Controlling Robotics Precisely With Haptic Technology
- Sunday, 01 August 2010
Robots are capable of very precise motion, but must be guided with precision in order to fulfill their potential. Consider the task of guiding a robotic surgeon’s arm to suture a wound or insert a catheter. A human surgeon, with all his or her knowledge and experience, is required to practice where to probe, cut, or sew before he or she can develop the necessary skills to make a clean suture with the right degree of tension at the right depth or an incision of the right depth. In contrast, a robotic surgeon’s arm can move more consistently and accurately than that of the best human surgeon.
The key is to guide the robotsurgeon’s arm with human expertise, and provide the surgeon controlling the robot arm with closed-loop motion control in order to ensure that the robotic arm does what it is supposed to do. With that in mind, a new robotic control technology called haptics is being developed to provide sensory feedback to the human surgeon while helping to guide the robotic motion.
Haptic devices are input-output devices that track a user’s physical manipulations and provide realistic touch sensations in coordination with a computer that, in turn, drives a robotic motion system. Haptics provide sensory feedback to the controls and allows users to effectively touch, feel, and manipulate three-dimensional objects. They also can precisely control the position of the robot’s end effector (the end of the robot arm that holds the tool). In addition, 3D boundary information can be factored into the robot’s control profile to prevent motion into restricted areas where it could cause harm, making the use of haptic controls ideal for robotic surgery applications.
The technology also enables the amplification, or “scaling,” of dimensions between the robotic end of the system and the human operator to enable the operator to make movement inputs that are comfortable at human scale while controlling robotic operations on a much smaller scale. This capability has powerful implications in the field of nanotechnology, including nanosurgery, but can also be extended to other operations to be handled at the molecular or near-molecular scale. Consider providing the ability to actually “feel” cells or molecules and manipulate them using precise robotic actuators.
“There are three technologies involved in haptics: the mechanics, the electronics, and the software,” said Sébastien Grange, PhD, co-founder and vice president of operations, Asia Pacific, for Force Dimension, a Nyon, Switzerland-based developer of haptic technology. “And it takes the latest innovations in all three, working together, to provide a realistic experience.”
The computer system that interfaces with haptic I/O devices must be able to exercise deterministic real-time control in order to provide realistic feedback to the operator. For example, in order to simulate the “feel” of a surface being probed, the computer must interact with the haptic I/O device at least 1,000 times per second to read control inputs and provide force outputs back to the haptic device. Otherwise, a human operator will feel vibrations or perceive nonexistent discontinuities (roughness) on the surface. Precise and time-deterministic feedback is also important to ensure safety of operation. As mentioned above, using boundary information coupled with realtime position feedback, the haptic device can be prevented from moving the robot end effector into areas that must be excluded.
Figure 1 shows a typical haptic input device. To provide tactile feedback to the operator, the device incorporates DC motors that are capable of responding very smoothly to control commands, with no inertia generated. Force Dimension uses motors that are very similar to the ones used on NASA’s Mars rovers. The company’s software monitors position information from a rotary encoder on each of the three axes and converts it into a set of coordinates in three-dimensional space. Then, based on the coordinates, the system computes how much force to generate back to the operator, and this process is repeated 1,000 times per second.
Because of the requirement for highspeed control and absolute determinism in timing of its operations, a real-time operating system must be used to control the motion. A general-purpose operating system couldn’t be counted upon to meet the deterministic timing requirements of the application. This is because these operating systems are optimized for data processing performance, not for I/O response. To handle the real-time requirement in its haptic control systems, Force Dimension provides a library of device control functions that work with different real-time operating systems (RTOSs), including INtime from TenAsys Corp. (Beaverton, OR).