By Dan Hebert, PE, Senior Technical Editor
There are a host of industrial machine motion applications that favor hydraulics over electromechanical solutions. But applications have been limited because hydraulic control systems could not match the precision, accuracy, and repeatability of electromechanical control systems. Recent advances largely have rectified this problem, and it now is possible to achieve precision control of hydraulic motion systems.
Hydraulic power can lift and hold heavy loads, move heavy objects at slow speeds or apply torque without gearing or braking, all while generating much less heat at the actuator. This makes hydraulics better than electric motors for press applications and heavy material transfer applications.
Hydraulic pumps need to be sized only for the average load since accumulators store energy between cycles. By contrast, electric motors must be sized for the maximum load. Hydraulic power's advantage is greatest when there are breaks in the motion, so the accumulator stores energy while the system is not moving. Breaks occur in many press, discrete manufacturing, and material-transfer applications.
"Applying hydraulic power with high-speed precision can be complex. Standard controls such as PLCs and other PID-based controllers often result in excessive error." |
The size and weight of hydraulic actuators can be much less than electric equivalents because the power-generating hydraulic pumps can be mounted separately in a base location. Sharing a pump between multiple-axis actuators can result in a cost per axis that is lower than an equivalent system employing electric motors. This makes hydraulics well-suited for multiple axes applications where actuators are moving other actuators including robotics, flying cutoff systems and most mobile uses.
In material transfer applications prone to binding due to mishandling of material, fluid powerwith its more compressible power transport mediummay be more forgiving of jams than electromechanical power.
Applying hydraulic power in these types of applications can be quite complex, especially if there is a need for high-speed precision control. Standard controls such as PLCs and other PID-based controllers often result in excessive error.
PID control is not optimal for hydraulic systems because of the asymmetric dynamics of the actuators, says Peter Nachtwey, president of Delta Computer Systems. Hydraulic actuators have different response characteristics in the extending and retracting directions because of the different surface areas on either side of the piston. To operate efficiently, two sets of gains must be used, one for each direction of travel.
The solution can be electrohydraulicselectronic controls designed specifically for hydraulics. These controllers supplement PID control with advanced control algorithms such as predictive and multivariable control so hydraulic systems can perform with speed and precision.
Predictive feed-forward controllers estimate the output required as a function of velocity, acceleration, and the rate of change of acceleration. The feed-forwards should be able to estimate the correct output to within 5%, so the PID component only has to compensate for the last 5% of the output, adds Nachtwey. Ideally the feed-forward should be right on the mark and the PID gains used only to compensate for changes in the load and other non-linearities such as environmental effects.
Most hydraulic controllers have position feedback for normal position and velocity control, and multivariable controllers add pressure sensors to provide force feedback. Force feedback is a leading indicator of what the load is going to do, concludes Nachtwey. Force feedback is necessary for force control, force limiting, active damping and the more extreme version of active damping called compliant motion.
Predictive, multivariable, and PID control often must be used in concert to optimize control of hydraulic systems. Application of these advanced control techniques can be quite daunting, especially for machine builders who only use simple PID loops for control of electromechanical systems. Some vendors recognize these difficulties and provide software tools that expedite hydraulic control system design and programming.
Delta provides automated tools called tuning wizards that allow a user to try multiple built-in mathematical models on the same control signal profile. The model with the least amount of error between estimated and actual positions becomes the basis for the control system, although further tweaking is often required.
Besides better control, electrohydraulics offer other benefits. Diagnostics, networking, remote dial-up for maintenance, and other features are important side benefits of electrohydraulics, says Tommi Kauppinen, managing director of Axiomatic Technologies Corp.
As hydraulic control systems improve, hydraulic solutions are beginning to replace electromechanical systems in applications where the dynamics of hydraulic systems are attractive. Electromechanical vendors are fighting back and improving their systems to deal with heavier loads, higher torques, and static loads.
The result is more options for optimal control of motion systems in industrial machinery.
