Choosing a Motor Type for Your Motion Control Rig

One of the more daunting tasks for your first motion control project is to decide on which kind of motor you need.  The right choice in motor can save you money and effort in the long run, not to mention increase your chances of getting the shots you're looking for.  Before selecting your motor, you need to first examine what your requirements are, and how they might change over time.  In this article we'll walk you through the process of selecting a motor for a motion control rig and the different factors that should play into the decision.

The basic principles you'll need to consider in motor selection, outside of cost, are: timeframe (how long your shoots will run for), power requirements, and repeatability.

Timeframe

One of the most important decisions you'll need to make about your rig is how long or how short the passage of time you want to shoot.  That is to say, there is a big difference between the speed one needs to shoot a sequence that occurs over several minutes to one that occurs over several days.  If you want an all-purpose time lapse rig, you'll need to cover both from fairly fast (full range of motion in an hour or less [even seconds]) to very slow (full range of motion over several days).  Based on your motor selection, you may limit yourself to a certain range of speeds.

Power Requirements

You'll need to consider how much power you'll need to run your rig. If it's designed to run in a studio, mains power will be readily available and your options will not be limited by availability.  Shooting on-location, and even deep in the field can present a real challenge for systems that consume too much power.  Every battery you have to carry is more weight to deal with - if you're intending to backpack your rig several miles to location, each battery reduces the amount of other useful gear you can bring.  (Including food and water!)  Whatever motor you choose, you're going to need to power it in some way - and relying on a little solar panel is a great way to hike home without that shot if the weather isn't just right.

Repeatability

If you intend to composit several shot sequences, repeatability is an essential part of a motion control rig.  True, guaranteed repeatability is rarely easy and cheap - some high count encoders or high quality servos can run into the hundreds of dollars, if not more.  Repeatability must be approached from an entire system design - perfect position of a motor doesn't matter if your gears have lots of backlash, belts are loosely held, or your movement relies on some string and a few pieces of wood.  The higher the accuracy and repeatability you require, the more expensive your rig is necessarily going to have to become.  You'll spend it in time or money.

Types of Motors

There are a number of different motors that could be utilized, but we'll focus on three basic types that are most common in motion control projects:

• DC Motors
• Stepper Motors
• Servo Motors

dc motorstepperservo

Each can be found in a bewildering array of sizes, torque, quality, and price.  Any kind of motor can be found in both hobby and industrial quality, generally speaking you're not going to see the value in buying the most expensive motor you can find as it will likely be designed to solve a problem you don't have.  We're not going to go deeply into the details of how to run these motors or build a system completely around them, but we will focus on how to select the class of motor that meets your needs.

DC Motors

Brushed DC motors are probably the most common types of motors to be used in a DIY motion control project.  They're readily available from numerous dealers - online and in stores. 

DC motors can be had for as little as a few cents for a used one down at an electronics surplus store, to hundreds of dollars for high-quality, geared DC motors. 

Timespan flexibility, how large of a range of speeds you can cover, is extremely limited with DC motors.  Generally speaking, you can get a range of from 10%-100% speed of a given motor through PWM control before it loses a substantial amount of torque or your motor stops responding to the signal being given it.  To achieve a speed very different from what a given DC motor produces requires a gear chain.  This gear chain comes with the same limitations as the motor its self - it doesn't increase the range of speed available, only moves that range further up or down the speed scale.  A flexible gear chain can become expensive increadibly fast - in most cases you'll get off easier choosing a different motor type if you want to cover large ranges of gears.  While it is possible to pause during movement to achieve a lower speed, the distance moved in total will not likely be very accurate as the motor moves with different efficiency once moving than when starting or stopping a move, and it will often take a small period fo time for the motor to build up enough power to move (on the order of fractions of a second, but in small moves enough to skew movement).

Power usage with DC motors covers a wide range.  For some very-low geared motors, you can move a substantial amount of weight with very little power.  A geared DC motor can often be your least power-consuming option for field work.

Repeatability is virtually non-existant with a plain DC motor.  With no feedback, and an unpredictable reponse to certain levels of power and load, any sense of repeatability will be hit or miss. Accuracy will need to be measured in "close enough" terms.

DC motors can be run in many ways, but one of the more common is to use a PWM (Pulse Width Modulation) driver.  These drivers can be had very inexepensively (under $10), and generally do what they claim.  A PWM driver works by turning the power on and off to the motor at a very high rate of speed (up to 20,000 times a second or more). In this way, it is able to run the motor at a slower speed without reducing its torque by reducing the amount of current it can draw.

Stepper Motors

Stepper motors are used in a wide variety of applications, from our vehicles to our printers and plenty of other devices for inexpensive and fairly accurate positioning. They are so named because they perform their rotations through a series of small, discrete "steps". 

There are a number of dealers of stepper motors, and they can easily be salvaged from old printers.  Be warned about harvesting/buying used: you get what you pay for, and you never know why that motor is not doing what it once was.

Timespan flexibility is extreme with stepper motors.  Since each rotation is made out of discrete steps, you can take steps at a slower rate instead of trying to slow down the rotation, as one would with a DC motor. A very common type of stepper is the 1.8' stepper, which moves 1.8 degrees every step - or 200 steps per revolution.  This means instead of trying to dial in a "speed" of 1 revolution per hour, you just have to slow down and step at a rate of 200 steps per hour.  Steppers can move very fast as well, if their power, current, and driver requirements are met, some can go as fast as 60,000 RPM with a much larger amount of torque than its DC motor equivalent.  Many stepper drivers can also do what is called "microstepping", and in doing so decrease the distance per step, starting at 2x (1/2 distance) all the way up to 64x (1/64th) and higher.  Microstepping increases resolution (addressable points in the revolution), but decreases accuracy in most situations.  However, microstepping can be used to greatly increase the speed at which the motor steps without incurring unacceptable vibration and noise.

Power usage tends to be higher for stepper motors than DC motors for the same amount of torque.  Expect more current draw, and often getting higher voltages for the same weight of gear than with a DC motor.  Most steppers tend to do better running at much higher voltages than their badges indicate.  In fact, with a proper driver you can run them on a wide range of voltages - however you may have to tune your driver when changing the input voltage.

Repeatability is moderate with steppers.  If using a good driver, well-written software for control, and running with clean power in the proper amount, you can often get great repeatability without any feedback mechanism.  Then again, a bad driver, poor pulse timing, or a weak battery and you can miss steps and you'll be no better than with a regular DC motor as far as accuracy and repeatability goes.  Microstepping increases resolution but can result in lower accuracy. For applications where linear or complex motion speed ramping is desired, the stepper is an easy choice for meeting this goal.

Every stepper motor needs a driver.  A stepper driver is very different from a DC motor driver, and they usually range from around $20 for a hobby-class driver, to several hundreds for a more industrial-class driver.  Generally speaking, you will get what you pay for in terms of capabilities, reliability, and performance, with some exceptions.  If you're driving a stepper that is run at an amp of current or less, you may not find much difference between a $20 hobby-level driver and a $60 mid-level commercial driver.  If you get a big NEMA42 stepper motor that uses 150 watts of power, you can expect to spend several hundred dollars on a matching driver.

Servo Motors

A servo is a special kind of DC motor that has a driver and a feedback mechanism.  You can either buy a servo with the driver and feedback already built-in, or create one from a DC motor by adding these two features.  Hobby-class servos are extremely easy to source, and range from around $15 to a hundred or more.  Industrial-class servos are more difficult to source, and generally much more expensive.

Low-torque, low-power servos can be bought at most hobby shops, but they are limited in their rotation, usually to less than 300'.  While it is possible to modify these servos to be continuous rotation, it removes all feedback, thus eliminating most of the benefits.  Industrial or self-made servos (with encoders for feedback) can rotate continuously and still maintain very high accuracy and repeatability.

Timespan flexibility is determined by the resolution of the driver and feedback mechanism.  In much the same way a stepper is used, a servo may be asked to go to a specific position which is very close to its current position.  While it still runs its movements at the speed it would in a continuous rotation mode, the driver stops moving the servo once the feedback indicates it has moved the given distance.  Of course, how much more effective this is than a regular DC motor in extending the time a movement occurs over is related to the granularity provided by the driver and feedback mechanism.  Hobby-level servos tend to move in 1' increments, but are usually limited to less than one full revolution while there are industrial-class servos that can rotate forever, with accuracy measured in arc-seconds.

As servos are basicaly just DC motors, their power consumption pattern tends to mimic that of the DC motor type.

Repeatability and accuracy are very easy to achieve with servos.  The feedback mechanism, especially when combined with a 'home' position feedback mechanism, can result in accurate, repeatable moves even between powering off your rig.  This accuracy comes with a price, as mentioned: to get many revolutions with high accuracy usually means shelling out hundreds of dollars or more on a motor for a single axis.

Summary Table

The following table compares each motor type based on the four basic principles for a motion control rig:

I hope this article helps you choose the right motor for your application.  Obviously, there is much more that can be said about the operation of each of these kinds of motors, but that is better left to other sites, and other articles.  Once you've chosen a motor type that seems like it is the right choice for your project, it is helpful to read up on that type and understand how it operates and what impact its going to have on what you want to do.

In future articles, we'll cover gear and drivetrain selection and overall system design.

- c. a. church