Digital Controls Conquer New Frontier

Digital controls did not infiltrate electrohydraulic drives as readily as it had electromechanical drives. But the continued onslaught of digital control into hydraulics may lead to hydraulics recapturing applications once lost to electromechanical drives

By Marco Agostani
Electronic division manager Atos spa
Sesto Calende, Italy

Digital electromechanical drives have found widespread use for multi-axis motion control in virtually all industrial automation fields. This is due in large part to their ready-to-use configuration, easy interfacing with other electronic controls, and standardization of mechanical-electrical interfaces and specifications.

Innovative digital axis controls are now available with precise position profiles, high repeatability, and full programmability to any application. These benefits not only make setup quick and easy, but ensure simplified repair and maintenance for the life of the machine.

Recent developments in semiconductor technology and new digital signal processing units make it more feasible to integrate digital electronics into proportional electrohydraulics, thus bridging the gap between electromechanical and electrohydraulic axis control. The combined benefits of this technology includes high power density and energy storage capability, intrinsic overload protection, low mechanical wear, long service life, and simpler mechanics from direct realization of linear motion.

As a result of a five-year research and development project, Atos has finalized its line of proportional electrohydraulics that has gained use in hundred of successful applications, some of which are described here.

Directional control valves

Digital proportional directional valves use on-board digital electronics to perform closed-loop control of position, speed, and acceleration
Digital control of proportional valves encompasses a variety of data acquisition functions to produce a driving current for the valve’s solenoid. In fact, an LVDT provides feedback of the valve spool position for closed-loop control within the valve itself.

Once provided with the appropriate position feedback signal from a linear displacement transducer on a double-acting cylinder, proportional directional valves use on-board digital electronics to perform closed-loop control of position, speed, and acceleration by regulating oil flow to the appropriate port of a double-acting hydraulic cylinder. The digital controls provide signal acquisition, and produce a driver current to the valve’s proportional solenoid. They efficiently perform complex data calculations, thus outperforming traditional analog valves in terms of dynamics, repeatability, calibration, auto-protection, and diagnostic functionality.

The valve’s functional parameters (external commands, spools position, solenoid current, etc.) are converted in numeric digital form by a 14-bit analog- to-digital converter with 200-μsec sampling rates by a 32-bit digital micro controller capable of 40 million instructions per second. The use of innovative algorithms, specifically developed for hydraulic applications in cooperation with Polytechnic University of Turin, helps overcome the traditional limits of general-purpose digital controllers.

Useful features
The user can freely adapt and customize the valve’s behavior to the specific application by setting the following main functional parameters in numerical form, then recording and reproducing them on other valves:

  • minimum and maximum flow regulation (bias and scale)
  • maximum acceleration ramps
  • compensation of valve regulation
  • characteristics (linearization)
  • settings of the control loop dynamics (proportional-integral-derivative)
  • anti stick-slip and friction compensation (dither), and
  • command signal range selection.

The digital proportional valves are equipped with Fieldbus electronic communication interfaces, with a choice of serial RS232, CanOpen, or Profibus. The basic RS232 data transfer gives access to the valve’s functional parameters and to external and internal signals by means of proprietary PC software. This software has a multi-level structure to simplify startup, tuning, and troubleshooting of the valve and the relevant electrohydraulic axis. This leads to a clear and immediate reading of any useful information, such as actual and trend values of command signals, feedbacks, control PID actions, and status or fault conditions. In other words the working characteristics and status of the valve can be easily focused and the functional parameters correctly set.

Digital valves with standard Fieldbus interfaces (CanOpen or Profibus- DP) can be integrated into the overall machine network to obtain additional benefits:

  • use of standard and modular wiring solutions,
  • direct access from the machine control unit to all configuration parameters, and
  • real-time digital exchange of all command, monitor, and diagnostic signals.
Basic RS232 data transfer gives access to the valve’s functional parameters and to external and internal signals by means of proprietary PC software

All these functions are basic requirements for remote diagnostic and troubleshooting for efficient on-site repair. Additional functionality can be incorporated for advanced users, such as:

  • life and working counters for preventive and predictive maintenance,
  • monitoring functions to check for the presence of erratic operation, such as a sticking spool, inadequate or absence of pilot pressure, extreme temperature, etc., and
  • real-time graphical display of all

important time-dependent valve operating parameters, such as set point, feedback, control action, alarms, etc.

Pressure-control valves
Digital electronics can improve performance of pressure-control valves, especially in closed-loop circuits using an integral pressure transducer. In these applications, digital control minimizes the influence of system transients on stability, hysteresis, repeatability, etc.

For example, anti-windup and limitation strategies of integral control take action, when a non-reachable pressure is requested. Also, differentiated damping factors allow changing pressure or multiple parameters to adapt valve dynamics to different oil volumes of systems.

Digital valves feature default factory presets and arrive ready to install. However, proprietary software makes it easy to change or adjust settings for optimum parameter tuning when required.

High-flow cartridge valves
Digital directional- and pressurecontrol valves are available also in cartridge versions. The integration of digital electronics in high flow cartridge valves boosts dynamic performance and makes it easy to change one or more multiple parameters just as in line-mounted valves.

Pilot-operated proportional cartridge valves are capable of controlling flows to 104 lpm with quick response — typically, better than 30 msec for NG80 size — and with repeatability similar to direct-operated valves. On-site adjustment of working dynamics may be required to avoid excessive mechanical shock. Once more, Atos proprietary software allows tuning of the valve for the optimum machine performance.

Pressure and flow control

High-flow directional- and pressure-control cartridge valves can exhibit the same benefits of digital control as their linemounted counterparts.

Fitting standard digital directional valves with pressure transducers enables them to control not only direction, but pressure, as well. This simplifies hydraulic circuits, which can extend their field of application. In addition, the following features can be incorporated:

  • regulation of line pressure from a single pressure transducer,
  • regulation of cylinder force using a pair of pressure transducers, and
  • flow regulation independent of pressure disturbances, by flow electronics compensation.

Multiple pressure and flow controls can also be applied to variable-displacement pumps to create an intelligent power unit that regulates maximum power and even compensates for internal leakage from pressure variations.

Digital servoactuators
Digital servoactuators (smart cylinders) integrate a hydraulic cylinder with the relevant linear displacement (and sometimes pressure) transducers and proportional directional valve to create a compact, pre-tested, and costeffective linear motion axis.

Smart cylinders integrate the appropriate transducers, proportional valve, and interfaces into a compact, pre-tested, and cost-effective package.

Smart cylinders are available in multiple configurations and with options to customize the electrohydraulic axis to specific application requirements. Common applications include injection molding, lumber processing, fatigue testing, motion bases for flight simulators or entertainment attractions, press brakes, etc.

With assistance from experienced application engineers, users can then select the configuration and size to achieve:

  • high speed and cycle rates to 50Hz and 2 m/sec with low-friction seals and high-response valves,
  • high resolution and repeatability (up to 1 μm) with high-resolution resolution encoders, and
  • working forces to 700 kN with standard cylinder execution.

A “plug & work” mode of operation simply requires entering some basic working data (mass to be moved, working force, etc.) to achieve quick startup and easy maintenance. Digital servoactuators with an internal motion profile generator manage all motion independently from the machine’s main control. This uncouples the machine automation system from the specific hydraulic axis management.

System technicians simply set the main motion parameters (position, speed, acceleration, etc.) and logic commands (start, stop, delay, wait, conditional jump, etc.) for each axis working phase. The same decentralized control architecture can be applied in troubleshooting and maintenance to implement safety functions, such as alarms; or fault or critical working conditions, such as disabling the valve, stopping the axis, holding a defined position, etc.

Digital controls are the present and the future of any electrohydraulic application from simple actuator positioning to sophisticated multiaxis system control with complex synchronization. Digital electronics provide strong advantages everywhere used, enhancing functionalities and flexibility of any element of machine and plant automation.

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