Linear Electromagnetic actuators consist of a hollow coil (solenoid) and a ferrometal rod. The rod is mounted loose in the coil and can move up and down. When current flows through the coil, the rod is pulled to the center of the coil. If the direction of the current is then reversed the solenoid will pull in the ferrometal rod. Due to Lenz's Law which states "For a current induced in a conductor, the current is in such a direction that its own magnetic field opposes the change that produced it." This means that an EMF will flow through the solenoid of the actuator to oppose the change in magnetic flux thus an electromagnetic actuator can never fully extend the full length of the rod.
Note to further modifiers: Feel free to tidy my Lenz's Law explanation up - i'm sure it could be written in a more elegant manner! --SamEEE 03:18, 3 February 2007 (UTC)
This type of actuator is useful for momentary linear motion. e.g. closing a gripper.
Solenoids use a large amount of power this requires more battery power which in turn requires more battery power to move the robot. A general rule of thumb is to use a electromagnetic actuators for small operations and for larger operations prehaps consider the use of Pneumatics the output power/weight ratio is usually more favorable. For more information on Pnuematics please consult the corresponding section of this book.
Calculating how strong the actuator is, isn't very easy. However it is possible by using a Newtonmeter to the line of action and measuring how many Newtons are exerted by the solenoid on the measuring device. There are also a way of measuring force though use of a formula however the forementioned method is usually the most practical.