beneficial and when the electronics are separate from the
Electric actuators consist of a ball, acme or roller screw,
connected via a coupler to an electric motor. As the screw
turns, it moves a piston, which is connected to the rod or
carriage that moves the load. Performance varies depending on the materials used.
Commonly used motors are steppers and servos. Step
motors are an economical choice for accurate positioning
at lower speeds. However, steppers can lose synchronization with the controller when employed open loop without
an encoder, or if they are undersized for the application
at hand. Servos by definition are closed loop and provide
superior performance at high speeds, albeit at a higher
Speed and thrust are related in the physics of an electric
actuator. Speed is surrendered for thrust and thrust is surrendered for speed. This is an important distinction from
pneumatic actuators. For a given electric actuator, more
thrust will be available at low speeds and less thrust at
high speeds. This characteristic is more pronounced with
step motors and less with servos. For this reason, accurate
sizing in an application is critical. Increasing thrust at
the same speed requires different designs using different
components and materials. An increase in thrust and speed
requires larger and more powerful components and materials, which increase costs. Understanding and evaluating
loading in the application under real conditions ensures
specification of the right actuator while minimizing
expense. Not understanding application loading leaves the
engineer vulnerable to poor performance and high costs.
Component Costs Vs. Operating Costs
Components of an electric actuator include the mechanical actuator that translates motor rotation to linear speed
and thrust, the motor, an electronic driver or amplifier to
power the motor and a controller to control motion.
Operating costs of electric actuators are largely due to
motor power draw. Controllers’ and drivers’ low voltage
circuitry consume far less power. While component costs
of electric actuators are high, operating costs are low.
Those high component costs often deter the use of electric
actuators, but the savings that can be realized in operating
costs compared to pneumatics are not adequately considered and in some instances are ignored.
For example, manual changeovers (adapting a production line to a different product) can be expensive in terms
of both lost production and man-hours required to implement the change. Over the course of a year:
• If changeovers are required once a week and each
changeover requires two people for four hours at $50
per hour, man-hour costs amount to $20,800 per year.
• If products are produced at the rate of one per minute
and the value of each product is $10, lost production
costs amount to $124,800.
• The total annual cost of changeovers amounts to
Electric actuators can substantially reduce changeover
costs. The annual cost savings must be considered as part
of the implementation decision.
When replacement cost, operating cost and process
efficiency are considered, electric actuators’ annual costs
are comparable to pneumatics’ costs. It helps when the
scale of deployment is moderate and when motion system
components can be replaced separately as they wear rather
than replacing an entire integrated actuator motion control
The initial cost of actuators is only one small consideration in a list of issues for intelligent and cost-efficient
automation implementation. The high cost of electric components can be misleading, but so can high compressor
operating costs. Neither tells the whole story. Pneumatic
operating costs can be controlled by sizing the compressor
to fit the scale of pneumatic device deployment. Electric
An electric actuator from Bimba Manufacturing.