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Static Friction

Static Friction. Load torques and Temperature dependency. Motivation. What is the most common researched phenomenon in robotics?. Why?. Robots eventually break down Increase of wear debris Increase of friction However! Many other things affect friction Temperature Load Position

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Static Friction

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  1. Static Friction Load torques and Temperature dependency

  2. Motivation • What is the most common researched phenomenon in robotics? • Why?

  3. Robots eventually break down Increase of wear debris Increase of friction However! Many other things affect friction Temperature Load Position Velocity/acceleration Lubricant, etc Motivation - Diagnosis

  4. Objectives • How does friction changes with • Position • Load torques • Manipulated • Perpendicular • Temperature • Scope • IRB 6620 • Joint 2

  5. Friction Curve Estimation • In steady-state one joint at a time • Average fwd and bwd movements • and use it as an estimate of the direction independent friction

  6. Joint angles • 50 static curves • Angles range • 8.5 to 60º Effects • Not significant • M0 should do it

  7. Perpendicular load torque • Tp range • [0.04, 0,10] • Small variations possible Effects • Not significant • At joint 1, small increase in Fs • For joint 2, M0 is enough

  8. Manipulated Load Torque • 50 static curves • Tm range • [-0.7, 0,4] Effects • (linear) Fc increase • (linear) Fs increase • cte vs • Proposed model, M0 +2 terms

  9. Temperature • 50 static curves (min/max) • T range • 34-80ºC Effects • (linear) Fs increase • (linear) vs increase • (exp-like) Fv increase • Load torque co-effects • Subtract surfaces • Subtract identified gravity terms from surfaces  Independent!

  10. Proposed Model • M0 + 4 terms • 7 terms (5 vel-weak, 2 vel-str) • Total of 11 parameters • Tm (3): • (linear) increase of Fc and Fs • T (4): • (linear) increase of Fs and vs • (exp) increase of Fv

  11. Validation • Parameters identified with previous data • Error reduced by a factor of 6 M0:mean=0.027,max=0.071 M*:mean=0.004,max=0.018

  12. Summary & Conclusions • Empirically motivated model to describe T and Tm • So far, 1 joint! • Tp might also affect • Tm affects vel-weak only (inc) • T affects vel-weak (dec) and vel-str (nonlinear inc) • 11 parameters (4nlin: 3 vs,TVo) • Id requires excitation over large range of T and Tm • Can T-params be directly related to lubricant props? • No T meas and Tm estimated* • Right now, more useful for design/evaluation • Can the model simply be plugged into a dynamic model (LuGre)?

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