Used in a wide range of automation equipment, including directional control valves and temperature control equipment, actuators are responsible for determining how and when a mechanism will function. All that an actuator needs to function is a control signal of some kind and an energy supply, typically an electric current. When the device in question receives its predetermined control signal, the actuator responds. It usually does this by process of changing the energy received into a mechanical motion, such as opening a valve. Where an electronic system is using an actuator, software can also be used to guide the function and this is especially useful for more complex operations like those used by robots or industrial machinery. An example of an actuator which is manufactured for use within industrial systems is a shock absorber. These are rated according to the maximum impact force they can handle as well as the energy consumption they require to run. A number of shock absorber accessories are also available, such as stopper nuts and foot brackets. Another type, solenoid actuators, work like any electromagnet to create a field when current is passed through it. They, therefore, allow electric systems to do physical work and are rated by their lifting force in Newtons. Holding solenoids and rotary solenoids are other versions which are also frequently used in industrial settings.