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Abstract
Whiskers provide a way to sense surfaces in the immediate environment without disturbing it. In this paper we present a method for using highly flexible, curved, passive whiskers mounted along a robot arm to gather sensory data as they brush past objects during normal robot motion. The information is useful both for guiding the robot in cluttered spaces and for reconstructing the exposed faces of objects. Surface reconstruction depends on accurate localization of contact points along each whisker. We present an algorithm based on Bayesian filtering that rapidly converges to within 1 mm of the actual contact locations. The piecewise-continuous history of contact locations from each whisker allows for accurate reconstruction of curves on object surfaces. Employing multiple whiskers and traces, we are able to produce an occupancy map of proximal objects.
A) Instruments robot end-effector with an array of 16 semi-curved whisker sensors. B)Illustrates the sensing region of a semi-curved sensor. Shaded region approximately defines a threshold or keep-out region based on the amount of deflection measured on the sensor. C) Shows the combined sensing region and keep-out region of a sensorized end-effector. Contacts within the threshold generate a repelling force Fr.
In our approach, we use soft, curved whiskers mounted along a robot arm and we use the robot’s accurate proprioception in combination with sensor measurements to localize contacts along the whisker and gather information about the environment.
We present methods for employing arrays of flexible whiskers for navigation and partial surface reconstruction. In this work we use pre-curved whiskers of super-elastic nitinol mounted along the arm of a robot; fabrication methods are presented in an earlier paper Lin et al. (2022). To evaluate the effectiveness of these whiskers to perceive nearby objects and surfaces, we conducted preliminary experiments comparing their performance to other proximity sensors.
We begin by summarizing the assumptions made to limit the scope of the problem.
Objects that come into contact are immobile in the world reference frame. In other words, if the robot does not move then contact locations do not change.
There is at most one contact point on a whisker with the environment at any time. This will be true for convex objects.
Frictional forces along the whisker are negligible in terms of their effect on the sensor. This again will generally be true given the nitinol whisker material.
To track contact locations from a sequence of base moment measurements, we use Bayesian filtering in a recursive algorithm that infers the state distribution from a history of sensor data and control inputs.
Process Model:
Sensor Model
In comparative tests, we report the accuracies of distance measurements to surfaces using these whiskers and with optical and ultrasonic non-contact sensors. These tests are relevant for obstacle avoidance and navigation.
Experiments Videos
Sensor Fabrication
Curved whiskers were fabricated by heat-forming nitinol wires in aluminum molds. The goal is to produce a desired curvature while maintaining the superelastic properties. The process is sensitive to temperature and timing as well as wire size. We used a controlled oven at 600◦C, placing the clamped molds with the nitinol wire and an embedded thermocouple. The temperature was monitored until it reached 500◦C, at which point the setup was quenched in a water bath. The process keeps the metal for no more than 4.5 minutes at temperatures above 450◦C to avoid aging and raising the austenite finish temperature.
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