The Cutting Edge of Haptic Research Graphical System Design

Have you ever played a car racing video game that shakes when you go off-road? If so, you have interacted with a haptic interface. The word haptic comes from the Greek haptikos, which means to grasp or perceive. With haptic robotics, a user can feel a remote or virtual environment. A haptic interface provides the sensory feedback so users can feel something with which they are not physically interacting. For example, a haptic interface may be used to provide a feeling of resistance in the rudder controls of a flight simulator. The feedback would help the pilot know when to apply more or less force to the instruments.

One of the cutting-edge areas of haptic research is called “passive haptics.” Typical haptic interfaces are active, meaning the system uses actuators such as motors and pneumatics to add force to the system with which the operator is interfacing. The risk presented by active haptic systems, however, is the actuators could add too much force and injure the user. Passive haptic interfaces offer a safer alternative. Instead of adding force to the system, passive haptic interfaces remove force from the system using passive actuators such as magneto-rheological brakes. Not only are passive haptic interfaces inherently safer, they can also use less power.

Researchers at the Georgia Institute of Technology Intelligent Machine Dynamics Lab. (IMDL) are studying the use of passive haptic systems. Dr. Wayne Book and graduate student Benjamin Black are exploring whether passive haptic systems can be as effective as active haptic systems for remote operation of a device, with the additional guarantee of safety. One of the main limitations of passive haptic systems, however, is the device cannot be placed in a specific position. Instead, the passive actuators must guide the operator to the desired position. Book and Black are trying to overcome this limitation by developing advanced control strategies for the passive actuators.

Using the Graphical System Design Methodology

The design of the system involved several steps made possible by the graphical system design approach. Graphical system design employs the combination of graphical development software tools and off-the-shelf hardware to rapidly design, prototype and deploy embedded control devices. Researchers used National Instruments LabVIEW, a graphical software development environment, to design and simulate the haptic control system and communication for remote operation. The design was deployed to real-time PXI control and acquisition systems to test the control strategies. The advantage of this approach is that Book and Black can iterate and create a better design by avoiding low-level embedded software development and custom hardware design when deploying.

The researchers were able to quickly import their master and slave controller algorithms into LabVIEW and then instrument these with actuators and sensors using a high-level programming interface. By instrumenting the algorithms with real hardware, they could verify theories with real-world data. Figure 1 (pS22) shows the graphical source code the researchers used to control the position of the slave. Moreover, the software tools provided high-level abstraction interfaces, such as the timed-loop feature. The timed loop is a LabVIEW programming structure that abstracts the details of priorities and multi-threading. With these types of abstraction, engineers and scientists can easily apply the performance advantages of multithreading to their application. This frees up the researcher to spend more time perfecting the object of the design instead of spending time developing low-level code.

Deploying the Design to Hardware

The researchers deployed the software algorithms to PXI modular hardware systems. These systems include a deterministic, real-time controller and appropriate I/O modules that interface with sensors of the experimental haptic devices. Using the LabVIEW Real-Time Module, the researchers deployed their algorithms to the PXI controller for headless operation. They used a plug-in PXI motion control module to control the linear slave motor and they used multifunction data acquisition devices to interface with the position sensors.

The test apparatus for this research uses a two degrees of freedom (DOF) manipulator serving as the master device to control a one DOF linear motor that acts as the slave. There is no physical connection between the master and slave; rather, there is a PXI real-time control system coupled to the master and another system coupled to the slave, as shown in Figure 2 (above). PXI System 1 executes a deterministic application programmed in NI LabVIEW that reads a gamma force sensor and two optical encoders from the master manipulator. The researchers use the data to determine the position of the master and then send that position to PXI System 2.

PXI System 2 uses the master position as the setpoint input to a 4 kHz PD (proportional-derivative) controller designed in LabVIEW to actuate the linear motor while reading position data from an optical encoder. The slave device encounters a physical constraint that resists its movement. The slave position is sent back to the master through UDP to PXI System 1, which feeds the data to a control algorithm that determines the haptic force that should be applied to the user to communicate the presence of the physical constraint. The force is applied using actuation of the magneto-rheological brakes. The goal of the system is the slave position tracks the master position.

Book and Black are now improving on this research by using dynamic system simulation based on LabVIEW. Using system identification techniques, the researchers can create a mathematical model of the master and slave dynamics based on real data acquired from stimulus and response tests. They use the resulting differential equations with the LabVIEW Simulation Module, which solves the equations in time to simulate the response to different control algorithms. This simulation process helps them iterate faster to optimize their algorithms before applying them to the haptic devices.

Summary

This research story once again shows how current technological advances are paving the way for future ones. By using the graphical system design approach, Book and Black have taken advantage of the democratization of embedded development in order to perform breakthrough research.

	An experimental haptic interface.