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Georgia Tech Develops Haptic Devices Using NI LabVIEW Real-Time, PXI
Benjamin Black, Georgia Institute of Technology; Wayne Book, Georgia Institute of Technology
Machines/Mechanics, Research, University/Education
LabVIEW Real-Time, PXI/CompactPCI
Investigating teleoperation methods for master and slave robots.
Using National Instruments LabVIEW Real-Time and PXI to control both an energetically passive master robot and an active slave robot, as well as manage communications between the two.
The master haptic interface control program reads the encoders, translates their angles into an x-y position, reads the analog force sensors, translates their x-y data into a global x-y coordinate system, commands forces to the output brake channel.
Haptics, also known as “force feedback teleoperation,” attempts to provide environmental interactions through a robotic system. Users mimic these interactions with robotic arms. By varying the amount of force these haptic devices exhibit, a user can achieve the sensation of interacting with the actual system. Haptic robotics can be viewed as a high-tech, multidimensional, force feedback computer input device - a not-too-distant cousin of the joystick or a mouse.
Active and Passive Haptic Devices
Haptic devices can be active or passive. The delineation is made by considering whether energy is added to the system (active) or removed from the system (passive). Active haptic robots have joints with motors; hydraulic actuators; or some other form of actuator that creates motion, adds energy, and reflects virtual forces. Instead of motors, passive haptic devices have brakes or dampers that provide the user with feedback forces. The passive haptic robot cannot force a user in a certain direction - it can only prevent or slow a user’s motion. The benefit of a passive robot over an active robot is that force spikes generated by the virtual environment cannot do any damage to the actual environment or the user.
At the Intelligent Machine Dynamics Laboratory (IMDL) at the Georgia Institute of Technology, we are using National Instruments PXI hardware and LabVIEW Real-Time software to research haptic teleoperation of master and slave robots. We apply NI PXI systems control both the master and the slave robots with communication between the two robots using the UDP Internet communications protocol native to NI LabVIEW Real-Time.
We are exploring two different aspects of haptic teleoperation. One aspect focuses on the effects of teleoperation over long distances via the Internet. One benefit of using National Instruments tools is the easy transition between the TCP/IP and UDP Internet communication protocols. In doing so, it is easier to compare the protocols on the same platform in the same system. Previous IMDL students, as well as preliminary results with NI hardware, have shown the UDP communication protocol is better suited (improved consistency and reduced time delay) for control applications where the important communication factors are small packet size and high speed.
The second aspect of the research concerns the problems of control and haptic interaction when the master robot cannot provide a restoring force to the user. The passive master used in these experiments is limited in that it cannot force the user in an arbitrary direction; instead, it can only guide the user with a magnetorheological braking system. The basic assumption is that the user is cooperative and is attempting to complete the task.
As a teleoperation experiment, the platforms (real-time operating system and hardware platform) of the master and slave are often independent. However, for simplicity, we have designed using NI PXI and LabVIEW Real-Time products to control both the master and slave. The control of the master involves reading two quadrature encoders and an analog force sensor, and then commanding forces to either three or four magnetorheological brakes through a pulse-width modulation PWM amplifier. This is simply acquiring an analog voltage input and producing a PWM signal for the brake. To simplify the control of the slave, we use the NI PXI-7344 motion controller. The slave’s target position is updated by the system software via a UDP connection so that it follows the position of the master.
To control the master, we use a NI PXI-8175 controller running the LabVIEW Real-Time OS using the I/O capabilities of the NI PXI-6070E multifunction data acquisition module and PXI-6713 high-speed analog output modules. The system reads force input and two encoder signals and sends force commands to the four magnetorheological brakes.
Previous IMDL experiments were limited by computing power, prompting a move to more capable systems such as the PXI hardware and LabVIEW Real-Time software from National Instruments.
Once we achieved the basic functionality of the master (reading position and force, translating those values into a global x-y coordinate system, and sending a control signal to the brakes), we extended the research and used the master to control a slave device. We chose a linear motor for the slave due to its simplicity.
Implementation of the slave device follows typical operation of a motor and includes a voltage input to produce a velocity output. We read the position using a very high-resolution linear encoder and implement a PID controller in LabVIEW Real-Time that runs on the NI PXI-8145 Real-Time controller using a PXI-6070E multi-function data acquisition card for I/O. The setpoint for the controller is provided by the x-position of the master and communicated to the slave controller via UDP. We automatically generated the UDP communication code using the RT Communication Wizard and later optimized it by hand.
The project objective is to investigate control problems induced by the use of a passive master with an energetically active slave. With an active master, typical haptic feedback can transmit a force to the user based on the difference in the position of the master and the position of the slave. This restoring force causes the active haptic device to track the position of the slave device. A passive haptic device cannot produce a similar restoring force; it can only resist the motion of the user. This difference makes haptic teleoperation an interesting control problem. Currently, few people are studying this facet of haptics, and nearly any control algorithm represents progress in the field. The next step is to investigate methods of providing feedback to the user.
Two basic types of feedback exist for teleoperated haptic systems that do not have force feedback as a measured quantity from the remote device. In the first set of solutions, the master system uses a model of the remote system to calculate virtual forces to be fed back to the user. The second group of solutions uses virtual coupling between the master and the slave to calculate forces to be fed back. With a passive master system, implementation of direct virtual coupling is very difficult because forces can only be applied to the user to oppose motion.