had a very significant influence on friction torque and operating temperature. The friction torque depends on the viscosity of the grease base oil, on its nature (mineral, ester, PAO, etc.), on the coefficient of friction in full film conditions, but also on the interaction between grease thickener and base oil, which affected contact replenishment and contact starvation, and thus influenced the friction torque.