Pneu robot to end electric era?

A specialist development team from Festo, used advanced pneumatics and closed-loop control at the core of a new and advanced robotic handling system.

In traditional handling applications, pneumatic drives have been used for a variety of motion functions from simple gripper actuation, through linear and rotary cylinders to position controlled servopneumatic drives. But until now, electrical drives have dominated for precision closed loop axis control – a fact evident in most industrial robots.

Against this background, the Festo team looked at modern development methodologies with the goal of designing a system that used pneumatics in a precision robotic positioning and handling system. The result is the Kinematic Robot – a unit that successfully demonstrates both the company’s specific capability in integrating automation components but also more generally illustrates the solutions becoming available through innovations in pneumatics.

Lateral thinking

As well as being a demonstration platform for the concept of intelligent servo pneumatic components, the Kinematic Robot provides the basis of several real-life customer specific solutions. The robot shows what can be achieved with cost-effective new technology through lateral thinking, with the potential for significant cost reductions being the bottom-line result.

Pneumatic drives are commonly used for motions between two fixed or manually adjustable end stops. A typical feature of these applications is fixed repetitive motion sequences. In contrast, an application that demands a high speed approach to variable intermediate positioning or flexible motion profiles requires freely programmable drives — and normally electromechanical units would be specified.

With drives that feature programmable setpoints, there is always a distinction between point-to-point motion and continuous path motion. For straightforward pick & place tasks, point-to-point control systems are usually adequate. However, continuous path applications, like paint spraying, coating, adhesive application or arc welding – where several axes must execute a co-ordinated motion – require more sophisticated technology. This is the new ground that Festo is breaking with its advanced closed-loop pneumatic solutions.

The move toward integrating pneumatic components into fully functional mechatronic systems was the start point for the Festo Kinematic Robot project. The critical success factor was to develop a handling solution that met a range of increasing market demands: shorter cycle times, higher accuracy and motion speeds, lower purchase and operating costs, and reduced time-to-market. No small task.

The robot itself is a mechatronics handling system that uses parallel kinematics to achieve higher accuracy and motion speeds than conventional Cartesian servoelectric configurations. An additional benefit is that the three-axis device offers low moving masses with very high rigidity thanks to its advanced closed kinematic design.

Off-the-Shelf

The drives deployed are three identical rodless pneumatic cylinders featuring integrated displacement encoders and pressure sensors. They are standard, inexpensive, off-the-shelf items. The drives also form the rigid frame of the structure while three pairs of ultra-lightweight but super-strong parallel carbon fibre rods connect the end effector to the drive slides. The simplified mechanical structure demands that more complex open- and closed-loop control be implemented. As a result, a multi-axis continuous-path controller is required. This uses application-specific motion programs to derive setpoints for individual axes.

To operate the Kinematic Robot, the operator simply programs the required motion sequence for the end effector in Cartesian co-ordinates. A multi-axis continuous path controller uses reverse transformation to process those co-ordinates on-the-fly, generating spatial points linked by straight lines or curved arcs. It then mathematically derives positions and setpoints for each individual axis. The setpoint derivation is also used to improve contouring behaviour. To provide collision detection and monitor contouring errors, the direct kinematics are implemented in the controller using forward transformation.

Independent acceleration and velocity profiles are also defined for each path to facilitate the high-speed movement and high accuracy positioning of the end effector. The path setpoints also assist the controller to compensate for gravitational, centrifugal, Coriolis and coupling forces in its complex acceleration force calculations. These are processed in the central component of the controller, using a method comparable to the ‘computed torque’ calculated in robot control.

The controller

In addition to acceleration forces, the controller features a status monitor that accommodates other variables such as interference and frictional forces and applies compensation by adjusting the cylinder piston drive force. This maximises positional accuracy and optimises contouring behaviour. The closed loop process also manages to compensate for inexactly-compensated coupling forces and deviations from the friction model, by monitoring these variables as interference and adjusting the drive force appropriately. Similarly, the controller is easily able to deal with problems that are otherwise difficult to resolve in servo-pneumatics, such as the approach of cylinders to their end positions or motions with very low load masses.

Key to the project’s development was a concurrent process of simulating the desired handling model in software. This software modelling process permitted various concepts to be analysed and feasibility studies to be rapidly undertaken, ensuring that the required specification could be achieved. During the project design phase, the simulation was used to test and optimise closed- and open-loop control strategies. Automatic code generation then allowed the real controllers to be tested without further delay once the prototype was ready. This type of simultaneous engineering was instrumental in reducing development timescales and, as such, helps to meet one of the critical success goals.

In the same domain

The result of the simulation was an impressive ‘right-first-time’ prototype build. The Kinematic Robot met its design criteria under test conditions and was demonstrated in both pick & place and continuous path applications. A typical test to confirm performance and accuracy comprised the end effector describing a circular motion track initially of 0.45m diameter at a path velocity of 1.0m/s, then at 0.675m diameter with path velocity up to 3.5m/s and acceleration onto the path of 50m/s. During these motion operations, contouring errors are less than 2mm with repeatability of 0.1mm.

Such performance and accuracy levels are in the same domain as precision electrical drives used in industrial robots and are consequently significantly higher than values usually achieved by servopneumatic drives. The Festo Kinematic Robot also retains the fundamental strengths of pneumatics, such as excellent performance to weight ratio, inherent resistance to overload, and robustness even in arduous environmental conditions.

Add to this the cost advantages and countless areas of application and market opportunities previously considered the preserve of electrical or hydraulic solutions are opened up.

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