Control techniques affect motor selection in positioning applications
Machine designers work hard to stay up to date on the latest electronic and software techniques that squeeze out the maximum possible performance from a given mechanical system. But the electronics are only as good as the motor that they are driving.
This article will look at the issues concerning the selection of the right type of motor in positioning control applications. The focus will be on understanding not just the motor, but how these motors are controlled, and how the choice of the control technique affects the system cost.
Give me a motor, any motor
Motion systems that control position precisely consist of a controller, an amplifier, and a motor. Figure 1 shows these elements.
Three major motor types are commonly used in positioning control systems. They are:
1) Step motors
2) DC-brush motors
3) Brushless DC (permanent magnet) motors
Let us start with a discussion of motor operation for each of the three motor types. Each motor type has particular characteristics such as preferred operating speed, smoothness, cost, etc. that provide many variables for us to compare and contrast.
Step motors are self-positioning, and therefore do not require an encoder to operate. This immediately gives step motors a cost advantage over servo motors which require an encoder to operate in a position mode.
Furthermore, step motors are sometimes constructed in such a way that they do not require any magnetic material in their rotor (the part of the motor that rotates) or the stator (the part that is connected to the motor frame). Instead, the ‘torque creating’ part of a step motor can be constructed entirely of iron or similar low cost materials.
Step motors are also ‘brushless’, meaning there is no physical contact with the electrical portion of the rotor. Step motors produce a relatively high torque for a given package size, and have a high holding (resting) torque.
Step motors have a few drawbacks though. Step motors create audible noise and induce vibrations which can disturb the load or affect parts of the system. Vibration can be reduced using microstepping techniques or even mechanical dampers, but these solutions seldom eliminate the problem completely.
Another limitation is that step motors have a low absolute high-end speed, rarely exceeding 5,000 rpm. and the torque that is available from a step motor drops significantly at higher velocities.
DC brush motors
DC brush motors are used in a wide variety of applications which require positioning. By themselves, however, DC brush motors have no sense of position. This means they must be connected to an encoder. The encoder provides the position information and the controller drives the motor using a PID algorithm or similar scheme.
DC brush motors are available in a large variety of sizes up to a kilowatt and beyond. They can operate at speeds of 10,000 rpm and even higher. They are known for their low cost relative to other servo motors such as brushless DC motors, and DC servo motors are smooth and relatively quiet.
DC brush motors have two primary disadvantages. The first is the mechanical device used to commutate the motor. These brushes can wear out or cause electrical arcing which generates electro magnetic interference (EMI).
Another disadvantage is that DC servo motors have a relatively low torque output for a given size, due to the fact that current is driven through coils located in the rotor. The rotor by definition, is not anchored to the motor frame and therefore the heat that can be extracted is limited.
Brushless DC motors
Brushless DC, also called brushless PM (for permanent magnet), or BLDC motors for short, provide a ‘no compromise” solution to servo control for many applications. They are smooth and quiet, and do not require mechanical brushes for commutation. In addition, brushless DC motors do not drive current through the rotor. Instead current is driven through the stator, which is solidly connected to the motor case allowing heat to be rapidly dissipated.
Brushless DC motors are available in a wide variety of power ranges up to and well beyond a kilowatt and they can operate at very high speeds, to 50,000 RPM and beyond.
Despite these important advantages, brushless motors have two main disadvantages. The first is cost. They are more expensive than DC servo or step motors due to the cost of the rare-earth magnetic materials in the rotor. The second disadvantage is that they must be commutated externally. This increases the complexity of the controls and requires the installation of Hall sensors, or equivalent phasing tracks in an optical encoder disk.
Now that we have been introduced to the characteristics of positioning motors let us examine some important issues related to how these motors are controlled. By developing a better understanding of motor control techniques we will be able to make more informed decisions about the best motor type for a given application.
Taking a look at ... motor phasing and commutation
Motor phasing refers to the number of phases that are externally supported by the motor and which must be driven by the control system. Some motors like a DC servo have only a single external phase while other motors like the step motor have two, three or five phases. Brushless DC motors nearly always have three phases.
Each phase of a multi phase motor requires an amplification circuit and therefore the number of phases is a contributor to the cost of the overall system. But with the cost of electronics falling, the difference in cost of (for example) a DC servo motor amplifier and a brushless DC motor amplifier is modest.
In the case of a brushless DC motors commutation is typically performed with the help of devices known as Hall sensors. These signals allow the controller to excite each of the three motor phase coils at the right time based on the shaft angle of the motor.
Another popular brushless DC motor commutation scheme is sinusoidal commutation, which uses the motor’s position encoder to generate continuously varying sinusoidal signals. This results in smoother motion in the resultant motor torque output. Figure 2 shows typical Hall-based as well as sinusoidal drive waveforms for a three-phase motor.
In the case of step motors, commutation is more properly called phasing, and techniques that the amplifiers employ are given special names such as full step, half step, or microstep drive. In any case, these different techniques refer to the number of different power levels that are applied to each motor coil during an electrical cycle.
Figure 3 shows various drive waveforms for a two-phase step motor.
Table 1 summarizes the most common commutation techniques for each motor type along with the relative advantage and disadvantage of each method.
Taking a brief look at ... motor position feedback
Servo motors such as DC brush or Brushless DC devices require position feedback at all times to maintain their position. These motors move through a controlled path by continuously comparing the actual motor position with the desired instantaneous position and generating a motor command which is amplified by the motor drive circuit. This is called a position servo loop and typically consists of a PID (proportional, integral, derivative) filter to generate the motor command value.
Step motors operate on a different principle, by continuously ‘rotating’ the phases through one electrical cycle after another the motor is driven to move a precise number of steps or microsteps.
Because the step motor does not require a feedback device to position itself most applications using this type of motor do not utilize a position encoder. Nevertheless there is an increasing trend toward use of position feedback devices in step motor applications because of the additional safety provided.
Table 2 provides a summary of the many types of encoders available.
There isn't, at least yet, a perfect motor for all applications. As I have tried to show in this article motor choice will affect a number of parameters including cost, noise, top speed, and reliability.
Table 3 provides a helpful summary of advantage provided by each motor type