Linear Actuator Position Control Using Sensors
Linear Actuator Position Control Using Sensors

Linear Actuator Position Control Using Sensors

Positioning sensors are devices that create a useful measurement signal based on a monitored physical process. Most often, this process is the linear movement of the actuator. This means that the sensor monitors the position of the movable element - the rod or carriage. The linear movement sensor can continuously transmit information to the control system or operate in certain positions. Thus, the control over the actuator is ensured during its production operation processes and other tasks. The use of actuators with sensors makes it possible to achieve accurate and efficient actuator position control and fully reveal the advantages of the actuator as an electric linear motion automation device.

What is Needed to Control a Position?

To control the position of the linear actuator can be used any sensor that simplifies the control over linear actuator. The target actuator that must be controlled and antenna can be mounted in the stock product in various ways, depending on specific conditions. In particular, it is possible using adhesive compositions and nylon screws.

Such linear motion automation sensors are suitable for operation in adverse environmental conditions when optical sensors or sensors with electrical contacts are unreliable. The linear motion sensor encoders work flawlessly even after continuous operation. Non-contact photoelectric readout from the scale and ball bearing guides of the reading head guarantee the long-lasting life cycle. Thanks to the housing, a special reading method, linear displacement sensors are especially well protected from contamination. Solid shielding helps to protect the signal from distortions. This is the most important aspect of the linear actuator with position control implementation. 

Types of the Linear Actuator Position Sensors

There are a few types of linear actuator position control units that are most frequently observed in the global industrial market. 

  • Limit switches, limit sensors - are adjustable mechanical sensors that open or close the electrical circuit in a specific position of the actuator. When triggered, a normally-open sensor closes the circuit, and current can flow through it, while a normally-closed sensor opens so that the current does not flow. These little assistants can be installed both on a moving part of the actuator and on rotating elements. Adjusting the limit switches allows limiting the actuator stroke length on both sides to the desired range.
  • Hall Effect Sensors - contactless semiconductor magnetic sensor that allows tracking the relative position of the actuator by measuring the magnitude of a magnetic field. The level of its output signal depends on the change in the magnetic field between the stationary sensor and the movable plate when they move relative to each other. With the help of a Hall sensor, position feedback can be realized and thereby provide flexibility for controlling the actuator.
  • Potentiometer - a three-terminal resistor, the resistance of which depends on the angular or linear shift between the movable and fixed elements. The fixed contact is fixed at one end of the resistive coil or plate, and the movable brush moves along it. A resistance voltage is applied to the potentiometer, and when its resistance changes, the current output, which is a measuring signal, changes. A potentiometer is an analog sensor, the resistance value of which is uniquely dependent on movement and unique in each position.
  • Reed switch (sealed contact) - contactless electromechanical magnetic sensor. Inside the hermetically sealed glass envelope of the reed switch are reed contacts that close when the magnetic field is applied. Reed switch is simple, durable, noiseless, but fragile and has moderate speed.

How to Control Linear Actuator Position

Linear motion sensors have widespread use. They are intended for use on machine tools and installations with adjustable feed axes, such as, for example, milling, turning, and grinding machines, machining centers, and horizontal boring machines. Good dynamic properties of linear motion sensors, their high speeds of movement and acceleration make it possible to use them with the ordinary highly dynamic axis, and with linear motion actuators.

Linear motion sensors detect the position of the linear axis without additional mechanical transmission elements. If the linear actuator positioning is made using a linear motion sensor then the control loop covers the mechanics of the actuator. This way the linear motion sensor can identify the mechanic’s transmission to the axis error and compensate in the control electronics. This method helps to exclude a number of sources errors like positioning errors caused by ball screw heating, errors caused by the presence of backlashes in the ball screw, kinematic errors caused by ball screw pitch errors. For machines with high demands on positioning accuracy and processing speed, the use of linear displacement sensors is the obligatory measure.

Mechanical Construction

Speaking of how to control the linear actuator position for projects where a smooth and reliable performance is needed. Closed sensors, scale, and reading elements of the sensors are protected from dust, filings, and water by an aluminum casing. An elastic seal covers the bottom case.

The reading element moves along the scale with minimal friction. A coupling mechanism connects the reading element to the reading housing heads, thus compensating misalignment between scale and caliper machine tools. Lateral and height offsets allowed between the read head and the scale from ± 0.2 to ± 0.3 mm, depending on the type of sensor used for the linear actuator position feedback improvement.

Linear actuator sensors with a small profile scale are oriented to be used with limited installation space. Long measurement lengths and increased acceleration loads are possible with mounting brackets and special fasteners.

Additional Ways to Control the Actuator Position

The linear motion sensors use the physical phenomenon of magnetostriction. The heart of the linear displacement sensor is a ferromagnetic measuring element that is called a waveguide. A movable positioning permanent magnet induces a magnetic field in the waveguide. A current pulse moving in the waveguide causes a second magnetic field radial to the waveguide to form a direction. At some point, two encountered magnetic fields cause the appearance of a torsion pulse. This pulse in the form of a material sound wave moves with constant ultrasonic speed from the measuring point to the end of the measuring element, where the receiving head of the sensor of the linear actuator with position indicator of the linear displacement sensor is located. Using this head, the received signal is converted into a proportional path.

The use of controlled electric actuators in various fields is justified by the broad range of benefits they give. This allows a more flexible, timely, and economical production process, therefore, the use of automated controlled electric linear actuators is currently prevailing and constantly expanding, which allows providing the best production quality indicators and to significantly reduce energy consumption and the implementation of other resources.

Sensor systems - are the sets of specialized devices, and tools necessary for programming a technological process, as well as for performing a technological process and individual technological operations. Using sensors the start-stop moments, the speed and amount of movement, the brake of the drive, the torque characteristics of the drive and various parameters that ensure the execution of the process are programmed.

Jake Hewer

AZ Engineer