Having something attached to a linear actuator in order to prevent it from moving more than you want or is required is something which can come in extremely handy when the task at hand requires more control. While some electric actuators are built with a programmable limit switch, most are not; these models come either with nothing limiting movement at all (therefore with no way for users to control the distance the actuator travels), or a limit placed at the extreme ends of the shaft only (to control the movement to some extent).
The advantage to using these actuators is that they are both cheap and easy to use, but it is still best to use them in tasks which do not require any overt control or finesse.
The linear actuators which come with limit switches are much more complicated to use, but the control which can be exerted over the movement grossly enhances the number of more delicate tasks which the actuator can be used in. The ability to program the switch means that the distance traveled can be adjusted according to the changing needs of the task.
While limit switches which are programmable are necessarily internal, it is also possible to install an external limit switch. This switch is non-adjustable, and so the abilities of it are limited, but it is a valuable addition to actuators which are otherwise uncontrollable.
A limit switch can control three things: the extension of an actuator, the retraction of an actuator, or both.
Types of Built-in Limit Switches
There are two main types of limit switches, whether they come already installed in the actuator, or as a customisation option:
- Non-adjustable switches – the shaft of the linear actuator stops at the end of each stroke, both extension and retraction, and the motor shuts off.
- Adjustable switches – the shaft of the linear actuator stop a pre-determined distance from either end of the stroke, and the motor shuts off. This can be done in two different ways – either by programming the actuator to do this, or by using a specific setting on an externally applied limit switch.
While people will notice that both types of switches turn the motor they are attached to off, remember that if a motor is left running – particularly against resistance – it will burn out much faster. Turning it off conserves power.
Cases in Which You May Need to Limit Travel on Linear Actuators
There are more reasons to limit the distance a linear actuator can travel than can be properly listed here, but some of the more common ones can be found in the list below (these are generally actuators which are non-adjustable):
- Opening or closing the boot of a car – the boot of a car is something which has very definite limits on either side – on the one hand, people need to be able to close their boots without denting the metal of the car exterior, and on the other, the boot door can only go so far without succumbing to the effects of gravity;
- Lifting a shelf in a confined space – this can be a shelf, or a drawer, or something else. In a confined space, opening more than the set distance would damage both the shelf and the surrounding area, possibly permanently. It makes sense to have the distance limited.
Some actuators can have their limit switches adjusted, so that the distances they can travel can change. Again, there are too many to adequately list, but some are found below:
- Extendable steps – these can be adjusted so that they can be used in a variety of situations, rather than there needing to be a separate set of steps for every occasion;
- Height-adjustable desks – particularly with the rise in popularity of standing desks, height-adjustable desks are becoming more used. Having it permanently set to only one or two heights would limit its effectiveness, so being able to adjust the distance traveled by the actuator would become necessary.
- On a hard stop – when there is a hard stop involved, the need for a limit switch is vital. A hard stop is identified as an immovable object or other force which will cause the actuator to jam and freeze up, by overshooting its own nuts and screw which hold it in place. Having a limit switch will keep this from happening.
- Fertiliser spreader – a machine to help spread the fields with fertiliser would need to have firm limits on how far the fertiliser could go. Not only will limiting travel help keep the fertiliser away from places where it shouldn’t go (therefore eliminating waste), it will also make sure that the majority of it goes where it should.
Ways to Control the Length of the Stroke on a Non-Adjustable Limit Switch
- Use an External Switch
- Use a dual-channel relay with a micro-controller
- Use an H-bridge with a micro-controller
There are other ways to control a linear actuator, but these are the main three ways which are normally used. The next three sections take each method separately:
How to Use an External Limit Switch with a Linear Actuator
An external switch is a device which can both break and recreate the electrical circuit which allows the linear actuator to move. Most external switches have a visible lever or button to press when it needs to be activated. It can either be activated by a human operator, or it can be moved as a consequence of the movement of the actuator itself, through an attachment which has been added to the actuator itself, or by a series of magnets which allow the external switch to affect the actuator without actually coming into contact with it. To use an actuator, either set the one which is already installed or install one yourself: decide what limit you want to place on the distance traveled by the actuator and set the limit switch to cut the motor at that point.
These switches are best used on linear actuators which only require unidirectional travel.
+ As a pro, external limit switches are cheap and easy to use.
– As a con, they only work in one direction – to have a linear actuator which is stopped in both directions, you will need to install two separate switches.
How to Use a Dual-Channel Relay and a Micro-Controller with a Linear Actuator
A micro-controller is a controller which includes a micro-processor, thus allowing for electrical control, and more finesse when it comes to controlling the equipment.
Dual-channel relay functions by sending a signal to the linear actuator to start or stop its process. To use it, the channel relay needed to be installed, and the parameters for how far the actuator should travel should be set. The micro-controller is used as a means of containing the programs which let the relay start and stop the actuator.
Using a dual-channel relay together with a micro-controller, there are more options for stopping and controlling a linear actuator. The use of both these components gives more control over both movements and allows the user to stop the actuator at certain distances.
+ Actuator can be stopped either way;
+ Timing of the stop can be controlled by the micro-processor, allowing for a greater degree of control than would otherwise be had.
– Dual-channel relay\micro-controller costs more than other methods, it requires more in-depth knowledge to operate.
– Another negative is that it does require two separate channels in order to work effectively – one to extend, one to retract the shaft.
How to Use an H-Bridge and a Micro-Controller with a Linear Actuator
How to use a micro-controller has been discussed above. An H-bridge is an electronic circuit which can be applied either way. To use it, apply the H-bridge to the actuator (by whatever means), and set the distances you want to traverse. Remember that an H-bridge allows a DC motor to move either way, so you can set distances on either side of your equipment. An H-bridge with a micro-processor is the same as a dual-channel relay and a micro-processor, except for the additional stability and functions it provides.
+ Pros to using an H-bridge and a micro-controller include an instantaneous response from the equipment, the equipment having no moving parts, and giving the current two paths to move through instead of simply having one.
– Cons include the fact that, while there are two paths, only one of these can be used at a time.