Selecting the Proper Motor for Linear Motion Applications

Linear motion systems are found inside countless machines including precision laser cutting systems, laboratory automation equipment, semiconductor fabrication machines, CNC machines, factory automation, and many others too numerous to list. They range from the relatively simple such as an inexpensive seat actuator in a passenger vehicle, to a complex, multi-axis coordinate system complete with control and drive electronics for closed-loop positioning. No matter how simple or complex the linear motion system, at the most basic level, they all have one thing in common: moving a load through a linear distance in a specific amount of time.

Figure 1. Brush, brushless, and stepper motors are available in many shapes and sizes.

One of the most common questions when designing a linear motion system centers on motor technology. Once the technology is chosen, the motor needs to be sized to meet the demands of load acceleration, overcoming friction in the system, and overcoming the effect of gravity, all while maintaining a safe maximum operating temperature. The torque, speed, power, and positioning capability of the motor are a function of the motor design, coupled with the drive and control.

What Motor Should I Start With?

There are a lot of application questions to consider when designing a linear motion system using a particular motor technology. An exhaustive explanation of the entire process is beyond the scope of this article. The intent is to get you thinking about asking the right questions when talking with a motor supplier.

There is no such thing as the best motor for every application, but rather the best motor for a particular application. In the vast majority of incremental motion applications, the choice will either be a stepper motor, brush DC motor, or brushless DC motor (Figure 1). The most complex motion systems may use linear motors coupled directly to the load, avoiding the need for mechanical power conversion; there’s no need for translation through a lead screw/ball screw, gearbox, or pulley system. Although maximum accuracy, repeatability, and positioning resolution can be achieved with coreless direct-drive linear servo systems, they are the highest cost and complexity when compared with rotary motors. An architecture using rotary motors is much less expensive, and will meet the majority of linear motion applications; however, some means of “rotary-to-linear” conversion (and as a result, power conversion) is needed to drive the load.

Figure 2. Block diagram showing a basic linear system using a BLDC gear motor.

Stepper, brush, and brushless motors are all considered DC motors; however, subtleties exist that will cause an engineer to favor one type over the other two in a particular application. It must be stressed that this choice is highly dependent on the design requirements of the system, not just in terms of speed and torque, but also the positioning accuracy, repeatability, and resolution requirements. There isn’t a perfect motor for every application, and all decisions will require design trade-offs. At the most basic level, all motors, whether they are called AC or DC, brush, brushless, or any other electric motor for that matter, operate under the same principle of physics to generate torque: the interaction of magnetic fields. There are dramatic differences, however, in the way these various motor technologies respond in particular applications. Overall motor performance, response, and torque generation depends on the method of field excitation and magnetic circuit geometry inherent in the physical motor design, the control of input voltage and current by the controller/drive, and method of velocity or position feedback, if the application requires.

DC stepper, brush servo, and brushless servo motor technologies all use a DC supply in order to power them. For linear motion applications, this doesn’t mean that a fixed source of DC can be applied directly to the motor windings; electronics are needed to control the winding current (related to output torque) and winding voltage (related to output speed). Listed below is a summary of strengths and weaknesses of the 3 technologies.

DC Stepper Motor

Strengths

Open loop positioning – No encoder required
Simple “pulse and direction” signal needed for rotation
High torque density at low speeds
Motor can be in a “stall” position without exceeding the temperature rating
Lowest cost solution

Weaknesses

No position correction in the event the load exceeds the output torque
Low power density – torque drops off dramatically at higher speeds
Motor draws continuous current, even at standstill
High iron losses above 3000 RPM
Noticeable cogging at low speeds (can be improved with a micro-stepping drive)
Ringing (resonance) at low speeds

DC Brush Servo Motor

Strengths

Linear speed/torque curve (compared with a stepper)
Low-cost drive electronics (4 power switching devices)
Many different configurations available
Highly customizable
Easy to control and integrate
Very smooth operation possible at low speeds (depends on the number of slots and commutator bars)
High power density – flatter torque at higher speeds (compared with a stepper)

Weaknesses

Motor will draw high current in an overload condition (same as the brushless motor)
Method of feedback needed for closed-loop positioning (same as the brushless motor)
Angular velocity is more limited due to mechanical factors in the armature design and brush system
Carbon brush wear and EMI generation
High thermal resistance (copper is in the armature circuit)

DC Brushless Servo Motor

Strengths

High power density – flatter torque at higher speeds (compared with a stepper)
Linear speed/torque curve (compared with a stepper)
Electronic commutation – no mechanical brushes
Low thermal resistance (copper is in the stator circuit)
Highest move response and acceleration possible (compared with stepper or brush DC motors)
Smooth operation possible (dependent on motor magnetic design and control technology)

Weaknesses

Highest cost among the 3 motor technologies
Motor will draw high current in an overload condition (same as the brush motor)
Method of feedback needed for closed-loop positioning (same as the brush motor)
Higher drive complexity and cost — (6 power switching devices)
Method of rotor position detection for electronic commutation

Power Conversion in a Linear Motion System

Figure 3: Motion profile for a motor for a linear rail mechanism that moves a load from point A to point B.

The design of the linear system begins with the load mass and how fast the mass needs to traverse from point A to point B. Motor type, size, and mechanical design begin with the power (watts) required to move the load. Starting with the load and ultimately working back through all the components to the drive power supply, the analysis is a series of steps to understand the power conversion from one part of the system to the other while considering the various efficiencies of the components in between. Watts in the form of voltage and current into the drive will ultimately translate to mechanical output watts moving a given load in a specific amount of time. Figure 2 shows a block diagram illustrating a basic linear system using a BLDC gear motor.

In order to get an indication of the output power needed at the load, a simple power calculation will help ballpark a motor (Figure 3). After understanding the average output power needed, finish analyzing the power requirements by working back to the motor and drive through the various power conversion elements. Manufacturers’ data should be referenced to take into account the efficiency of the various components, as this will ultimately determine the size of the motor and the power supply. It is personal preference regarding what units to work with, but SI units are highly recommended. Working in SI units avoids the need to remember multiple conversion constants, and the end result can always be converted back to English units.

How Much Power is Needed to Move the Load in the Required Time?

Here’s a typical list of application requirements.

A 9-kg mass lifted against gravity will require a force of about 88N. Calculating the watts needed to move the load will provide a starting point for determining the components in the rest of the system. This is the average power needed to move a mass of 9Kg vertically from point A to point B in 1 second. System losses such as friction are not included. The motor shaft power required will be somewhat higher and depends on the other components used in the system such as the gearbox and lead screw.

P = (F × S) / t

P = (88N × 0.2m) / 1.0s = 17.64w

This is different than the peak power that will be required from the system. Once acceleration and deceleration are taken into account, instantaneous power during the move profile will be somewhat higher; however, the average output power needed at the load is about 18 watts. After a thorough analysis of all the components, a system like this one will require about 37w peak power to accomplish the job. This information, along with the various other application specs, will now help choose the most appropriate motor technology.

What Motor Technology Should I Consider?

Excellent positioning capability and relatively simple controls would lead a designer to look at the possibility of using a stepper motor first. A stepper motor, however, would not meet the requirement of a small mechanical footprint while meeting the load demands. A peak power requirement of 37 watts would require a very large stepper motor. Although stepper motors possess very high torque at low speeds, the peak velocity and thus power requirement of the move profile exceeds the capability of all but the largest stepper motors.

A brush DC servo motor would meet the load requirements, a small mechanical footprint, and would have very smooth rotation at low speeds; however, due to the strict EMC requirements, it’s probably best to avoid the brush motor for this particular application. This would be a less expensive alternative compared with a brushless system, but it might present difficulty in passing any stringent EMC requirements.

The brushless DC motor using a sinusoidal drive system would be the first choice to meet all the application requirements including the load and motion profile (high power density); smooth, cog-free motion at low speeds; and a small mechanical footprint. In this case, there will still be the potential of an EMI signature due to the high frequency switching of the drive electronics; however, this can be mitigated using in-line filtering due to a narrower frequency band. A brush DC motor exhibits a broader band EMI signature, making it more challenging to filter.

Motor Sizing is Just the Beginning

This article was a brief discussion to introduce a designer to various considerations when choosing a motor technology for a relatively simple linear motion application. Although the principles are identical for a more complex system such as an X-Y table or a multi-axis precision pick-and-place mechanism, each axis will need to be analyzed for load independently. Another consideration outside the scope of this article is how to choose an appropriate safety factor in order to meet the desired life of the system (number of cycles). System life isn’t just a function of the motor size, but also the other mechanical elements in the system such as the gearbox and lead screw assembly. Other factors such as positioning accuracy, resolution, repeatability, maximum roll, pitch, and yaw, etc. are all important considerations to ensure the linear motion system meets or exceeds the application goals.

This article was written by Dan Montone, Director of Marketing at Haydon Kerk Motion Solutions/Pittman Motors, Harleysville, PA. For more information, Click Here.