# What are the key properties to consider when choosing a DC electrical motor for linear movement?

I would like some help defining the specification of my motor, as I have little experience in this field. I've tried researching through Google, but the information is insufficient or unrelated.

The projects involves horizontal movement of a platform with a load of up to 10kg along at a speed of 10mm/sec. As you can see this is a large load and very low speed. The movement has to be pretty smooth. I'm unsure how to specify this when searching for motors (I think it's referred to as jitter). I have yet to decide on the sliding mechanism if it will be using wheels or just gliding on a cylindrical rail, but I'll be trying to minimise the coefficient of friction to reduce the motors force needed.

So I was wondering if anyone can help me identify the key properties of a motor so it provides very smooth movement? Or if people have experience designing supporting components to the system and why I should focus on that aspect instead of the motor.

You need more specifications to decide what you need. Power is force times speed, but force is mass times acceleration. You want to move at a constant speed, so there is no force. If you assume a frictionless surface, then any actuator could be used because the required power is zero.

You have to choose how quickly you want to get from zero to top speed to determine an acceleration, multiply that by your load inertia (if you're going through gears/belts/other reductions) to get a force, then multiply that force by your top speed to get a peak power. Add a margin for friction, but I think you'll find that actuators come in certain "bins" where one class of actuators will be insufficient or marginal and the next class up will be sufficient.

You don't say how far you need to move your platform, which would be another spec in finding a linear actuator or in designing an actuated platform.

I would suggest considering looking at an electric linear actuator and putting your platform on wheels or carriages. Electric linear actuators are relatively cheap, easy to power and control, and can have modest ranges of motion (up to a meter would probably be feasible). Beyond that, you may want to consider some kind of pulley or cart system with a motor and encoder, but again, you'll still need to perform the calculations above - peak force (or torque if you're going with pulleys), and peak power from the force (or torque) and linear (or angular) velocity.

Not sure where you are starting, this question may be deleted as too broad.

1) Start by understanding the physics involved - forces, energies etc. Look at hyperphysics.org.

2) Get some practical advice by looking to see how others do it in your area, on Google etc. Types of rails, friction, drive mechanisms etc.

3) Now you need to control the movement - from a switch to a highly complex arrangement. This depends on what you need to do , which you will have learned from the physics above.

4) Go to your friendly retail motor supplier, hardware, electronics store and speak to them - they are amazingly knowledgable, helpful and informed and will help you if you have done your homework.

SE is also a great place to bring questions you have researched thoroughly.

If you haven't researched, you WILL get downvoted and perhaps even banned.

Research Papers and Journals? How many Millennia do you have?

• Hi yeah, the question is broad because I am inexperienced in this area. I study Mechanical Engineering, and do understand the underlying physic. I'm working on a project for the University and just wondered if people who have more experience with motors could pitch in. Thanks for the information anyway! – Kiran Jan 28 '16 at 12:52
• I've tried, everything I find is either hobby robotics or EV related. which don't fit the scope of my project. – Kiran Jan 28 '16 at 12:58
• try adding 'industrial', 'conveyor' 'timing belt', belt, rack etc to your search. Go see a local bearing supplier and suck their brains. – ChrisR Jan 28 '16 at 13:12

If you use a worm gear arrangement, you could use almost any gear that was fast enough, given the constraints of your speed vs. torque.

We don't know the torque requirements, of course, but a reducing gear could make many worm gear or pulley arrangement work.

In terms of jitter, a worm or rack and pinion arrangement can be very stable.

I would look into a threaded rod with a nut fixed to the platform: that way, you can have a motor that spins very fast and gives you smooth, linear movement.

You'll often have problems getting a motor to give you very smooth movement since most controllers give you a jerking motion at lower speeds. If you add in any sort of gearing (such as an ACME rod) so that the motor can spin at higher speeds with less resistance, you will get better results. It won't help you with jerking, but you can ensure a consistent speed using an encoder (rotary on your motor/transmission or linear on your platform) to give your controller feedback, so that it can adjust its output based on if it's moving too fast or too slow.

In summary: you'll never get smooth movement unless you use a fast motor geared down to be slow, and the only real way to guarantee a consistent speed is to measure it.