Electromechanics vs. hydraulics vs. pneumatics: Choose the correct power source for your motion task
By Richard Meyerhoefer and Ben Schmidgall
Conventional electric motors are well suited to applications where the predominant form of motion is rotational.
They are generally easy to control and can be the least expensive power source in small systems that have few axes or light loads.
Linear electric motors have an advantage in positioning applications where motion is linear and the applications require quick direction changes, although they can be more expensive than conventional motors.
Hydraulic motors and actuators can do virtually everything electric motors can do and have several advantages in heavy machinery applications that may be the key to meeting productivity goals. For example, hydraulic actuators can lift and hold heavy loads without the need for braking, move heavy objects at slow speeds, or apply torque without the need for gearing, while consuming less space and producing less heat at the actuator than electric motors.
When sizing electric motors, one must make sure they can handle the maximum load that will ever be necessary. Hydraulic pumps need only to be big enough for the average load. Hydraulic actuators are comparatively small, even for applications that involve heavy loads.
The hydraulic advantage is greatest when there are pauses in the motion as the accumulator (a hydraulic fluid reservoir) stores energy while the system is not moving. On the other hand, electric motors make sense in applications with continuous motion such as conveyor applications.
An electric motor is typically located close to or directly on the motion axis. In fluid power systems, the air or hydraulic pump—along with its noise and weight—may be located remotely. Only the accumulator and pressure control valves need to be located near the actuators.
This can make fluid power an ideal motive force for robotics applications with many axes. The pump can be in a base location, keeping the weight on the robotic arms as low as possible. In addition, sharing a pump between multiple axis actuators can result in a cost per axis lower than the equivalent system employing electric motors.
Hydraulics has the additional advantage that pressure can be constant without applying significantly more energy. By comparison, driving an electric motor to apply constant torque could cause the motor to overheat.
In material transfer applications prone to binding due to mishandling of material, fluid power—with its more compressible power transport medium—may be more forgiving of jams than electromechanical power.
Pneumatic grippers and rotators, along with vacuum devices, are common components of many industrial robotic systems. Pneumatic motion axes requiring precise positioning are less common, but they do have a place. The natural “give” of pneumatics, while a detriment to fast pick and place applications, can be of benefit in other applications.
One example is in physical therapy, where robotic actuators help to retrain muscles. Another unique example is automated milking machines. In these applications, the inherent “give” of the pneumatic axes is an important safety benefit.
Acceleration limiting algorithms, including active damping, can make electric and hydraulic axes mimic this behavior, but the behavior then becomes dependent on proper functioning of various sensors and algorithms. If something fails, the axis could move suddenly with full speed and force.
Pneumatic actuators are safer since the acceleration limiting is a function of the technology. A futuristic application is with humanoid robots, where specialized pneumatic actuators, called artificial muscles, are showing promise for better duplication of natural movements.
Fluid power actuators are also more forgiving of overloads. Hydraulics has an advantage when heavy or unpredictable loading can overload the actuator, while pneumatics can have an advantage when softer motion is desirable to prevent damage to the manipulated objects.
Electric actuators do not do well in either case—having the possibility of harsher motion than pneumatics and handling overloads less gracefully than hydraulics.
Machine designers often have many significant decisions to make, driven by fundamental tradeoffs between the system elements that are available.
A case in point relates to the development of a new concrete delivery robot by the Hawkeye Group, a supplier of machinery that makes concrete components for sewers and culverts.
The company recently completed the development of a new machine for making large concrete box and pipe sections. This concrete machine is composed of a central hopper, a conveyor, and a chute feeder mechanism. The hopper and chute move, using hydraulics, about pivot points, enabling the machine to fill molds of different shapes from 4-ft wide up to 20-ft wide, with walls between six inches and a foot and a half thick.
In order to cause the end of the chute to follow the shape of whatever concrete mold is in use at the time, an electro-hydraulic motion controller directs the two rotational axes to follow profiles defined by user-developed mathematical formulas.
In addition to regularly shaped boxes and round pipes, elliptical and arch-shaped pipes are possible by using the appropriate motion profile programs. The system incorporates an operator interface to provide key x-y coordinates to a PLC, which passes them off to the motion controller. The PLC also handles general operations including the transfer belt and other control functions. The motion controller calculates additional points to complete the shape and then calculates the main pivot angle and chute angle needed to hit each point. Then the motion controller moves the axes to trace the path.
The new system is actually a second-generation design. Hawkeye originally used a high-performance PLC to compute the coordinate points for the motion, and electric motor drives to move the hopper and chute pivot axes.
The company made the change from electric to hydraulic power because the electric motors could not handle the heavy and varying loads of concrete. The feeder mechanism can weigh up to 10 tons when full.
With heavy loads such as this, the electric motor drives would “trip out”—shut down due to overloading. The system could not achieve the desired motion while staying within the motors’ power curve.
Hydraulics has the distinct advantage of being much more forgiving with heavy loads and overloads than electric motors.
By coupling robust hydraulics with closed-loop position control from the motion controller, Hawkeye was able to move widely varying loads smoothly and precisely. In this system, position feedback comes from a quadrature encoder mounted on each rotational axis.
We used a two-axis controller with Profibus communication capability from Delta Computer Systems.
Because of the motion controller’s computational ability—it can calculate the complex motion paths using its internal software—Hawkeye was able to switch from the high performance PLC model to a lower-cost one with more moderate but adequate performance.
The Hawkeye engineer did the programming of the motion controller. Since complex mathematics would be involved in the application, it was important to prove out the numerical methods to prior to the construction of the machine.
To do this, the engineer used an embedded simulator that was a part of the motion control software. The engineer was able to generate sample points for the moves and watch the simulator graph out the target and actual positions.
He began by tracking simple shapes and then added features. To track the motion around a square, the engineer started with a consistent linear speed. Then he went on to slow down the speed of motion near the corners of the box, still just using the simulator.
Though he had never done any complex motion control programming before, he was able to simulate all the motion of the system while sitting at his desk. Then, once he got the machine and motion controller hooked up on the manufacturing floor, it only needed to be tuned.
As Hawkeye’s engineer discovered, an advantage of using the Delta motion controller is its simulation and graphing capability. Using the old system, there was no way of telling if the system was properly tuned or not because there was no way of monitoring the actual motion.
The graphs show not only when the system was well tuned, the system graphs also serve to document the development progress with the project. The new Hawkeye system has a modem for remote programming and maintenance, which can remotely diagnose system problems or do fine tuning of systems, even after installation at remote concrete plants.
The best motion controller cannot compensate for poorly selected and specified hydraulic components, however, and designers developing fluid power-based robotic systems for the first time will have to deal with some different design issues, however.
Hawkeye’s experience indicates the new system enables pouring the molds faster than before because the hydraulics are more powerful than the old electrical system, and the new control system enables him to track faster than before.
The faster the concrete delivery system can go around the mold, the thinner the layer it can lay down, and the better the quality of the concrete products the machine produces.
We learned a number of lessons from this project.
Hydraulics can be better than electric motors where heavy loads need to be precisely positioned.
Simulating the motion before constructing the system shortens development times.
Using best of class system components, tailored for the job they perform, such as electro-hydraulic motion controllers with complex and powerful math capability, provides the maximum system functionality and value for the price.
Robotic system designers should choose the right power source for the job. Often, folks choose electric motors without thinking about the benefits of fluid power sources, hydraulics, or pneumatics.
For applications where precise control of large forces and smooth motion are required, or applications that require “forgiveness” in the motion, fluid power can deliver significant benefits compared to electromechanical motion.
As in all good design, it is important to carefully consider the selection and sizing of the fluid power system elements and to tune the motion controller for optimal performance.
ABOUT THE AUTHORS
Rick Meyerhoefer (firstname.lastname@example.org) works at Delta Computer Systems in technical sales and marketing. Ben Schmidgall (email@example.com) works at Hawkeye Pipe as the senior automation engineer.
Sizing motion systems
The most important factor in planning sizing motion systems is sizing the actuator cylinders.
Clearly, the cylinder selected needs to be long enough for the stroke required, but oversights sometimes occur in specifying the diameter of the cylinder. The cylinder choice is crucial, since the natural frequency of the system is roughly proportional to the diameter of the cylinder.
The natural frequency is fundamental in determining the maximum acceleration rate the system can achieve under control.
Therefore, if a system needs to accelerate twice as quickly, the natural frequency of the system must be twice as high; to do this, the cylinder diameter must be twice as big.
After choosing the piston/cylinder diameter for the desired acceleration, one must calculate the pump size to provide air or fluid flow for the speed and acceleration needed. If the pump is too large, however, fluid and the power required to pump it may be wasted.
Fortunately, the calculation is relatively simple: The required volume of oil flow in a hydraulic system matches the required change in internal volume of the cylinder over time. To achieve double the acceleration requires double the diameter or four times the surface area. With four times the area and twice the speed, the oil flow must be eight times higher.
After determining the pump size, the next step is to size the accumulator. The accumulator in a fluid power system serves two purposes. First, it serves as a buffer, allowing the power requirements from the pump to be time-averaged. Second, it allows the system pressure to remain relatively constant, so the effects of motion control inputs remain relatively constant.
This avoids the need to continually change the control input-response relationships used by the motion controller to maintain precise control. A good rule of thumb is to make the accumulator large enough so the pressure does not change by more than 10% during the system’s operating cycle.
Further, the physical location of the accumulator needs some consideration in order to minimize system pressure losses. It is important the accumulator be close to the valve rather than close to the pump.
The next step is to select valve(s). Precise hydraulic motion control will often require the use of servo-quality proportional valves that allow a linear increase in fluid flow through the valve in response to a linear increase in drive current. For maximum system responsiveness to control inputs, valves should match the cylinder’s flow needs by providing the required flow plus another 10 to 20%. If the valve is too large compared to the size of the cylinder, control of the valve will be coarse because only a small part of the control range is in use.
For controlling pressure, sensors should be at the bottom of the cylinders on either end where trapped air will not affect them and where there is less oil motion. A common mistake is to mount the pressure sensor in the manifold, where the Venturi effects of moving oil can decrease the accuracy of pressure readings.
Linear transducers such as linear magnetostrictive displacement trans¬ducers (LMDTs or MDTs) provide absolute position information to the motion control¬ler and do not require homing. MDTs also have pressure and temperature specifications that allow them to be inserted directly (gun drilled) into hydraulic cylinders.
Magnetostriction is a property of ferromagnetic materials to undergo a change of their physical dimensions when subjected to a magnetic field.
Venturi effect is a special case of the Bernoulli effect and in the case of fluid or air flow through a tube or pipe with a constriction in it. The fluid must speed up in the restriction, reducing its pressure and producing a partial vacuum.
Hydraulics is a branch of science and engineering concerned with the use of liquids to perform mechanical tasks. It is part of the more general discipline of fluid power.
PLC is a programmable logic controller, a small computer for automation of real-world processes, such as control of machinery on factory assembly lines.
Concrete is Portland cement, mineral aggregate—gravel and sand, and water.
Pneumatics is the use of pressurized gases to do work in science and technology.
Electromechanics combines the sciences of electromagnetism of electrical engineering and mechanics.
Mechatronics is the discipline of engineering that combines mechanics, electronics, and information technology.
Mechanics is the science dealing with the motions of material bodies, including kinematics, dynamics, and statics.