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.
By Paul Stavrou November 3, 2015

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

Tip 3: Manage force (pressure) with specialized hydraulic controls

Figure 2: Many hydraulic control valves do not have a linear flow vs. signal characteristics, so the control loop gain for the hydraulics axis can vary widely, resulting in poor performance or instability when not understood. Courtesy: Bosch RexrothForce control, whether alone or associated with a positioning control, is an increasingly important requirement in hydraulic drives. In a typical application that uses a conventional controller, such as a press, the full press force would be available up to the given positioning point. While suitable in some applications, the ability to control or limit force (or torque) based on various process requirements is becoming a key requirement today. As flexible production cells become more common in Industry 4.0 factories, having the ability to change and control different levels of applied force based on production application requirements, as well as controlling velocity and position, will be a future "must have" for many hydraulically driven systems.

Controlling force, or adding force-limiting to a hydraulic actuator or drive, can be the most challenging hydraulic control function. This is due to the fact that in high-performance systems the available rate of pressure change (change in pressure per unit time) at the actuator can exceed 100,000 psi/sec.

This rate makes the response of the controlling valve and the control electronics a critical factor. If the control valve or control electronics are too slow for the desired application, the result can be poor pressure control performance. The resulting issues that occur include overshoot and undershoot of pressure, pressure instability, and the inability to reach the desired accuracy.

Many control engineers and technicians have been frustrated trying to control pressure, or force, using PLCs as the basis of control. Poor throughput (throughput = PLC scan time + analog input conversion time + analog output conversion time) can result in update rates taking as long as tens of milliseconds, which can allow the pressure to exceed the desired set point before the next output value is generated, resulting in over/undershoot or instability.

Using a controller that is designed for hydraulic-control applications can eliminate these problems. High throughput speeds, combined with control algorithms that go beyond conventional proportional-integral-derivative (PID) control loops, can directly or predicatively control pressure and force under the most extreme performance demands and provide accurate, stable pressure and force control. 

Tip 4: Remember that hydraulic drives are non-linear devices

Figure 3: For the high-performance requirements needed in industrial machinery, it’s important to consider the low inherent damping characteristics of hydraulics – which can be addressed using a control system that can provide additional damping compensatControl of hydraulic actuators is often not a linear process. Control valves are resistive devices, similar to an electrical resistor, where the flow (current) is a function of the pressure drop (potential drop) across the device. The result is a flow proportional to the supply pressure (Ps) minus the load pressure (Pl). Therefore flow for a given valve command will vary as the load and supply pressures vary. In addition, many hydraulic control valves do not have a linear flow vs. signal characteristics (Figure 2). The result is a control loop gain for the hydraulics axis that can vary widely. This non-linear gain term usually results in poor performance or instability when not understood.

Control is often a compromise between accuracy and stability. For improved control, the controller must be able to compensate for these non-linear characteristics so that gain, and therefore accuracy and response, can be optimized. Advanced controllers designed to work with hydraulics will have the ability to compensate or linearize the valve characteristics to greatly improve system performance. Additional control capabilities also monitor load forces or pressures and adaptively adjust the controller gain to maximize available performance.

Tip 5: Position integrators can help optimize endpoint accuracy

When a hydraulic actuator is being positioned by a controller, fundamental control characteristics can change. A valve-controlled hydraulic positioning system is characterized as an integrating term while moving. This means a change in control signal to the valve results in a change in actuator velocity. Therefore position is defined as: X= ∫ (signal input) dt

However, as the actuator reaches its desired position, the control characteristics will change from an integral to proportional control. This is due to internal leakage flows within the hydraulic valve. The result is that the desired position may never be reached. Additionally, the greater the load, or force required at the final position, the greater the potential positioning error.

Using an integrator for the positioning control allows the final position to be achieved with high accuracy, but can also cause significant instability during motion portions of the move. The solution is to use a control that can dynamically precondition and switch on an integral term in the control during a small window around the positioning point, and off during other portions of the move. This greatly improves the accuracy of the final positioning without instability during the movement.

The switching point of the integrator is generally based on several conditions, which are selected to optimize the control. This option is available when using modern high-performance controllers designed for hydraulic axis control.

Hydraulics offers Industry 4.0 advantages

Figure 4: Using hydraulic drives in industrial production machinery, especially for new Industry 4.0 applications, requires high performance, high accuracy and flexible configuration capabilities that conventional PLCs and PID controllers may not offer. CUsing hydraulic drives in industrial production machinery, especially for new Industry 4.0 applications, requires high performance, high accuracy and flexible configuration capability. Conventional PLCs and PID controllers used by many OEMs and end-users today for hydraulic drives do not provide specialized control features needed for high-performance hydraulic drives. Future demands will require controllers specifically designed and optimized with these features. These "hydraulic-optimized" controllers will allow the full range of features and performance to be realized for tomorrow’s factories.

Tips for Using Hydraulics in Mechatronics

  • Use velocity sensors or derived velocity values in hydraulic drives to improve the stability and accuracy
  • Damping is critical in modern high-performance hydraulic drive systems; increase damping using one of three methods, each with its unique advantages: transducer-based, observer-based and real-time derivation
  • Controlling force, or adding force-limiting to a hydraulic actuator or drive, can be challenging; there are high-performance controller technologies
  • Using advanced algorithms designed specifically for hydraulic drive applications
  • Since hydraulic drives and valves have non-linear characteristics, the drive controller must be able to provide compensation so that gain, and therefore accuracy and response, can be optimized
  • As a hydraulic actuator reaches its desired position, the control characteristics will change from an integral to proportional control; use a control that can dynamically precondition and switch on an integral term to provide high endpoint accuracy while maintaining stability during motion.

– Paul Stavrou is manager applications engineering at Bosch Rexroth. Edited by Eric R. Eissler, editor-in-chief, Oil & Gas Engineering, eeissler@cfemedia.com.

Key concepts

  • Most electromechanical drives use a velocity feedback sensor, tightly integrated into the function of the drive’s controller.
  • Using observer-based damping, a simplified mathematical model of the drive exists within the controller.
  • Force control is an increasingly important requirement in hydraulic drives.

Consider this

Hydraulics can provide some unique advantages. They are increasingly being teamed up with advanced controls to match electromechanical drives in accuracy and flexibility.

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