Linear actuators are undeniably the star of the modern-day motion control industry. These elegant devices have widespread applications in a range of industries such as aerospace and health. They are equally important for the domestic consumer, finding applications in everyday-use equipment, for example, lawn care.
Owing to their extensive utility, linear actuators boast a myriad of customization options. They come in various sizes, performance ratings, protection grades; you can essentially have anything you want when it comes to their capabilities.
A crucial decision to take when selecting a linear actuator for your application is the choice of installing a limit switch or an overcurrent protection device. We will be discussing what these two are, their merits and demerits, and why would you choose one over the other for your application.
Limit Switches & Overcurrent Protection: Why Do You Need Them?
Limit switches and overcurrent protection devices are both responsible for stopping the actuator when it needs to stop, at the stroke-end, and sometimes mid-stroke as well.
From an electrical point-of-view, they serve the same critical purpose of motor overcurrent protection. Having an overcurrent protection circuit in your linear actuator is absolutely necessary as overcurrent issues can lead to significant damage to electrical components like the motor and wiring.
Overcurrent is a direct outcome of an actuator getting more load than it can withstand. At the end of its stroke, the restriction provided by the frame’s fixed stroke length is the cause of this elevated load. For mid-stroke situations, any physical obstruction in the motion pathway can cause overcurrent. The graph below is a visual representation of how to load and current are related. It shows that current is directly proportional to the force on the linear actuator.
The aim is to stop the motor from drawing current for any load beyond the encircled point, as all load-current combinations above this point are not safe and are to be avoided at all costs.
What is a Limit Switch?
Limit switches are electronic devices that define the ends of a linear actuator’s stroke. Upon reaching either end of its stroke, the rod makes contact with the limit switch, which instantly turns off power to the motor, thus stopping the rod from traveling further and potentially damaging something.
Limit switches are usually adjustable over the stroke. This means that if you have a 1’ linear actuator but the desired stroke length is 9”, you can achieve it by moving one of your limit switches 3” towards the other.
There are three types of limit switch configurations you will come across in the market. Let’s see what each of them entails:
Configuration 1: No Feedback
A limit switch without feedback is a sole showrunner. It is capable of cutting off power to the motor directly, it does not need any other device to perform its function. Technically speaking, it is not part of a control system, just a component in the circuit.
This is the standard, default option in most linear actuators and it is the simplest one of all three.
Configuration 2: Independent Feedback
This option is the exact opposite of the first one mentioned above. The limit switch acts as a feedback device in the control system loop that administers the supply of power to the motor.
It differs from a ‘No Feedback’ limit switch in a way as it cannot cut off power to the motor by itself, the decision lies in the hands of the controller.
An advantage of this setup is that the feedback loop is independent of the motor wiring, giving the user control over its signal voltage. There is also a drawback, however. This configuration is not as reliable as the others since an additional control system is now part of the design, adding complexity and margin of error.
Configuration 3: In-Line Feedback
In-Line Feedback combines the functionality of the other two options. The limit switch acts as a feedback unit and the feedback signal itself is responsible for turning the motor off.
The linear actuator limit switch wiring is different here as the feedback signal is fed in the motor’s power supply line. This is quite a reliable setup as the overcurrent protection is feedback-based and the limit switch autonomously shuts off the motor.
One thing to consider while going for this option is that the feedback signal has to be the same voltage as the power supply as they share a line. Provision must be provided to ramp up the signal to the desired voltage.
What is Overcurrent Protection?
Linear actuator overcurrent protection is exactly as it sounds. It is a specialized device that monitors current levels in the line and turns off the motor if current levels are too high.
The best way to grasp this concept is to imagine a fuse, which is the simplest overcurrent protection device. It sits silently as the circuit works normally but jumps into action the moment current values reach unsafe levels.
Which is Best for My Application?
The choice of overcurrent protection differs across the board and depends entirely on the nature of the application. It is important to know that while having both of these options simultaneously seems beneficial, it is not what manufacturers usually offer. Most linear actuators allow only one of these, so you have to make up your mind on which option you need.
We have highlighted some of the main For and Against points for both of these systems below. You can then accurately decide which one suits your application better.
Pros & Cons of Limit Switches
Limit switches are a useful thing to have in your linear actuator. However, like everything, buying a linear actuator with a limit switch has its own merits and demerits.
The pros lie in the variety of options you have for a limit switch. Depending upon your specific application, you can opt for any of the three aforementioned configurations. Another positive aspect of this option is that limit switches are part of intelligent control systems, which means you can program it according to your needs.
The cons of having a limit switch is that they only provide stroke-end protection. If your actuator encounters a problem mid-stroke, a limit switch can do nothing about it, and this leads to problems. If you accidentally miscalculated your stroke length and set the limit switches accordingly, the actuator can potentially destroy itself. Similarly, any unwarranted obstruction can yield the same disaster.
Pros & Cons of Overcurrent Protection
The biggest advantage of having an overcurrent protection device in place is that it provides mid-stroke protection. Unlike limit switches, it is capable of shutting off the motor anywhere along its path. For example, if you are using a linear actuator for your garage door opener and something gets stuck in it while the door is closing, this device would prevent damage by turning the motor off. With a limit switch, the motor would keep working until either the door, the actuator, or the obstruction gives way.
A major disadvantage of a linear actuator overcurrent protection system is the inability to pair it with an intelligent feedback-based system. This makes your actuator a bit less accurate during overload conditions. The other drawback of overcurrent protection stems from the first one. These devices are designed only to trigger at maximum load, which means they only shut off the motor when current and load have maxed out. Consequently, it may wear out the mechanical end-stops as they are subjected to maximum force for each cycle, requiring some extra maintenance. However, our linear actuators have been rated to deal with repeated stress on the end stops with overcurrent protection.
What Does the Market Offer?
We hope that you are now much more enlightened about the topic and that our article helped you out in deciding what you need. Progressive Actuators offers an entire range of state-of-the-art linear actuators that can remarkably complement your products. We have the top of the line PA-13 that incorporates both limit switches and overcurrent protection, and actuators such as the PA-07 if you have settled on what you need.
If you feel you need some professional advice before buying one, feel free to contact us to learn more!