End stops & Limit switches

The most common way to avoid running the actuators to the end of the stroke is to integrate limit switches.  A limit switch is a circuit that stops the motor at the desired or programmed stroke position and only allows it to start in the opposing direction. This means the actuator will avoid 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, some 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 as well as stopping the motion with an integrated mechanical end-stop buffer.

Position feedback sensors and systems

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.

 The position sensors can be divided into “incremental” or “absolute”. The main difference is that the incremental version 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. 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.

Based on this it may seem like an easy pick which position feedback system to choose. But with all things considered, such as precision, reliability and cost, the choice may not be that obvious.

Here follows a short compilation of various solutions:

Feedback type

Comment

Hall sensors (Incremental)

 

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.

Optic encoders (Incremental)

 

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.

Softpot/linear potentiometer (Absolute)

 

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.

Multi-turn potentiometer (Absolute)

 

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.

DigPot ( Absolute to some degree)

 

The DigPot is a position control system with position feedback, power control and programmable electrical end stops, start/stop ramps etc. It’s fully integrated in the actuator. The DigPot allows accurate and advanced control of multi-axis systems. It has a bus communication and all actuators can share the same 3-wire bus. The position sensor is “solid state” and very reliable and it can accept one half turn of the screw in the power-down state without losing position. This, in combination with the location (on the actuator) means that wiring to the actuator can be disconnected, even during full speed, and position will not be lost. This is not possible with an incremental type of feedback.  Therefore it can’t be said to be neither incremental nor absolute – but rather something in between.