Five tips for improving hydraulic control in a mechatronic system

Hydraulics tutorial: Advanced machine designs and new Industry 4.0 technologies combine mechanical motion with sophisticated electronic controls in a synergy known as mechatronics.


Figure 1: Using an “observer” can enhance control capability. Courtesy: Bosch RexrothIn certain areas of industrial production, hydraulics technology is still king, by virtue of the speed, force, and power density available. In those applications it is unlikely that electromechanical drives will effectively displace hydraulics, given foreseeable technologies. Since hydraulics can provide some unique advantages, they are increasingly being teamed up with advanced controls to match electromechanical drives in accuracy and flexibility.

Modern hydraulic drives are an integral link in the mechatronics chain and a vital element in overall Industry 4.0 machine concepts. However, working with hydraulics poses a number of challenges, based on physical properties and limitations that need to be addressed. Be sure to consider these important tips to improve functionality and performance when incorporating hydraulic drives into a machine design. 

Tip 1: Use velocity to stabilize and improve performance

Nearly all electromechanical drives incorporate some type of velocity feedback sensor, which is tightly integrated into the function of the drive's controller. The result is a well-defined velocity-per-unit-signal (velocity gain) for the drive. However, in a hydraulic drive, velocity of the actuator is a function of the command signal and also is strongly influenced by the load on the actuator.

This change in velocity has a direct effect on the loop gain of the drive and ultimately affects stability and/or accuracy of the motion. Adding a velocity feedback sensor to the control can eliminate this problem, but is rarely done, especially by controls engineers who may be unfamiliar with hydraulic drives. For example, using CNC controllers designed for electromechanical drives, do not typically have the ability to add velocity feedback and will result in sub-par performance in response, accuracy or both.

To achieve full system capability, the controllers used with the hydraulic drives must have features that compensate for changes in system gains. These features usually incorporate some sort of velocity estimator to calculate estimated velocities and stabilize the overall gain.

Specialized filters, or an "observer" can use the available feedback signals and derive an estimated velocity signal to use in the control. An observer uses a simplified mathematical model of the system to derive the calculated velocity signal. 

Tip 2: Use control-based damping for hydraulic drives

Damping is critical in modern high-performance drive systems. When using an electromechanical drive, embedded torque and velocity loops within the drive's controller provide a high level of stiffness and damping for the drive.

In contrast, hydraulic drives use fluid that is compliant when compared to mechanical drive elements. The fluid's compliance, or springiness, when combined with the load inertia or mass, results in a relatively low-frequency "spring-mass" system, with low-inherent damping. The drive system may have a tendency to oscillate as the performance requirements are tightened.

The only option for improvement is to increase the damping of the hydraulic drive to increase bandwidth and performance. Traditional methods of damping include controlled internal-fluid leakage, or resistive frictional elements to dampen oscillations. While effective, decreased drive efficiency and reductions in stiffness are the undesired side effects when using these methods.

Today's control technology can provide advanced methods of damping for hydraulic drives.

These fall into three general categories: transducer-based, observer-based and real-time derivation. In transducer-based damping, transducers are added to the drive. These can be velocity, acceleration or pressure sensors, or a combination. Using electronic filtering techniques, these sensors measure undesired motions resulting from the onset of instability, and then offset the undesired motion by applying corrective signals. The disadvantages are the additional cost and mechanical complexity of adding the transducers.

Using observer-based damping, a simplified mathematical model of the drive exists within the controller. This model includes the resonances and inherent low damping of the drive. Using available feedback signals (generally a position transducer), the observer predicts the presence of undesired motions and provides compensating signals to remove them (Figure 1). This eliminates the need for additional transducers but at the expense of greater computing power and complexity required, as well as detailed knowledge of the system dynamics required to generate the observer model.

Real-time derivation uses available transducer signals, similar to the observer, but discerns information about undesired motion by deriving information about the frequencies present in the feedback. Using methods such as Inverse-FFT, signals not associated with the desired motion are used as corrective signals. Since a mathematical model of the system is not required, the setup is simpler and flexibility of control is greatest when using this method, but computing demands and software complexity limit the use to specialized systems.

For the high-performance requirements needed in future industrial machinery, low-inherent damping of hydraulic drives will become a focus area that can be best addressed when using a control system that can provide additional compensation. Most of today's standard-performance computer numerical controls (CNCs) and programmable logic controller (PLCs) do not incorporate these capabilities, so there are advantages to using high-performance controllers engineered specifically for hydraulics. 

See additional tips on managing force with hydraulic controls and tips for using hydraulics in mechatronics

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