Robust and Smooth Force Sensor-less Power Assist Control

Robust and Smooth Force Sensor-less

Power Assist Control

V. Salvucci S. Oh H. Fujimoto Y. Hori

The University of Tokyo

Outline

1 Power Assist Robots without Force Sensor?

2 Proposed Force Sensor-Less Power Assist Control

3 ResultsHuman Input Based EvaluationRigorous Experimental Verification

4 Conclusions

Outline

1 Power Assist Robots without Force Sensor?

2 Proposed Force Sensor-Less Power Assist Control

3 ResultsHuman Input Based EvaluationRigorous Experimental Verification

4 Conclusions

Power Assist Robots without Force Sensor?

Can we control these robots by estimating the user force with cheap sensors?

Why without Force Sensors?

Control key point

Knowledge of user’s applied force

1 Force Sensor

2 Force Sensor-less Control(encoder based)

Why without force sensors?

Advantages

Low cost

Low weight

Compactness

Faster Response: No delay in forcemeasurement

Safety: the robot is a sensor

Difficulties

Robustness (ex: friction non-linearities)

Difficult to provide smooth assistance

Outline

1 Power Assist Robots without Force Sensor?

2 Proposed Force Sensor-Less Power Assist Control

3 ResultsHuman Input Based EvaluationRigorous Experimental Verification

4 Conclusions

Force Sensor-Less Power Assist Control (FSPAC)

Constant gain [Oh 2008]

Based on disturbance observer for:

Disturbance rejection

Force-to-assist estimation

Variable Gain [Salvucci 2010]

VD-TG = Velocity DependentTriangular Gain (i.e. variableimpedance)

VD-TG = Velocity Dependent Triangular Gain

|va| VD-TG why

|va| < |v1| 0 no assistance stability in unknown environment

|va| ≈ |v2| high high assistance at low speed to overcome inertia

|va| → |v3| decrease low assistance at high and dangerous speed

Outline

1 Power Assist Robots without Force Sensor?

2 Proposed Force Sensor-Less Power Assist Control

3 ResultsHuman Input Based EvaluationRigorous Experimental Verification

4 Conclusions

Human Input for a “Feeling” Evaluation

No assistance

Low backdrivability

Conventional FSPAC

Not smooth(oscillatorymovements)

Unstable

Not Robust

Proposed FSPAC

Smooth

Stable

Robust

Experimental Apparatus

High Back-drivable System

Actuators: 1 linear motor

Sensors: 1 linear encoder

Motion is transmitted to the doorthrough the linear motor→ low friction, and highback-drivable

Low Back-drivable System

Actuators: 1 AC motor

Sensors: 1 rotary encoder

Motion is transmitted to the doorthrough the ball screw system→ high friction, and lowback-drivable

Experiment Input

Constant-force (in absolute value) input

Applied on the doors’ end from position 1 (left) to 2 (right)

Experimental Results for Low Friction Systems

Experimental Results for Low Back-drivable Systems

Outline

1 Power Assist Robots without Force Sensor?

2 Proposed Force Sensor-Less Power Assist Control

3 ResultsHuman Input Based EvaluationRigorous Experimental Verification

4 Conclusions

FSPAC with Variable Impedance Works

Proposal

A simple-to-implement Variable Impedance Controller for Power AssistRobots with low cost Position Sensors

Advantages

1 Higher robustness:Experimentally stable for different humidity and room temperatureconditions (i.e. friction modeling error)

2 Smoother assistance:Higher assistance to the user at low velocities (=assistance when needed!)Better input tracking

3 Higher safety:No oscillatory torque references and velocities that can cause instability

Thank you for your kind attention

Hori-Fujimoto Lab— Eco and Human-friendly Motion Control—

References

S. Oh and Y. Hori. Generalized discussion on design of force-sensor-less power assistcontrol. In Advanced Motion Control, 2008. AMC ’08. 10th IEEE InternationalWorkshop on, pages 492–497, 2008.

V. Salvucci, S. Oh, and Y. Hori. New approach to force Sensor-Less power assistcontrol for high friction and high inertia systems. In Industrial Electronics (ISIE),IEEE International Symposium on, pages 3559–3564, 2010.