# Creating a fast, uniform, linear actuator

Most of the linear actuators I've seen are nonuniform and/or slow. Those using a cam or crankshaft-like mechanism (and nearly anything hydraulic/pneumatic) cannot be moved at a constant speed without some programming. Those using a screw-like mechanism are uniform, but slow.

Aside from a rack and pinion/rope wound around a stick, what other fast, uniform linear actuators exist? By uniform, I mean that the speed is uniform (Or the distance moved is linearly dependant on the angle rotated by the motor)

• Solenoid? If I have a pneumatic/hydraulic/solenoid actuator that I put a spring on (activation compresses the spring, deactivation lets the spring relax), and design the spring to change diameter as the length changes, wouldn't this accomplish your goal? It would allow for quick changes, and having the spring change diameter in relation to length should linearize your change in force (eliminating the shorter/greater resistance problem). Commented Oct 28, 2012 at 7:08
• @Everett: Hmm, interesting, though designing such a spring may not be that easy.. Commented Oct 28, 2012 at 8:34
• Could you define what you mean by fast? What is the application? I've seen fast linear actuators based on screw drives. Commented Oct 28, 2012 at 8:48
• @Jakob: Well, with a screw drive, generally high-rpm motors are required to get a decent speed--unlike a rack and pinion. Basically, it should be as fast as a rack and pinion using the same motor. I've not yet seen a screw-based linear actuator that fast, but you're right, they exist :). My bad... Commented Oct 28, 2012 at 11:04

It would be easier to give an answer if we knew what you want to use this for. That would give an idea of your requirements in terms of speed, cost, and space requirements.

As Rocketmagnet has pointed out, you can use a belt drive, or long pitch screw. And you have already mentioned a rack and pinion system. These are all involve a conventional (DC) motor along with a mechanical system to convert angular motion into linear motion:

• belt drive
• screw
• rack & pinion
• rigid chain actuator (this is also cool, it can be folded into a small space)

Other methods include:

• Linear motor - This is quite cool, it involves a line of magnetic poles, which accelerate the rotor in a linear direction. These can be very fast and precise! This can also be controlled in open-loop (like a stepper motor).

Above is a cross-section of a linear motor

• Pneumatic cylinder - the speed can be controlled using a control valve, pneumatic actuators are also quite popular for robotics applications because of its compliance.
• Hydraulic cylinder

Wikipedia has an article on linear actuators, and also a comparison table with advantages and disadvantages!

• I mainly wanted to use it for fast, accurate retraction of an arm. The linear motor is pretty interesting, I'll look into that. Thanks! Commented Oct 28, 2012 at 12:06
• Not all linear motors can be used open loop. In fact, I think that the type you have shown here requires not only closed loop control for position, but also a motor driver similar to a BLDC motor. Commented Oct 28, 2012 at 12:52
• A BLDC motor can be controlled in open loop like a stepper motor if you want. It just requires that you know its initial position, and that you leave each phase on long enough to be confident it has reached the next step. If you don't know the initial position, you can cycle through all phase states once. Commented Oct 28, 2012 at 13:02

I don't know if this qualifies as a rack & pinion, or rope wound round a stick, but a simple belt drive actuator is very fast and smooth. I'm using one for a SMD pick and place machine where I need very good speed and smoothness of movement.

Linear actuators based on screws don't need to be slow at all, it just depends on the pitch of the screw you use. Use a long pitch screw, and you get a lot more travel per revolution of the motor.