LIMIT SWITCHES: Limit switches allowing of Actuator’s movement within a limited range defined by the stroke. A limit switch is a circuit, that depend of it’s type power or signal, can work in two ways. Power limit switches cut off the current to the motor at the desired stroke position and only allows it to start in the opposing direction. Signal limit switches gives an information to the power/control system that stroke positions have been achieved. This system is directly responsible to cut off the power to the Actuator.
END STOPS ON LIMIT SWITCHES OR OVERCURRENT: When the actuator stop on ends position using limit switches avoiding the mechanical stress that would occur if it ran into a physical or mechanical stop, e.g. if the actuator collides with other mechanical parts in the application. Although many low-cost actuators on the market are not designed to handle this stress. Actuators such as REAC’s RE25 and RE35 are designed to cope with these mechanical collisions throughout their entire life span. This is accomplished by monitoring the increased current by controller as well as stopping the motion with an integrated mechanical end-stop buffer.
POSITION FEEDBACK SENSORS AND SYSTEMS: Feedback gives valid information about system’s behavior which could be taken into account during it’s operation. The position feedback is best explained with a simple example: When a Power Wheelchair (PWC) seat lift system is raised above a certain height, the maximum top speed of the PWC must be reduced. This is achieved by a mechanically activated switch at a predetermined programmed height in the PWC control system. As complexity and requirements rise, there will be an increased need of knowing more precisely the position and other parameters. Then continuous feedback signal is required, based on sensors like Potentiometer or Hall effect types.
The position sensors can be divided into “incremental” or “absolute”. The main difference is that the incremental version (e.g. Hall sensors) gives information about relative movements when the system is powered up. Meaning that, when powered down, the control system must remember the position and the position must not move/change. Failure of either will result in loss of position. The absolute version, on the other hand, will not lose track of position if moved during power down, and the control system will therefore not have to remember position at power loss.
With all things considered, such as precision, reliability and cost, the choice may seem like an easy pick which position feedback system to choose.
Here follows a short compilation of various solutions:
Feedback type: Hall sensors (Incremental)
Comment: Generate a pulse train that is connected to some form of control system. Usually they are a low-cost solution integrated into the motor and generate one pulse each motor turn. The two-channel version can also detect rotational direction, and is recommended for position tracking (at least over the one channel version).
Good reliability.
Adds 3-4 extra wires between actuator and control system.
Feedback type: Optic encoders (Incremental)
Comment: The same pulse-train principal as for hall sensors, but can achieve very high resolution.
Good reliability.
Adds 3-5 extra wires between actuator and control system.
Relatively expensive solution.
Feeback type: Softpot/linear potentiometer (Absolute)
Comment: A linear potentiometer measures the actual position of the piston.
A low-cost absolute sensor usually suffers from poor accuracy.
Must be customized for different stroke lengths.
Adds 2-3 extra wires between actuator and control system.
Feedback type: Multi-turn potentiometer (Absolute)
Comment: A potentiometer connected through a gearbox to the screw, measuring screw rotation over many turns.
Absolute sensor with average resolution and accuracy.
Adds 2-3 extra wires between actuator and control system.
REAC designs and manufactures electrical linear actuators. Learn what linear actuators are and how linear actuators work, or the REAC product line
