Collision Detection and Safe Reaction with the DLR-III Lightweight Manipulator Arm
Summary (3 min read)
Introduction
- Safety issues are of primary concern when robot manipulators are supposed to operate in an unstructured environment, sharing the workspace with a human user and allowing a close physical cooperation [1], [2].
- Along a similar but simpler line, robot manufacturers are providing hw/sw facilities for prescribing safe Cartesian areas that should not be accessed by the robot in any operative condition.
- Tuning of collision detection thresholds in these schemes is difficult because of the highly varying dynamic characteristics of the commanded torques.
- A detection scheme that works under similar conditions and avoids the above drawbacks has been recently proposed in [15].
- Collisions are viewed as faulty behaviors of the robot actuating system, while the design of a detector takes advantage of the decoupling property of robot generalized momentum [16], [17].
II. PRELIMINARIES
- The authors first consider robot manipulators as open kinematic chains of rigid bodies, having N rigid joints.
- The dynamic model is M(q)q̈ + C(q, q̇)q̇ + g(q) = τ tot, (1) where M(q) is the symmetric, positive definite inertia matrix, the Coriolis and centrifugal terms are factorized using the matrix C(q, q̇) of Christoffel symbols, and g(q) is the gravity vector.
- In the absence of friction and other external torques acting from the environment, τ tot is just the motor torque τ .
- The motor inertia matrix B and the joint stiffness matrix K are diagonal and positive definite, while D ≥ 0 is the diagonal joint viscosity matrix.
- The authors note that the analysis presented in this paper applies, with minor modifications, also to the more complete model of robots with elastic joints considered, e.g., in [20].
III. DETECTION AND REACTION WITH RIGID ROBOTS
- During normal operation, the robot arm may collide with a standing or moving person/obstacle in its workspace.
- Accordingly, the Cartesian collision forces and moments are denoted by F K = [ fK mK ] ∈ R6.
A. Collision detection
- Note that σ can be computed using the measured joint position q and velocity q̇ (possibly obtained through numerical differentiation) and the commanded motor torque τ .
- Not all possible collision situations are detected by this scheme.
- With the robot at rest (q̇ = 0), the instantaneous value of τK does not affect σ, whereas this will happen only when the robot starts moving.
- On the other hand, when the robot is in motion, collision can be detected provided that the Cartesian collision force produces motion at the contact.
- In fact, no possible robot motion would be able to reduce the force loading in this case.
B. Collision identification
- The previous scheme does not provide any directional information on the Cartesian collision force, nor is able to identify which robot link has collided.
- In view of the structure (15), the authors call r a collision identification signal, or simply a residual bearing this term from the fault detection literature.
- More in general, the sensitivity to F K of each of the affected residuals (proximal to the robot base) will vary with the arm configuration (see also [15]).
- Thanks to the properties of the generalized momentum, this dynamic analysis can be carried out based only on the static transformation matrix JTK(q) from Cartesian forces to joint torques.
- In fact, the residual dynamics in eq. (14) is unaffected by robot velocity and acceleration.
C. Reaction strategy
- Enforcing such a zero-gravity condition is useful for guaranteeing a safe behavior [1], as robot motion will not be biased along the gravity direction.
- During normal operation (pre-impact phase), it is convenient to apply a control law that provides accurate trajectory tracking in free motion while displaying passive properties (springdamper type) in response to unexpected collisions.
- The simplest reaction strategy to a collision would be to stop the robot by using its brakes [6].
- Instead, the directional information embedded in the residual vector r can be used in the post-impact phase, by switching control to a more friendly behavior.
- Equation (18) is clearly an active control scheme, as additional energy is feed into the system after the collision has been detected.
D. Energy dissipation
- The robot operation states and their transition conditions are shown in Fig 1.
- Note that as long as the collision flag is up, any further collision will keep the robot in the reflex reactive state.
IV. EXTENSION TO ROBOTS WITH JOINT ELASTICITY
- The extension of the collision detection and identification schemes developed in Sect. III for the rigid case can be made in different ways.
- In view of the application of their methods to the DLR-III lightweight manipulator, the authors take advantage of the specific sensing devices available on board of this robot.
- In particular, every joint is equipped with a high-resolution incremental position sensor on the motor side and an integrated joint torque sensor, so that θ and τ J are directly available.
- More specifically, substituting τ with τ J + DK−1τ̇ J in the expressions (11) and (13), the authors obtain similar collision detection schemes and identification properties.
- This leads again to the linear and decoupled residual dynamics ṙEJ = −KIrEJ + KIτK .
A. DLR-III controller
- The authors recall first the form of the robot control law used for general tasks with the DLR-III arm, in which reaction strategies to collisions have been inserted as additional control modalities.
- The authors note that the passivity properties are preserved provided that the following condition on the introduced damping matrices is satisfied [18]: D ≥ 1 4 (Ds −D)T Dθ (Ds −D).
- Equation (25) represents a full-state feedback with respect to suitable, possibly time-varying, reference values.
- This fact will be used for implementing different strategies of robot reaction to collisions in a single framework.
- Realizing a zero-gravity condition for an elastic joint robot, with a choice equivalent to eq. (16) of the rigid case, is not as immediate.
B. Reaction strategies
- The residual rEJ in eq. (22) is used for detecting a collision and identifying a safe direction for the reactive motion of the robot.
- As baseline behaviors in the performed collision experiments, the authors have taken the case of no reaction at all (Strategy 0) and of immediate stop of the trajectory generation with simultaneous high-gain position control (Strategy 1).
- With reference to the general control law (25), three reactive strategies have been considered.
- This strategy leaves the robot floating in space in response to the collision force, while motion is damped at the motor side.
- This strategy is the closest to eq. (18) of the rigid case.
V. EXPERIMENTS WITH THE DLR-III MANIPULATOR
- The authors have performed several tests on collision detection with the DLR-III lightweight manipulator and using the robot reaction strategies defined in Section IV-B.
- The authors report here numerical results obtained on repeated collisions with a balloon (see Figs. 2–6) and qualitative results for collisions with humans (see Fig. 7).
- Fig. 2. Collision with a balloon (motion at 100◦/s) and robot reaction Collisions occurred between the hand or the outstretched arm of different test persons and different locations between the 4-th and 6- th link of the robot, with linear speeds at the contact up to approximately 1.5 m/s.
- When the time interval of contact is relatively long, the detection and reaction capabilities of the robot are enhanced.
VI. CONCLUSIONS
- The authors have presented a complete approach, from detection to reaction, for handling human-robot collisions without the need of external sensing.
- Collision detection and identification signals can be efficiently generated resorting to energy arguments or based on the robot generalized momentum and by using only proprioceptive measurements.
- The robot retracts itself safely and rapidly away from the collision area, using the local directional information collected during the impact.
- On-going work is concerned with acceleration-driven collision detection and the reduction of control communication delays in their robotic set-up, as well as with a more accurate evaluation of several severity indices of the impacts and of the beneficial inclusion of compliant coverages.
- Furthermore, robot redundancy will be exploited in order to devise reaction strategies that try to complete a given Cartesian motion task, despite of the detected collision.
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Citations
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...In particular, they will serve for a new method of scaling time increments in the trajectory generation, which allows the user to push the robot intuitively forth and back along its desired path even though the robot is still under position control....
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...Furthermore, it will be showcased how the collision detection and reaction can help to prevent damage to the robotic structure and thus additionally contribute to an increase in safety due to fault protection....
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...The collision detection mechanism proposed in [10], which provides a filtered version of the external collision torque τext, along with improvements concerning detection sensitivity and alternative detection schemes are presented and compared to each other....
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...The monitoring method based on the generalized momentum observer introduced in [33], [34], and [44] was motivated by the desire of avoiding the inversion of the robot inertia matrix, decoupling the estimation result, and also eliminating the need of an estimate of joint accelerations....
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...The first method that achieved simultaneously collision detection, isolation, and identification was proposed in [33] and [34]....
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...The following approach is an extension of our previous works [34], [40] by systematically deriving a threshold with higher robustness against model uncertainties and disturbances A collision detection function cd(....
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...We start from our early works [34], [35], [40], [43], [44] and take advantage of the extensive experience gained over the years in developing, using, and refining our original methods....
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...The monitoring signal r is also called residual vector (see [34] and [35])....
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"Collision Detection and Safe Reacti..." refers background or methods in this paper
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...In fact, no possible robot motion would be able to reduce the force loading in this case....
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...A detection scheme that works under similar conditions and avoids the above drawbacks has been recently proposed in [15]....
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...Note that σ can be computed using the measured joint position q and velocity q̇ (possibly obtained through numerical differentiation) and the commanded motor torque τ ....
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