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Low-Cost DIY Linear Actuator

UPDATE Oct 2011 - The DIY Linear Actuator plans are now also available as part of the Full Download Plans Set available from the plans order page. Previously they were only available bundled with motion software orders.

(For some movie clips and pictures of the actuators fitted to a motion platform check the Platform 2 pages here. NOTE - v2 of the actuator plans now released.)

Linear (push/pull) actuators are a common means of drive for commercial motion platforms. Their use in DIY systems seems less frequent, although there are a few examples around which utilize trapezoidal type lead screws as their primary drive transmission elements. The preferred drive method for electrical actuators is by ball screw which offers better speed performance and overall efficiency but have the unfortunate characteristic (at least for most DIY'ers) of being horribly expensive.

Lead/ball screw drives are not the only means available for linear actuation however, industrial equipment routinely uses both roller chain and toothed belt type transmissions for generating linear movement and packaged actuators are available that use these methods. These don't match ball screw performance in some applications but for those of us interested in a low-cost alternative which may be useful in DIY motion platform design they are worth looking at.

I've had a go at designing an electrically powered linear actuator which uses a standard roller chain drive. Looking at the design it would be possible to do a ball-screw version but this would at least double the cost of the parts - including the drive motor, the cost of parts for my roller chain prototype was about UK150. It has a 24V 200W drive motor, stroke length of 425mm, max speed of 390-450 mm/s (load dependent) and a theoretical static rod thrust of 50 kgf at the motor's rated torque output.

Needless to say the cost to build will vary depending on where you get parts and the performance and strength of the actuator is materials and build quality dependent - so this one is for competent DIY'ers with good build skills. The design is such that highly specialised tools are not needed but there are holes in both wooden and metal parts that need to be drilled accurately and a good bench drill (drill press) is really needed for this. The builder will also have to be able to follow a few important build procedures to ensure the actuator is assembled and set up properly. Note also that the actual final actuator performance is heavily dependent on what the control system driving it is telling it to do.

The Design

As with most linear actuators there are two distinct functional elements - one is the provision for guidance/constraint of the moving rod and the other is the means of motion generation or actuation. The guidance method I used is a standard approach in linear motion systems design ie linear guide rods with a ball bushing mounted moving block. The thrust rod is fixed to the moving block and carried on a further ball bushing where is passes through the fixed "neck" block. The rods are all 16 steel which gives the guidance element good rigidity. Ball bushings are preferred to plain sliding bushes to minimise the friction levels and provide a free-stroking slide system.

The motion generation is by closed-loop roller chain attached to the moving block and driven by sprocket attached to the drive motor output. The idler sprocket shaft also provides a convenient position for mounting your position or speed feedback device, I used a 5-turn potentiometer for position feedback - approx 4.5 turns are used over the full stroke. I have used 3/8" pitch simplex roller chain which is widely available and fairly inexpensive. Both the drive and idler sprocket shafts are deep-groove ball bearing mounted. The drive motor has a through-shaft worm gearhead and is mounted on the drive sprocket shaft and secured to the actuator wooden casing. It's current draw at its 9.35 Nm rated output torque is about 12 Amps.

Actuator mounting is by spherical bearing rod ends on the thrust and tail rods. There is plenty of scope for fitting limit switches on the casing which are triggered by the moving  block - important for implementing drive cut-off when the actuator approaches its end-stops - see below! Driving onto the end stops at full power is likely to damage the device - it is a DIY design.

Performance

You can get an idea of this from the two movie clips below. On the prototype build the sliding guides work well and run freely. The chain drive can also be set up so that there is very little backlash and movement reversals are smooth. There may be a small amount of backlash arising from the low-cost drive motor's worm gearing but this doesn't appear to be a problem. You can always amend the design to fit a higher quality motor if you want - you get what you pay for with these.

Clip 1 (unloaded), Clip 2 (load "testing" - don't do this at home!)

(Feb '08 - more movie clips of three actuators running together can be seen on the Platform 2 page here - this shows the actuators driven by the single chip SPU with the PID servo controller in a 3-point platform setup.)

I was slightly concerned about the possibility of vibration coming through the drive rod from the chain transmission but this doesn't appear to be very noticeable and would tend to be present only at higher loads. The noise level is mainly motor dependent - as is the norm with roller chain drives the chain itself produces little noise. I made the mistake of driving the actuator onto its end stops without functional limit switches fitted and shattered the motor/gearbox shaft coupling (see right). I had to replace this with a home-made one and that's the main source of the noise on the prototype. Moral of the story - don't power up the actuator until your limit switches are fitted, wired and functional!

With the build of the second motion platform I've been able to see the actuators run under load and the performance is not bad. They are capable of driving the balanced platform motion without significant motor heating which suggests that the motors are not overloaded - this is platform mass related off course. I've modified the design (see V2 of the plans) to add a chain tensioning facility which allows any chain slack to be adjusted out.  Backlash in the low-cost motor worm gearing is noticeable during frequent heave reversals - higher quality gearing would be better. However the effects could be reduced by setting the platform up so that a net downward load on the actuators is present - ie don't balance the weight perfectly.

I've also made up a preliminary design for a modification that would allow the Unite MY1018Z motor to be used as an alternative. This is not worm geared but is more readily available in the US than the MY7712NZ used in the original design. I've not tested this modification - so use with caution. The mod is shown in this .pdf sheet -

ACT1-S004-1 - Preliminary MY1018Z Modification - Provided for Information "AS-IS"

Plans

I've been able to make a detailed set of plans available for the prototype design for information purposes. These are bundled free with the motion drive software and are available to customers who buy an unlock key for the BFF Motion Drive software. AND they are included in the Full Download Plans Set available from the plans order page.

They are in the form of protected pdf engineering drawing sheets and include a detailed materials and components list. Note the plans are provided "AS-IS" and for information only.

You can view the Overview drawing of the actuator here - the password is: actuator

 

 

 

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