The use of industrial robots is well established in applications where the robots are in controlled environments where they are separated by fences or cages. A collision between a robot and a contacted object is a complex situation, where one of the most severe situations is when a collision with clamping occurs, entrapping the contacted object between the contacting portion of the robot and a static fixture, such as a wall. In this case, the severity of the collision may be indexed using the maximum contact force, which can then be used as a reference for unexpected collisions during the design process.
Control and dependability are characteristics needed from a robot to allow it to interact in a minimally controlled, e.g., unfenced and uncaged, environment with other objects, which may be moving or stationary. Roboticists may typically use three different strategies to develop these characteristics. First, roboticists may develop algorithms that use vision systems, proximity sensors or the like to anticipate and avoid potentially harmful contacts between robots and objects. Secondly, methods may be developed to detect a collision by monitoring joint torques or a robot skin and to quickly react to manage the contact forces under a certain level. Thirdly, roboticists pursue robot designs that will intrinsically prevent damaging contact.
Avoidance, reaction and design strategies can be combined together to improve robot design. However, the first two options alone may not fully guarantee the desired result. Consider that a robot intended to interact physically with a contactable object will require the ability to distinguish desirable and undesirable contacts, e.g., good and bad contacts. This can be done either by disabling sensors on the robot parts intended to interact or by running an algorithm that will decide if the upcoming contacts are desirable or not. In either case, control is compromised either by unprotecting certain parts of the manipulator or by giving the robot some sort of “judgment capability” which may in some situations be wrong. Furthermore, avoidance and reaction strategies rely on electronic components that can fail. Finally, one could argue that an operator may feel insecure working with a machine protected only by an algorithm. Thus, a third strategy may be employed to obtain compliant and dependable robots, which is to use a design strategy, e.g., to design robots that intrinsically prevent damaging contact.
To design robots to intrinsically prevent damaging contact, a typical approach is to make the robot compliant, to reduce the peak contact force attained during a collision. Compliance may also extend the duration of the contact, allowing the controller to sense a collision and react to reduce potential damages, within certain constraints, i.e., reaction time. However, adding compliance may limit the precision and stiffness of the robot, compromising performance and precision.
Some robots designed to avoid damaging contact incorporate a flexible flange with breakaway function that links the tool to the manipulator. This device triggers an emergency stop when the contact force at the tool control point exceeds a certain threshold, which may be a breakaway torque measured at the flange. This device therefore limits the moment, not the force that can be transmitted by the manipulator to the end-effector, which means that the threshold depends on the location of the collision point. Therefore, for a breakaway system, the design of the system must be sub-optimized for the worst case moment arm, which may result in a system which is overly sensitive and prone to false triggering in high inertia non-collision situations, which may require limitations on robot velocity.
Active compliance systems are in some aspects derived from admittance control techniques, e.g., efforts are measured at the effector and processed to command a displacement equal to the contact force divided by a virtual spring stiffness. Thus, the robot behaves like a spring around its trajectory. However, the response time of traditional actuators is larger than what is required to accommodate high frequency forces applied during collisions. Consequently, during a collision, the robot may not achieve a compliant behavior and thus this technique is not optimal as a design strategy.
Techniques may be used to provide passive compliance, at each joint of the robot, which may be programmable or non-linear. Programmable passive compliance consists of using a compliant joint for each axis of the robot and a supplemental set of actuators to allow the adjustment of the stiffness of each joint. Either two antagonistic actuators or a second actuator that adjusts the stiffness via a mechanism may be used, to allow high stiffness and precision at low velocity and low stiffness at high velocity, i.e. when contact with the manipulator may be more severe. This gives the controller the ability to continuously adjust the compromise between control and performance. However, using this type of passive compliance system adds weight and complexity to the manipulator. Also, for many mechanisms, the ratio between the largest stiffness and lowest stiffness is not sufficient to obtain high precision at low velocity, when collisions are less severe and high precision is required for acceptable robot performance.
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