Fluid power adds muscle to robotics and mobile equipment

Paul Boughton

Machine builders, material fabricators, hydraulic and pneumatic equipment manufacturers and robotics users all have to grapple with the issue of controlling the brute force of hydraulic and pneumatic cylinders and actuators. Boris Sedacca reports.

Fluid power has been used since ancient times but recent developments have seen a demand for safer, quieter pneumatic systems and leak-proof hydraulics. The sometimes brute force of fluid power needs to be tamed and accurately targeted to applications.

Today's sophisticated feedback control valves have come a long way from the days of early fluid power systems, which were either on with full flow, or off with no flow. Flow control was achieved by choking off the output with a faucet, lever or similar mechanism to reach the desired flow. Then along came proportional servo valves which controlled flow at the input using the inherent mechanical features of the valve.

A key technological breakthrough came later with the use of electrical control in the form of the solenoid. Since then, the sophistication and complexity of valves has evolved at a dizzying pace leading to the latest proportional integral derivative (PID) electronic feedback devices and computer based systems of valve islands linked by fieldbus networking.

Hydraulic consultant Stephen Barrett says the boundary between mechanical, electrical and control technologies have become so blurred as to be indistinguishable. "Clearly cost comes into it because one of the main requirements is accuracy, and generally speaking, you get the best accuracy with electronic control," explains Barrett.

"Embedded electronics also simplifies the mechanical complexity of a valve, so it is possible to have closed loop control system that comprises only of the cylinder or actuator, and the valve. You need a command signal, which at its simplest would be a lever, knob or slider, with closed loop control inside the actuator.[Page Break]

Explosive atmospheres

"At the other end of the scale, mechanical feedback systems are simple and reliable, and may be needed in explosive atmospheres (ATEX) for example, where the use of electricity is an issue. Generally, there is more intrinsic safety in a mechanical setup where an actuator is moving until it hits a mechanically operated cam instead of an electrically operated device.

"Otherwise, more systems are electrical or electronic nowadays and the trend is away from mechanical feedback control. PID feedback is easily implemented in electronic systems, whereas in mechanical systems, the gain is determined by a spring, and to some extent that gives you a complex mix of integral and derivative rate control. You are introducing some form of time element into it because a spring has a variable stiffness rate the more it is compressed."

In Barrett's experience, many fluid power engineers that are too mechanically minded are not catching on to the fact that they need to have some understanding of closed loop control and basic analogue electrical principles. As a medium fluid power can be controlled in terms of actuator position, force or velocity, which all correspond to the three PID parameters.

Whether the system is digital or analogue, the same parameters have to be controlled from a finite gain within the control loop. Digital processing has the advantage of speed and enables sophisticated handling of the error signal at the output of the loop, as well as the ease of setting up.

Aside from control considerations, the need for quality fluid power components is crucial to any application, whether it be factory floor and robotics, mobile hydraulics or aerospace equipment. Wilhelm Stoll Maschinenfabrik GmbH manufactures front loaders for tractors with outputs of 25-300 HP. Its new ProfiLine front loaders are often used in continuous operation and under extreme conditions, where the entire structure is subjected to vibration caused by the drive and by uneven ground.[Page Break]

When designing a new product line, Stoll engineers also wanted to optimise the interfaces in the hydraulic system with the aim of achieving minimum leaks with maximum process and assembly reliability. The specifications required that the ProfiLine be equipped with hydraulic lines made of steel tubing.

Uwe Ockert, design engineer at Stoll says: "Lines made of steel are of higher quality and more durable. Our competitors mainly work with flexible hydraulic hoses. In contrast to flexible hoses, however, steel line connection calls for absolute process reliability during manufacturing - from cutting, deburring and shaping of the contour, all the way to bending.

"The steel tubes are safely accommodated in the loading arms of the front loaders, and practical experience shows that minimum leaks really have been achieved. The feedback from our distribution partners and users shows that have found a high-performance, functionally reliable system that more than proves its worth in practice - even when subjected to severe stresses and operating under very tough environmental conditions."[Page Break]

Rigorous testing

Turning to pneumatics applications, critical components in the aerospace and defence industries, such as engine housings, wings and control surfaces, undergo rigorous testing to ensure quality and safety. Marietta Nondestructive Testing (NDT) historically used X-ray to check the strength of welds and look for evidence of defects and faults in both large and small components. However, large, complex shaped carbon fibre parts and other advanced composite materials require a different approach.

"X-ray is limited in determining the size and depth of composite parts. However, they can be accurately inspected using ultrasonic technology," says Curtis Cooper, director of engineering for Marietta NDT.

The majority of automated ultrasound testing machines employ immersion tanks filled with water. These act as a medium through which sound waves travel. However, complex parts make immersion tanks impractical with technicians resorting to scanning each component by hand, resulting in a slow and labour-intensive process. This often leads to overlapping scans, which provide inaccurate or inconsistent test results. The challenge was therefore to create an automated system which would avoid these issues.

A revised ultrasonic inspection equipment developed by Marietta NDT and Bosch Rexroth is custom-designed for each client, based on the unique specifications of the parts being inspected. To overcome the challenges of scalability, reliability and speed, the engineers at Marietta NDT designed a gantry style AG2 overhead scanner - a rigid, multi-axis, automated testing machine capable of scanning large, sophisticated parts and intricate shapes, without the use of immersion tanks.

With a scanning envelope of up to 60 x 20 x 16 feet, the machine can easily be configured to test a wider variety of components for each customer, instead of being specifically designed for one part. The scanner's ultrasonic scanning system utilises two sets of jets that face each other. During inspection, the jets stream water - the medium that the sound wave travels through - around the part.

The precise servo motion control of the system became a critical factor in the final design. In order for the machine to offer up to 12 axes of motion, component synchronisation had to be tightly controlled so the testing would be accurate. "Each nozzle is roughly five inches from the face of the part," adds Cooper. "Since the two nozzles face each other, they have to be lined up. We were able to make streams of water, which are each manipulated by five axes of servo motion, concentric within 0.02 inches."[Page Break]

Path planning

Marietta NDT used drive and control components from Bosch Rexroth. Digital servo drives and controllers, profiled guide rails and pneumatic components allow the machine to follow intricate path planning for scanning complex, curved objects with tightly controlled motion tolerance.

"The high-quality controllers are reliable and easy to programme. They can also accommodate the large number of interpolated axes of the machines," continues Cooper. In addition, the controllers generate minimal overall electrical noise to minimise ultrasound interference, producing a higher quality image than was possible in the past. The pneumatic components include a vacuum generator to remove air from the nozzle which also improves the quality of the ultrasonic scans.

Linear guide rails also contribute to the overall rigidity and accuracy of the machine. If the machine is not stiff enough to handle the squirter system, it will begin to vibrate, which adversely affects the testing. From a maintenance perspective, the linear guide rails are an ideal solution due to their longer lubrication interval, dual rail datum and interchangeable runner blocks.