Electric Motors vs. Fluid Power: Robotic System Designers Have a Choice
- Friday, 01 June 2007
Robotic system designers should choose the right power source for the job. Often, electric motors are chosen without thinking about the benefits of 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.
Conventional electric motors are well suited to applications where the predominant form of motion is rotational. They are easy to control and can be the least expensive power source in small systems that have few axes or light loads. Linear electric motors, although more expensive, have advantages in positioning applications where motion is linear and where quick direction changes are required. Hydraulic motors and actuators can do practically everything that electric motors can and have several advantages in heavy machinery applications.
Hydraulic actuators can lift and hold heavy loads without braking, and can 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. Electric motors must be sized for the maximum load applied; hydraulic pumps need to be sized only for the average load. Hydraulic actuators are also comparatively small. The hydraulic advantage is greatest when there are breaks in the motion, as the accumulator stores energy while the system is not moving. Electric motors make sense in applications with continuous motion.
An electric motor is typically located close to or directly on the motion axis. In fluid power systems, the air or hydraulic pump may be located remotely. Only the accumulator and 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 mounted in a base location, keeping the weight on the robotic arms low. Sharing a pump between multiple-axis actuators can result in a cost per axis that is lower than the equivalent system employing electric motors. With hydraulics, pressure can be held constant without applying significantly more energy. Driving an electric motor to apply constant torque could cause overheating. In material transfer applications that are prone to binding due to mishandling of material, fluid power 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 less so. The natural “give” of pneumatic motion technology, while a detriment to fast pick and place applications, can be of benefit elsewhere, such as in physical therapy. The inherent “give” is a safety benefit. Acceleration-limiting algorithms (active damping) can make electric and hydraulic axes mimic this behavior, but such is dependent on proper functioning of various sensors and algorithms. If something fails, the axis could move suddenly with full speed and force. Hydraulics have advantage when heavy or unpredictable loading can overload the actuator; pneumatics can have advantage when softer motion is desired. Electric actuators do not do well in either case, having the possibility of harsher motion than pneumatics and handling overloads less gracefully than hydraulics.
Fluid Power & Design
The most common use of fluid power is linear motion and the most important factor in planning linear motion systems is sizing the actuator cylinders. Clearly, the cylinder selected needs to be long enough for the stroke required. 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. 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.