Making Sense of Electrohydraulic Controllers

April 10, 2008
Here’s how to match controller to machine for top-notch performance without overpaying.

Michael Liedhegener
Manager, New Technologies Development
Bosch Rexroth Corp. – Industrial Hydraulics
Bethlehem, Pa.

Edited by Kenneth Korane

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Machine builders and equipment operators sometimes have misconceptions about hydraulic-based motion. Although valued for its ability to move, crush, or lift heavy loads, hydraulics is often not considered exceptionally precise, or able to support complex, synchronized multiaxis operations.

The advent of closed-loop electrohydraulic (EH) motion controllers has changed the situation, making practical hydraulically powered equipment that is extremely precise, repeatable, and adaptable.

Today, EH controllers come in a variety of platforms with wide-ranging capabilities, so designers need to assess several factors to choose the right one. Start with the application requirements, including motion complexity and the number of controlled axes. Then move on to machine design and operational factors to finalize the decision.

Control basics
Hydraulic motion control is the coordinated control of acceleration, velocity, and position as well as the force or pressure a cylinder or hydraulic motor exerts. Previous-generation machines were typically open loop, sending a simple signal to a directional or proportional valve to control only one of these factors. This limited precision and sophistication, and open-loop systems often required manual fine-tuning that added time and costs to machine commissioning or updating.

Closed-loop control adds feedback to the hydraulic circuit. Position and pressure sensors provide real-time data on parameters such as position, its derivatives, and forces. This opens the door to rapid, precise, repeatable and, most importantly, automated position and force control of hydraulic actuators.

Open-loop controls are great for simple applications. But evertighter quality requirements combined with demands for faster throughput increasingly make closed-loop motion control the preferred approach.

And digital EH controllers offer significant advantages for OEMs and end users alike. They simplify and speed commissioning for machine manufacturers . And when building a series of machines, a control program created for one can be applied to others with little extra effort or expense. For end users, process monitoring and machine diagnostics are built into the controller at no extra cost.

To choose the best controller, start with four key decision points to assess and define during machine development. These include positioning accuracy, the number of axes, programming preferences, and bus architecture. Here’s a closer look at each.

Positioning accuracy
Most closed-loop hydrauliccontroller lines include low-cost analog versions that support basic motion requirements such as simple position or flow control. However, because analog controllers do not incorporate digital feedback and processing, they generally do not provide accurate positioning at the end point.

If an application requires positioning accuracy greater than 0.1% of the total stroke — for example, no more than a 0.1-in. deviation over a 10-in. stroke — it is almost always better to rely on the precise accuracy and stability of digital feedback and control. Even if the motion is simple and the travel path does not require high accuracy, if the end point demands fine positioning — such as micron-level accuracy for machine tools or plastics processing — then it’s wise to eliminate analog controllers from consideration.

Axis requirements
Next look at how many axes the controller must support. Most digital EH controller manufacturers offer a range of systems, with options that include single axis, combination (two to four) axis, and multiaxis (two to 32) units.

Digital controllers pay off

Accurate position and pressure feedback lets controllers precisely coordinate the overall motion of every axis. Advantages include:

• Consistent motion wastes less time and causes fewer rejected parts.
• Operators can change positions and forces while a machine is running, giving them more flexibility in making parts.
• Controllers can reduce or eliminate pressure spikes that damage sensors and cause leaks, reducing maintenance costs and extending machine life.
• They can significantly reduce machine downtime during product ion changeovers.
• Data from controllers can be used for diagnostics and process monitoring.
• Machine motion can automatically adapt to different material consistencies and changes in temperature and humidity.
• Closed-loop pressure control ensures consistent product quality from different machine operators.

Single-axis controllers commonly replace analog controllers and are suited to a wide range of equipment, including clamps, presses, material handling, test stands, steelworks, and machine tools.

A common misconception is that single-axis controllers are “simple” devices, supporting less than the full range of motion control. In reality, most single-axis controllers support position, pressure, velocity, and force control; bumpless transfer from position or flow control to pressure control; and active damping for excellent dynamic performance.

Single-axis controllers also perform complex operations. For example, a honing tool working an engine-block cylinder cavity follows a complex, oscillating single-axis path. The motion must have an extremely precise end point and tight control during honing. If friction increases or some other material factor affects the operation, the controller can adjust velocity, force, or both in real time to keep the actuator within microns of the target.

Combination-axis controllers are recommended for machines with multiple axes but space constraints, or those that require cost-effective control architectures. They handle up to four axes and provide the full range of position, pressure, velocity, and force control.

They also provide a “masterslave” architecture. This avoids redundancies such as multiple bus interfaces, I/O devices, and spaceconsuming enclosures. It can also improve communication between axes for coordination, synchronization, and safety purposes.

In this setup, a master controller card handles bus communications and power distribution with up to four axis cards controlled by the master. This provides several efficiencies. It avoids hardware redundancy, uses less equipment, is more cost effective, and makes better use of space. Changes or updates are uploaded to the master card, so technicians do not have to connect and update four separate controllers. And it provides realtime communication and control between the axis cards.

Multiaxis controllers are top of-the-line systems capable of handling up to 32 interpolatable axes. For example, the Bosch Rexroth mac8 controls synchronization and interpolation of axes through an NC interpreter, which lets it process up to 32 NC programs in parallel. It is suitable for both machinestand- alone machines and integrated systems.

Multiaxis units typically serve high-level applications such as presses, material-handling cranes and truck lifts, steel and rolling mills, sophisticated test rigs, and special machinery such as coal distributors and engine-turning systems.

Programming options
After settling the axis requirements, next evaluate the controller’s programming tools to best fit application and operation requirements. Most product lines offer three options, with ascending levels of complexity. Choosing the most suitable is often based on the skills and experience of programmers and machine operators.

Single-axis controllers are configured using menu-driven ladder logic that is relatively easy to learn and operate. They also have built-in programs to speed implementation of most standard movements. For example, the Rexroth VT-HACD lets users select prestored sequences for linear-gain characteristics and position-dependent braking. No programming is required.

Other applications need additional input on the part of users — for instance, a plastic injectionmolding machine with bumpless transfer from velocity control to pressure control during a motion loop. Prestored commands make configuring such sequences straightforward. For many operations, this basic approach makes controller setup a snap and helps smooth the transition from one machine design to another.

Motion complexity at this level is limited to the programs the controller supplier provides. Obviously, other sequences can be more sophisticated, such as NClevel functions where precise position or force throughout a motion loop requires real-time algorithm processing in response to feedback signals. This often takes a moreflexible controller (either single or combination axis) that supports G-code programming, like the Rexroth HNC100, for example.

Unlike setting up controllers by configuration, this requires a Basic-like programming language in combination with G-codes. Gcode commands are a machinetool industry standard that lets users define axis-motion sequences within the NC program.

Besides standard commands, the best EH controllers in this category support special NC commands tailored to the particular features of hydraulic axes. These include closed-loop force/pressure control and limits, transitions from closed-loop position to pressure control, and adaptive-controller functions.

The most-effective and flexible controllers for multiaxis synchronization and complex control loops support C-level programming. For instance, applications that call for active synchronization of multiple axes with force limitation, or dynamically coordinating multiple axes, need advanced algorithms written in higher-level languages.

This programming approach also lets controllers process complex motion sequences for moreefficient machine control. For example, when a CNC machine cuts a cam profile, the controller accesses a look-up table with extremely fine variations in command values, guiding the actuator through a complex series of motions to generate the cam.

And for applications needing millisecond sampling and processing of feedback data in the control loop, it is more efficient and reliable to process motion-control sequences at the controller, rather than constantly communicating back and forth to a machine-level PLC. Therefore, choose a controller that supports C-level sequence programming and is powerful enough to handle 1-msec or better sampling and loop-sequence processing.

Buses and I/O
As a final step, do not overlook communications and power interfaces. Electrohydraulic controllers support a number of bus architectures, including fieldbuses such as CANopen, DeviceNet, Profibus, Interbus-S, Sercos Drive bus, and various forms of Ethernet. In fact, most EH controller lines support several buses on each product, so the choice often depends on the communications backbone of the machine being built or upgraded.

For analog and digital I/O, make sure the controller has enough I/O to support the application. For example, the Rexroth HACD singleaxis controller offers nine discrete inputs and eight outputs, while the mac8 supports 32 inputs and 24 outputs.

Selection tips

Electrohydraulic controllers are available from various manufacturers, in many versions and with multiple options. What’s most important? If you ask engineers, their priorities are ease of use and performance parameters such as accuracy and repeatability. If you ask project leaders with budget responsibility, issues like price, support, and maybe design and appearance are important factors.

Ideally, users should buy the controller in a package, together with a drive, from one source — that is, an axis that combines a drive and controller. A single source generally means a dedicated partner who takes responsibility for making it work right, troubleshooting, commissioning a system so it’s market-ready, as well as support it in the field.

No doubt, users can mix and match products from different vendors — provided they have the requisite performance and common interfaces. But there is a risk in integrating and operating components from multiple sources. For example, if performance issues arise with a closed-loop system, which company do you call for troubleshooting and support? Viewing controllers as commodity items and making third-party components work together, while possible, can lead to costly time and development delays.

Another caveat: Using a general-purpose electromechanical controller for hydraulic axes is not recommended, for several reasons. Hydraulic drives typically have nonlinear characteristics and the sometimes- challenging dynamics inherent in hydraulics. Axis controllers for hydraulic drives are designed to improve the dynamics (state control) and compensate for nonlinearities. Using a general-purpose controller would probably entail much more time-consuming setup and tuning to, hopefully, match performance inherent in an electrohydraulic controller.

It also makes sense to look for a supplier with in-depth experience engineering a wide range of motion-control platforms across many industries, combined with extensive experience in open and closed-loop hydraulic systems. Suppliers that understand systems and have the software to simulate the static and dynamic behavior of machine axes are best positioned to help select the right electrohydraulic platform that meets performance requirements.

Make Contact
Bosch Rexroth,

Hydraulic servocontrol offers benefits such as precision and coordination of multiple axes, diagnostics, and processmonitoring capabilities. But to get the most from a system, the controller and drive must match the application.

Single-axis controllers such as the Rexroth VT-HACD support position, pressure, velocity, and force control, as well as position or flow control with bumpless transfer to pressure control. It also offers active damping that improves dynamic performance.

The Rexroth mac8 is a highend, multiaxis controller. It can control and synchronize up to 32 axes and supports 32 inputs and 24 outputs.

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