MOTION CONTROL FEATURE


ELECTROMECHANICAL ACTUATION: W


ith hydraulic systems, barely compressible hydraulic fluid amplifies electrical energy


Electric actuator Hydraulic cylinder


to move a load. In a typical dual-action system, an electric motor drives a pump, which delivers the hydraulic oil to a cylinder containing a tightly sealed, but easily movable, piston attached to a piston rod. As the intake fluid arrives from the reservoir, it exerts pressure on the piston rod, which moves the load movement. As the piston moves, it forces fluid out the other end of the cylinder to the reservoir for eventual return to the chamber. For this, an external system of hoses,


connectors, filters, switches, valves and pumps that cycle the fluid to and from the cylinder, are needed. Even the smallest system would require at least eight separate moving components. In addition, maintaining consistent pressure is critical and depends on proper valve settings, connections and switching. As a result, there can be problems – from


filters clogging, oil levels dropping, and seals and gears deteriorating; to valves not opening or closing properly, changes in oil viscosity, and oil overheating. Added to this, hydraulic cylinders consume space, and noise can be an issue, with some users resorting to noise mitigation products and services, which add even more design challenges and costs. Furthermore, the need to have hydraulic systems idling in wait makes them inefficient consumers of energy. This is especially true when the system must hold a static load in place, as can be the case for mobile equipment. Most applications involve multiple hydraulic cylinders. The more cylinders in use, the greater the complexity, ambient noise, energy consumption and related costs.


POTENTIAL ISSUES There is also an additional layer of maintenance related to the hydraulic fluid itself. The following are some of the potential hydraulic fluid handling issues that have been identified: Toxicity. The U.S Agency for Toxic Substances


and Disease Registry reports that some types of hydraulic fluids can irritate skin or eyes and that ingesting certain types can cause pneumonia, intestinal bleeding or even death in humans. The effects of breathing air with high levels of hydraulic fluids are not known, but some countries have set hydraulic fluid exposure limits. Contamination. In addition to being potentially


toxic to humans, hydraulic fluids can be harmful to the environment. The U.S. Environmental Protection Agency (EPA), for example, includes hydraulic fluids on its list of oils that require special handling. Slips and falls. Fluid leaking or spilled during


maintenance can present slip and fall hazards. Burns. During routine operation, hydraulic systems can reach 180˚F or even higher.


Figure 2. Hydraulic and electromechanical wiring comparison. / DESIGNSOLUTIONS DESIGN SOLUTIONS | JULY/AUGUST 2020 35


Figure 1: Illustration comparing electric actuators and hydraulic systems


Cleanup. Indoor spills may also be subject


to local regulation or require investment in absorbent substances or other cleanup services. Combustibility. Petroleum-based hydraulic


fluids are less flammable than petroleum middle distillate fuels such as jet fuel, kerosene or diesel fuel. Hydraulic fluids can be a fire hazard if sprayed. Special care during storage and fluid changes is needed.


DIFFERENCES Because hydraulic cylinders are designed only for simple end-to-end movements, instructing one to stop at a certain position, change location, or attain a specified speed requires adding control components. Likewise, ramping the speed up, slowing it down or following a consistent motion profile would require external components, which can be expensive to integrate, operate and maintain. Obtaining accurate position readings, for example, would require an external measuring device such as a rotary encoder. Speed control would require sophisticated valve assemblies. Figure 1 (above) compares simple ways


to run electric and hydraulic cylinders back and forth. Electric actuators are simpler and more compact in design, ensuring no contamination risks. Hydraulic cylinders, on the other hand, are more complex, requiring external support infrastructure as well as more


For powerful and smart linear motion For many years, hydraulic systems have been the only


option for some applications. But with their ability to offer a high-power, zero


maintenance solution, there are a number of reasons to consider electromechanical systems as an alternative, writes Thomson Industries


space-demanding solutions that add more weight to the complete system. Electromechanical actuators are therefore


more suitable for the new generation of intelligent linear machines. These have advanced to provide the high-load, compact, advantages of hydraulics but without the drawbacks – they can handle loads of 16kN or more without the need for oils and complex hoses, valves, pumps and other assemblies. In electromechanical actuators, all functionality


can be embedded in the actuator housing itself, which connects an electronic control unit (ECU) with only a few wires (Figure 2, below). Leveraging this capability are built-in microprocessors that can be programmed to report position, provide diagnostic feedback that improves performance, and handle complex functions like synchronising multiple actuators. Electromechanical actuators can accept


commands and, in return, provide status information such as position and speed, and safety-related data such as load or temperature. Furthermore, because functionality can be switched on instantly, there is no idling. Considering all these factors, electromechanical


actuators are increasingly being specified for the next generation of mobile equipment, industrial machinery, aerospace systems and many other applications where simple, powerful and smart linear motion is required.


Thomson Industries www.thomsonlinear.com/smart


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