Give Our Hydraulics a Lift

Tips for Ensuring a Well-Tuned Hydraulic System With Emphasis on Controls and Embedded Sensors

By Control Design Staff

We work on older, hydraulically powered, multi-axis systems, so we have a big interest in them not all converting entirely to electrically powered alternatives. By sharing a pump across many axes of motion, we believe we can keep our per-axis cost in line with any solution. We'd like to hear what tips readers have for ensuring a well-tuned hydraulic system with emphasis on the controls and the embedded sensors.

—From April '12 Control Design

Answers

Hydraulic or Electric?
Hydraulic vs. electric is an age-old question, further complicated by technological advances made by both sides. Hydraulics now can be quiet and highly responsive. Electric drives now deliver higher torque and power. So the first design criterion to be established is whether the drive will be electric or hydraulic. This usually has to do with power, response and control needed on the axis, but can involve other considerations as well.

Sharing a pump's flow across many axes of motion can be accomplished by way of valving to split or combine that flow as needed, such that one pump and drive motor can provide all the necessary motions, rather than individual drive motors on each axis. This would be fairly typical for a hydraulically driven machine. Individual electric drives, servo motors or ball screws can't be combined mechanically into one, but hydraulics can. A single drive pump can be directed to various control valves, so that any number of sequential or overlapping functions can be achieved. It normally wouldn't be feasible to have an individual pump for each motion desired.

The perceived advantage of an electric drive on each axis is that an idle axis has an idle drive, whereas a common pump is being driven whether the valve controlling the axis is idle or not. Even if the pump compensates, there is still some drive power to the motor and leakage from the pump to maintain lubrication.

Advances in servo and variable-frequency drives now allow servo-driven pumps to be used, either fixed or variable displacement, to most effectively combine electric drives with hydraulic response and power.

The controls and sensors involved in closing the loop around whichever parameter is critical to the process (pressure, flow, position, speed) must be compatible with the system and capable of operating with the required characteristics. Duty cycle and ambient conditions must be considered.

It's best to work with an integrator or parts supplier who understands systems and has complementary products to offer, so they're not just pushing individual items with no concern of how the integrated system will perform. Also be careful that purchasing doesn't "cost optimize" your engineering requirements down to the lowest bidder. We all know the adage, "You get what you pay for," and it's as true in professional and commercial situations as it is at home. More robust components and system-savvy expertise don't come inexpensively, but the return on investment is generally worth it.

As far as a well-tuned hydraulic system, overall — that can lead to an entire discussion on fluid conditioning: filtration, temperature, aeration. Even the most highly tuned system can be operating very differently just hours after commissioning if the fluid gets contaminated or changes chemistry. We've seen many "controls" issues turn out to be air in the system, or filters in bypass, or insufficient cooling.

Dave Wenner,
hydraulics business unit manager,
Airline Hydraulics

Flexibility of Software
Depending on your application requirements, there are a couple different approaches for multi-axis control with a single pump.

If multiple axes need to run simultaneously, they can be individually controlled through servo valves connected to a single pump system.Instead of using a dedicated hydraulic controller with rigid firmware structure and limited diagnostic capability, consider a flexible software-based solution, which can offer software-based pressure, position and force control algorithms along with functions to compensate the inherent non-linearities of hydraulic systems.

The hydraulic control software will be an integral part of the overall machine control. This allows a tailored and cost-optimized hardware configuration of controller and I/O modules to be designed for connection of sensors and controlling the regulating devices like valves in any application using standard I/O modules.

Because the control software and the I/O network run fully synchronized, all valves receive their control value at the exact same time, allowing for the highest possible precision and coordination between multiple axes.

With today's energy prices, developing energy-efficient solutions becomes more important. Conventional swash plate pumps running at constant speed are extremely inefficient. Hybrid drive technology can realize significant energy savings especially in partial duty cycles by using a constant displacement pump controlled by a servo motor and drive. Displacement volume is now set by adjusting the motor speed depending on the hydraulic volume needs of all axes combined.

If the application requires only a single axis to run at any point in time, the servo valves can be eliminated altogether, thus further reducing cost and increasing efficiency. Now only simple switching valves are used to direct the hydraulic fluid to the axis in motion, with the volume generated as needed by the servo pump.

Robert Muehlfellner,
director of automation,
B&R Industrial Automation


Design Considerations
Having a well-tuned hydraulic system is critical for maintaining efficiency. Paramount to tuning the system is the initial design. In the design phase, the system should be engineered to produce a specified and desirable result. Hydraulic systems have the advantage of power density over electrically powered alternatives, and are less sensitive to contamination and mechanical wear. Using accumulators to eliminate peak current draws also contributes to the system's efficiency. By minimizing current draw during peak periods, infrastructure power upgrades common to electrically powered systems are greatly reduced. The line noise of these voltage spikes can wreak havoc on embedded sensors and controls.

One key to helping your hydraulic and control system stay well-tuned takes place before your system is even up and running. Choose a system integrator that not only has designed multi-axis systems, but has successfully installed and commissioned them as well. Hundreds of variables and potential pitfalls can damage a system over time. Parameters to consider:

  • Proper flow sizing of all pressure, tank, pilot and drain lines
  • Proper sizing and location of valve manifolds and accumulators
  • Minimization of system shock
  • Protection of low-voltage electronics from electrical noise
  • Oil temperature and cleanliness control
  • Pressure test points for diagnostics
  • Peak flow demand analysis
  • The design of "outs" should alternative circuit options be necessary.

Robert Ferrara,
president,
Atlantic Industrial Technologies

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