Quasi-Direct Drive for Low-Cost Compliant Robotic Manipulation

David V. Gealy, Stephen McKinley, Brent Yi, Philipp Wu, Phillip R. Downey, Greg Balke, Allan Zhao, Menglong Guo, Rachel Thomasson, Anthony Sinclair, Peter Cuellar, Zoe McCarthy, Pieter Abbeel

★ Must read April 2019

TL;DR Blue is a human-scale 7-DoF arm built from quasi-direct-drive motors and timing belts, so it is backdrivable and force-controlled without torque sensors. It carries 2 kg, has a 7.5 Hz nominal position bandwidth and 4 mm repeatability, and is projected to cost under $5000 at volumes above 1500 arms.

What they do

Force-controlled arms that are safe around people cost tens of thousands of dollars. This paper argues that quasi-direct-drive (QDD) actuation, low-ratio transmissions that stay backdrivable, can give a compliant, human-scale arm at the price of a research computer. The prototype, Blue, is a 7-DoF arm with a 2 kg payload, designed for teleoperation and learning rather than industrial precision.

How it works

QDD actuation. Brushless outrunner gimbal motors drive each joint through a single-stage 7.125:1 timing belt reduction. Motor current is measured for torque control, so the motor also acts as the torque sensor.
Differential links. Each link combines two belt transmissions in a geared differential to produce pitch and roll. Belts let the motors sit closer to the shoulder, cutting gravity torque and flying inertia by about 30%.
Burst-power sizing. Like a human arm, Blue is sized for peak loads at a limited duty cycle instead of a continuous rated payload.
Control. A central computer sends torque commands over RS485 at 170 Hz; each motor driver runs current control at 20 kHz. A VR interface teleoperates the arm.

Results

Repeatability: over 133 return-to-home motions, all trials fell within a 3.7 mm radius, with 2.6 mm average deviation.
Bandwidth: 7.5 Hz nominal position bandwidth and an estimated 13.8 Hz torque bandwidth, against a 2.3 Hz human limit.
Thermal: 20 W RMS in normal motion without payload; a 2 kg payload can be held at full extension for about 2 minutes before a 7 minute rest.
Cost: $3328 bill of materials per arm in a 7-unit run; contract-manufacturer quotes keep the end-user price within $5000 above 1500 arms.

Limitations

Step responses are underdamped, with up to 2.8 cm of end-effector overshoot at a 2 kg payload. Friction and motor cogging give a torque hysteresis band of up to 2.6 Nm, which bounds the accuracy of commanded torque. Full payload is thermally limited to short bursts. The authors list timing belt skips and long-term creep of the plastic shells as open problems. Teleoperation tasks are evaluated only qualitatively.

Videos

Figures

From Figure 1: The Blue manipulator, a human-sized, compliant 7 degree-of-freedom arm with a 2 kg payload that can co... — From Figure 1: The Blue manipulator, a human-sized, compliant 7 degree-of-freedom arm with a 2 kg payload that can cost less than $5000 per arm to end users at scale.

From Figure 3: Internal view of a single 2-DoF geared differential module, with the motor, timing belt, tensioner, di... — From Figure 3: Internal view of a single 2-DoF geared differential module, with the motor, timing belt, tensioner, differential and structural shell labeled.

From Figure 2: Position bandwidth of the elbow joint under 0, 1 and 2 kg payloads, compared to a similar test of the ... — From Figure 2: Position bandwidth of the elbow joint under 0, 1 and 2 kg payloads, compared to a similar test of the human biceps brachii. Curves entering the grey region are beyond effective control (-3 dB) for that load.

From Figure 6: Repeatability test. The arm moves to one of 8 end poses and returns home (a); home-pose repeatability ... — From Figure 6: Repeatability test. The arm moves to one of 8 end poses and returns home (a); home-pose repeatability in the plane of highest variance (b); torque hysteresis caused by friction (c); end-pose repeatability for each target (d).

Abstract

Robots must cost less and be force-controlled to enable widespread, safe deployment in unconstrained human environments. We propose Quasi-Direct Drive actuation as a capable paradigm for robotic force-controlled manipulation in human environments at low-cost. Our prototype - Blue - is a human scale 7 Degree of Freedom arm with 2kg payload. Blue can cost less than $5000. We show that Blue has dynamic properties that meet or exceed the needs of human operators: the robot has a nominal position-control bandwidth of 7.5Hz and repeatability within 4mm. We demonstrate a Virtual Reality based interface that can be used as a method for telepresence and collecting robot training demonstrations. Manufacturability, scaling, and potential use-cases for the Blue system are also addressed. Videos and additional information can be found online at berkeleyopenarms.github.io

Summaries are written automatically from each paper's text. Figures and videos come from the paper and its project page; all credit goes to the authors.

Matt Strong