|Concentric linear actuator without feedback, opened to show gears.|
The LD Series of linear actuators by Concentric International (formerly Iowa Export-Import) are 12V DC gearmotors that use a worm drive to move a shaft back and forth along its length. The worm drive ensures that the shaft will hold its position even when unpowered. Two limits switches safely stop the motor at either end of its range, while integrated diodes allow it to reverse direction after reaching a limit point if the supplied voltage is reversed. The actuators are mostly metal, and the entire case is sealed to protect against dust and water (rated IP63).
|Connecting the bracket to one end of a Concentric LD linear actuator.|
The Concentric LD series linear actuators are available in a variety of lengths and with optional potentiometers that are linked to the shaft position, for use in feedback systems. Several lengths are also available in two gear ratios: 5:1 and 20:1. The 5:1 versions have lower load ratings—34 lbs dynamic, 450 lbs static—but allow for higher speeds, up to 1.7 in/s. The 20:1 versions are slower—up to 0.5 in/s—but are rated for dynamic loads up to 110 lbs and static loads up to 500 lbs.
|20:1||20:1 w/feedback||5:1||5:1 w/feedback|
We also have generic linear actuators that can generally be used as less expensive alternatives to the ones from Concentric; the table on Pololu's linear actuator category page can help you find the actuator with the combination of speed and force that best meets your project’s requirements.
Mounting brackets are available for attaching the actuators to a structure; two are required for each actuator. Pololu's bracket for generic linear actuators also work with the LD actuators, although they are a little loose.
|Concentric linear actuator with feedback, 4" Stroke (LACT4P), shaft fully extended.|
Using the actuator
To test-drive the actuator, simply connect a power source of up to 12 V to the motor leads. Reversing the applied voltage will reverse the direction of motion. A motor controller or motor driver is required for electronic speed and direction control. Pololu recommend Pololu's Jrk Motor Controllers for use with the feedback actuators and the Pololu Simple Motor Controller 18v7 for controlling the actuators without feedback, though many of Pololu's other motor controllers and motor drivers are capable of powering this actuator.
These actuators have a stall current of 10 A at 12 V, but they will, on average, draw far less than this when used within their load ratings. They draw around 500 mA with no load and can exceed 3 A at their maximum rated dynamic load, so they have found they generally work well with Pololu's lower-power jrk 21v3 motor controller with feedback (see the bottom of this page for more information using this controller for closed-loop linear actuator position control). Note, however, that the actuators can briefly draw close to their full stall current when abruptly started or on a sudden change of direction. Such current spikes can be dampened if you take steps to limit the acceleration of the actuator (many of Pololu's motor controllers offer optional acceleration limiting).
Diagrams of the linear actuators are shown below. The versions that include potentiometers have a larger gearbox, so their overall size and weight is larger. For more detailed information, including the retracted and extended lengths of each version, see the appropriate datasheet (click the Resources tab above).
|Dimensional drawing for the Concentric LD linear actuators with feedback.|
|Dimensional drawing for the Concentric LD linear actuators without feedback.|
Each actuator has a 3-foot-long cable. On actuators with feedback, this cable is terminated with special female connectors as shown in the left picture below, one for the three potentiometer leads and another for the two power leads. These connectors match male versions on the extension cable for LD linear actuators. Actuator versions without feedback have two unterminated, stripped power leads as shown in the right picture below. Linear actuators without feedback do not have cables that are compatible with the linear actuator extension cable.
Using a jrk motor controller with a linear actuator with feedback
The feedback feature included with Pololu's jrk motor controllers make them a great solution for precisely controlling Pololu's linear actuators with feedback. Pololu's settings file for the jrk configuration utility makes setup easy, eliminating the need to calibrate the feedback and tune the PID constants. To get started, follow the steps below:
|Connecting a linear actuator with feedback to a jrk 21v3 motor controller.|
- If you have not already, read through the Jrk USB Motor Controllers User’s Guide and download its drivers and configuration software.
- Connect your jrk to a PC with a USB cable and launch the configuration utility. The red LED should be on, and the green LED should be flickering quickly.
- Download the jrk 21v3 settings file for use with LACTxP (1k txt). The settings in this file work with any length actuator that has a feedback potentiometer (model LACTxP), and they were tested on the jrk 21v3, which is powerful enough to drive these actuators in typical applications. These settings should also mostly work for the higher-power jrk 12v12, though a few of them (e.g. current calibration) might need to be adjusted.
- In the configuration utility, choose File → Open settings file (Ctrl + O), and navigate to the location of the settings file you downloaded in step 3.
- Click on the PID tab of the configuration utility and verify that the proportional and derivative coefficients are not zero. If they are zero, the settings file was probably not loaded properly and you should try performing the previous step again.
- Click “Apply settings to device”.
- With your power supply off, connect your linear actuator to your jrk using the connections shown in the picture to the right. The picture shows a jrk 21v3, but the connections will be the same if you use a jrk 12v12.
- Turn on Power.
- On the Error tab, choose “Clear Errors”. The “No power” and “Feedback disconnect” errors will clear. The red LED will turn off, and the yellow LED will blink slowly.
- On the Input tab choose “Set Target” to move your actuator to the target position.
While this setting file gives precise control over most of an actuator’s range, you might find decreased performance very near the extremes due to the limit switches. If your project requires better control near the actuator’s limits, you might need to recalibrate the feedback settings for your particular actuator.
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