5 Applications for Micro Linear Actuators

5 Applications for Micro Linear Actuators

By: Lasse Jespersen

The Morai Motion Inline Nexus micro linear actuator is beguiling. I have shown it to a few technicians I know, and all responded as most people do when they see a new Apple product – they touch it, look at it from all sides, and wonder how it could be used in their lives.

It’s a far shot from the shoddy micro linear actuators I’ve used previously… This one has limit switches which prevent it from consuming power when it is fully protracted or retracted, it can be left unpowered without losing its position, and it is _much_ more powerful. You can always rely on the movement speed of a micro linear actuator to tell at a glance how much force it can provide. The shoddy one I used previously was very fast, but could not move any decent load – despite consuming 18 watts when unloaded. The micro linear actuator from Morai Motion reliably moves at 0.51 inches per second (or 1.3 cm per second in metric). I would estimate that it exceeds 4.4 lbs of force, and consider that a safe value for any calculations that go into planning a project.

Previously we looked at how to make a universal H-bridge to drive many micro linear actuators in parallel – but for the applications below, it is sufficient to use one of the H-bridges from our shop.

The Morai Motion micro linear actuator does not feature any way of reporting back to its controller the _position_ of its shaft, whether fully retracted, partially out, or fully protracted. You can use an awful hack to ‘know’ its position, by keeping track of the time spent moving it in either direction. A 6 inch MLA which moves at 0.51 in/s will be extended to 1/4 of length after 6/0.51/4*1000 = 2941.17647 milliseconds. Or 2941176.47059 microseconds to be more precise, but not as precise as I would like it. In any case, keeping track of the micro linear actuator by keeping track of time is hell, and the longer the the MCU has been running, driving the micro linear actuators, the larger the skew of the ‘best estimate’ will be.

It is not needed, because everyone interested in using micro linear actuators will find it doable to mount their own magnets and Hall effect sensors, read a bit about the subject, and proceed to program an Arduino and be quite happy with the result. A Hall effect sensor is a clever little sensor which reacts to the presence of magnetic fields. When one is present it will pull either LOW or HIGH depending on whether you use a pull-up or a pull-down resistor. The sensor pins are numbered from left to right, with the flat bit facing toward you. The pinout itself may differ, so refer to the datasheet.

An example of wiring it up follows:

– Wire sensor pin 1 to VCC/5V of your Arduino.
– Wire sensor pin 2 to GND.
– Wire sensor pin 3 to D2.
– Wire a 4.7 kilo-ohm pull-down resistor between VCC/5V and sensor pin
3. It will pull it HIGH when a magnetic field is nearby.

Put this in your setup() function:

— snip —
Serial.begin( 9600 ) ;
pinMode( 2, INPUT ) ;
— snip —

And put this in your loop() function:

— snip —
byte reading = digitalRead( 2 ) ; // either LOW (0) or HIGH (1)
if( reading == HIGH )
{
Serial.println( F( “loop(): at position marker 1 ) ) ;
// Do whatever is needed.
}
delay( 1000 ) ; // Don’t flood the serial monitor
— snip —

You can now detect the various positions of the shaft, for example to know if the landing gear for a drone has been fully retracted or fully protracted. Reed switches function in a similar way, but for most applications you will do fine with a Hall effect sensor.

Applications for Micro Linear Actuators

1. SOLAR TRACKER

5 APPLICATIONS FOR MICROLINEAR ACTUATORS

A solar panel is not effective if it gets no sunlight – in fact it consumes current rather than producing it. A low-power method for tracking the sun (which does not rely on ugh… timekeeping, and the sun’s known course in the sky) involves using 4 photo-resistors – LDRs – to make an “eye”, and two micro linear actuators to control vertical and horizontal movement of the panel. The resistance of an LDR to current drops as its exposure to light increases. Each LDR should be wired in series with at least a 1 kilo-ohm resistor.

The eye itself is made by making a “+”-shaped object from plywood, plastic or aluminium sheet where the lines each are constructed to be a few inches high – little walls separating the LDRs, which occupy the corner positions. The “+” shape guarantees that you will see the lowest resistance on all 4 LDRs _only_ when the eye is toward the sun. By mounting the eye perpendicularly to the solar panels themselves, the reading it gives you will ensure that whatever it “sees”, the panels also see.

Each LDR can be read with a simple:

— snip —
int reading1 = analogRead( ldrPin1 )
— snip —

in an appropriate function, and the values compared to each other, to determine the correct way to swivel and tilt the panel.

In addition, use bypass diodes (Google “solar panel bypass diodes”) on each panel to prevent any shady areas of your solar panels from devouring precious current.

The Arduino can be put into sleep mode, and only wake up occasionally to scan for a better angle. The “Narcoleptic” library for Arduino is perfect for this.

You can get it here: https://github.com/brabl2/narcoleptic .

Now for the motors – two are preferred. Each will control its own axis, so the panel can tilt up/down and move left/right. Micromechtronic sells a 6 inch micro linear actuator which if placed properly can handle all required movement for tracking the sun across the sky. It is best to power the entire circuit from a 12V supply that is NOT connected to the solar panel itself, since the charge voltage for a car battery is ~14.4V, a bit above the rated voltage for Morai Motion micro linear actuators. It is also initially a bad idea to run from a car battery, since 1) it is bad for it to be placed in the sun, and 2) we must not over-discharge it, i.e. cause it to drop below a charged voltage of 10.4V. That’s the no-no voltage limit for car batteries, and now you know.

The construction of the tracker itself, and the mounting details of the micro linear actuators, H-bridges, panels, “eye”, etc., are outside the scope of this overview. But rejoice, and hallelujah: I have ordered a bunch of small solar panels and will be building a solar tracker when they arrive from the Orient.

There is no better time for building such a device than darkest winter, in anticipation of the first tender days of spring.

2. ATTIC VENTILATION

Many attics contain a passive or active ventilation system. If the house does not have a controlbox to adjust the amount of air the pipes should pass into each individual room – based on temperature for example – it can be bothersome to get up into the attic just to adjust the damper. A single Arduino can be programmed fairly simply to respond to different temperature thresholds by opening/closing a damper.

The ideal motor for this application is not a servo, but a micro linear actuator. A DHT11 sensor is reliable once calibrated, and will run off 5 volts from the Arduino, while the Arduino and a single micro linear actuator will run fine off a single 12V power supply. To drive the micro linear actuator you will need a single H-bridge module.

3. HYDROPONIC GARDEN

5 APPLICATIONS FOR MICROLINEAR ACTUATORS

If you or your better half would just love to grow tropical fruit in the basement or the guest bedroom, a hydroponic garden is no longer out of reach.

While some old-timers rely on the sun to heat a greenhouse of GLASS, and control humidity and temperature in a sloppy manner with pneumatic oil-piston tomfoolery, you can have a true hydroponic garden, automated through and through, complete with low wattage LED growlights, soil moisture sensors to let your Arduino know when it’s time to turn on the watering pumps, and temperature/humidity sensors which help the Arduino know when to trigger the opening/closing of windows, 12V computer fan arrays to bring in fresh air and push old air out… It really would not be too expensive, in parts or electricity, once built and calibrated to your needs.

The LEDs themselves must be a good mix of warm white (40%), 630 and 630nm red (10%), 430 and 450nm blue (20%) , and green (30%). Green light propagates deeper into plants than red, and blue is used to kickstart photosynthesis. A 30-40W array of LEDs hand-built is better than anything I have seen on the market. I made a small 6.6W LED array with 1 x 450nm blue LED, 1 x 630nm red and 2 x 660nm red and achieved photosynthesis in a terrarium. This is very doable.

The airflow itself from IN to OUT should cross the area where you keep your plants. A good airflow is essential to prevent mildew in your pots at higher temperatures and humidity. There is no better way to open/close windows the right amount than one or two 6 inch micro linear actuators depending on the force required – remember you have 4.4 lbs available per micro linear actuator.

With regard to generating cheap heat for this hydroponic garden, I cannot recommend that you get a dozen lasers for DVD/CD burners, mount them on heatsinks, and pump 400mW through each of them after having focused their beams into small drilled holes in large copper heatsinks… Do NOT do that. And if you do, be extremely careful not to expose your eyes – EVEN INDIRECTLY – to the dangerous light of the laser diodes. Just don’t do this… At least not unless you have lots of experience working with this type of dangerous light. If you do it, it’s certainly something you do at your own risk of serious injury.

All this hydroponic gardening is particularly interesting for all those preppers that have proliferated in later years. Nothing makes an old mineshaft or a bunker quite as cozy and homely as a hydroponic garden full of strawberries. And it is very possible to scale it up.

Who knows? These days, anything goes. And if you were to spend the next half century in a bunker, would you not like to eat strawberries every once in a while? ;)

4. TABLE LIFTER

Elevating or lowering a table sounds like a silly idea – but it is not bad in practice.

Workstations (i.e. desks) with several users of different stature will benefit greatly from being adjustable. A very user-friendly way of crafting the control mechanism involves two simple pushbuttons, each with a 4.7 kilo-ohm pull-down resistor, an Arduino, the universal H-bridge from a previous article here on Micromechtronic blog – a note on wiring the H-bridge onto perfboard: Make sure that all the transistors and MOSFETs are aligned in a row, with their flanges all facing outward from the perfboard. This ensures you can attach even a very large heatsink if it is required.

By raising the tabletop as long the up-button returns HIGH (remains pressed), and lowering the table as long as the lower-button returns HIGH you achieve the desired result easily. Include a check to see if both the up-button and lower-button return HIGH simultaneously, and if that is the case, do NOTHING. Only lower or raise the tabletop if one is HIGH and the other is LOW.

There is no problem with over-protracting/retracting the micro linear actuators involved, since they have limit switches, and will not respond when they should not respond.

Here we assume a desk which must elevate/lower synchronously across all micro linear actuators. The tabletop should be weighed, the weight multiplied by 2, and then divided by 4.4, and the result rounded up. My desk’s tabletop weighs ~16 lbs, so I would need 16*2/4.4 = 7.3 -> 8 micro linear actuators. That gives me a decent ( 8 * 4.4 ) – 16 = 19.2 lbs of lifting force, which is plenty for my laptop and a few other items I keep within reach. I have 4 mount-points, so in this example, I should then mount 2 micro linear actuators on each leg of the table. I would then place the pushbuttons discreetly on the right side, on the table’s frame, beneath the tabletop.

Each micro linear actuator consumes 2.76W, so when driving 8 in parallel we get 2.76 * 8 = 22. 22 watts means we should use a heatsink. Only the IRF630 and IRF9630 power MOSFETs require heatsinking here, but you may as well do it on the TIP120 NPNs as well. This is easy, don’t worry – attach thermal silicone pads to the flanges of the MOSFETs and transistors on the universal H-bridge, and mount a medium-sized _copper_ heatsink onto the silicone pads. The flanges will only get hot when the micro linear actuators are active, so there is no need to worry about dissipating the heat from the heatsink itself.

If the flanges and heatsink are all electrically connected when the H-bridge is driving your micro linear actuators, it will all short out – but now you know how to prevent that.

5. DRONE LANDING GEAR

5 APPLICATIONS FOR MICRO LINEAR ACTUATORS

For a medium to large hobbyist drone you will benefit in several ways from having landing gear. Your takeoffs and landings will be made with a reduced risk of things on the ground getting into your rotors and causing damage. It will look cool when it goes up and down. It is fairly straightforward, although you will need a plastic- or aluminium harness for the micro linear actuators to be mounted on.

There are innumerable designs available via a Google search, but most which ‘go up and down’ are basically 3 or 4 micro linear actuators wired in parallel to an H-bridge. Sometimes the designs include one or more Hall effect sensors to report if the landing gear is up or down. Some I have seen have even automatically pushed down the landing
gear based on a reading from ultrasonic distance sensors. Very nice and automagic.

Building landing gear is one of the easiest mods you can upgrade your drone with, and it only adds a little weight. You can get by with a fairly small stroke length (2-3 inches) if you position the micro linear actuators just right.

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