Linear actuators are very useful power conversion devices. The market is growing rapidly.
Internally, a linear actuator consists of an electric motor with a reducing mechanism that consists of several gears and an endless screw. The screw pushes a piston or rod, which can be extended or retracted depending on the direction of rotation of the motor.
The main selling point of linear actuators is that they can exert large forces and large displacements. Depending on the model, they can exert from 20 to 150kgf, with displacements between 100 and 800mm.
One disadvantage of linear actuators is that they have slow travel speeds, typically of the order of 4-20 mm/s, so that the total extension time can be several minutes in the longest and slowest models.
There are linear actuators of different supply voltages. Most models have 12V or 24V supply voltages. Current constraints are generally high, in the order of 3A to 5A in the 12V actuators and 2A-3A in the 24V models.
There are three main types of actuators:
- Linear actuators without control.
- Linear actuators with two limit-switches, which allow to detect when the piston has reached a terminal position.
- Linear actuators with potentiometer, which provides an analog measurement of the piston position.
- Linear actuators are widely used in all types of industrial applications, due to the great ability to exert forces and good precision, which have allowed to replace hydraulic equipment, comparatively much more expensive and complex to maintain.
In our electronics and robotics projects we can use linear actuators when we need to lift large loads, for example, lift tables, unfold an awning or expand a robotic arm.
Factors to Look at When Choosing a Linear Actuator
Here are some of the most important factors to evaluate when choosing a linear actuator.
Linear actuators are expensive devices, typically between $22-$89. The price depends entirely on the length, maximum force, and speed.
Thus, we can find a linear actuator from 90Kgf to 100mm for $30, 100kgf to 300mm for $55, and from 150kgf to 150mm for $67. We got these estimates by looking at international eBay or AliExpress dealers.
First, we will see the connection of the actuator motor line, as this is common to all types of actuator. Later, we will see the position control of the actuator, which depends on the model of actuator that we have (it is even frequent that it does not have any).
LINEAR ACTUATOR POWER SUPPLY
All actuators will have two conductors to activate the electric motor that drives the worm. In order to extend and retract the plunger we need to be able to reverse the direction of rotation of the motor, i.e. reverse the direction in which the current passes through the motor.
As we know, we can use an H-bridge for this. However, the DC controllers we have seen (L298N and TB6612FNG) do not have enough power to control a large linear actuator, and in most cases we would burn the controller.
A common alternative is to use two stages of SPDT (single pole double throw) relays in a configuration similar to an H-bridge. The disadvantage of using conventional relays is that we will lose speed control, since mechanical relays have long switching times.
If we want to have speed control, we must use solid state relays, or build our own H-bridge using BJT transistors or MOSFET transistors.
LINEAR ACTUATOR CONTROL
As for the control, if we had digital pushbuttons to simply connect them to any two digital pins as we saw in the input reading a pushbutton with Arduino.
If the linear actuator has a potentiometer to determine the position of the plunger, we would connect the output to an analog pin as we saw in the input reading of a potentiometer with Arduino.
EXAMPLE OF CONNECTION OF A LINEAR ACTUATOR
The following diagram shows the connection of a linear actuator with potentiometer for controlling the angle position. If your linear actuator does not have a potentiometer, simply skip that part and connect only the connector.
With the relays we set each of the phases of the linear actuator to Gnd or Vcc. When the relays are off, both phases are in Gnd and the actuator is stopped. When activating one of the two relays, we put one of the phases in Vcc, keeping the other one in Gnd, so that it turns in one direction or the other.
The connection would be as follows. The potentiometer connection appears in dashed lines. Again, if your linear actuator does not have analog position control, skip this part.