Comparing induction motors, permanent-magnet motors, and servomotors

Designers and motor personnel advantage from discovering a supplier that's an skilled resource of info to assist in pragmatic motor choice. Involve application specialists as early as you possibly can, as they are able to assist create prototypes, custom electrical and mechanical styles, mountings, and gearboxes. This also reduces expenses related with shorter lead occasions and rush delivery.

Servo motors can offer higher performance, faster speeds, and smaller sizes. PM synchronous motors offer advantages on high-energy- consuming and high-dynamic applications, compared to induction motors. Variable frequency drives used with asynchronous motors also can be used with synchronous servo motors, producing higher efficiencies than an asynchronous motor, using perhaps 30% less energy in positioning applications.

Induction motor systems (lower cost, rugged, reliable, and well known) can offer an alternative to servo motor systems (the traditional, established solution) for certain applications. This, of course, is based on similar electronic controls being used (with the latest technology and approximately the same cost), leaving the cost of motors the differentiating issue.

Overview of the pros and cons of each motor type

Induction motor

SPEED - Less speed range than PMAC motors • Speed range is a function of the drive being used — to 1,000:1 with an encoder, 120:1 under field-oriented control
EFFICIENCY - Even NEMA-premium efficiency units exhibit degraded efficiencies at low load
RELIABILITY - Waste heat is capable of degrading insulation essential to motor operation • Years of service common with proper operation
POWER DENSITY - Induction produced by squirrel cage rotor inherently limits power density
ACCURACY - Flux vector and field-oriented control allows for some of accuracy of servos
COST - Relatively modest initial cost; higher operating costs

PMAC

SPEED - VFD-driven PMAC motors can be used in nearly all induction-motor and some servo applications • Typical servomotor application speed — to 10,000 rpm — is out of PMAC motor range.
EFFICIENCY - More efficient than induction motors, so run more coolly under the same load conditions
RELIABILITY - Lower operating temperatures reduces wear and tear, maintenance • Extends bearing and insulation life • Robust construction for years of trouble-free operation in harsh environments
POWER DENSITY - Rare-earth permanent magnets produce more flux (and resultant torque) for their physical size than induction types
ACCURACY - Without feedback, can be difficult to locate and position to the pinpoint accuracy of servomotors
COST - Exhibit higher efficiency, so their energy use is smaller and full return on their initial purchase cost is realized more quickly

Servomotor

SPEED - Reaches 10,000 rpm • Brushless DC servomotors also operate at all speeds while maintaining rated load
EFFICIENCY - Designed to operate over wide range of voltages (as this is how their speed is varied) but efficiency drops with voltage
RELIABILITY - Physical motor issues minimal; demanding servo applications require careful sizing, or can threaten failure
POWER DENSITY - Capable of high peak torque for rapid acceleration
ACCURACY - Closed-loop servomotor operation utilizes feedback for speed accuracy to ±0.001% of base speed
COST - Price can be tenfold that of other systems

In the end, all industrial motor subtypes have strengths and weaknesses,plus application niches for which they’re most suitable. For example, many industrial applications are essentially constant torque, such as conveyors. Others, such as centrifugal blowers, require torque to vary as the square of the speed. In contrast, machine tools and center winders are constant horsepower, with torque decreasing as speed increases. Which motors are most suitable in these situations? As we will explore, the speed-torque relationship and efficiency requirements often determine the most appropriate motor.

Mechanical Overview of Servo Motors

Servo motors have serval distinct characteristics that sperate them from their stepper counterparts. The biggest is the lack of direct gearing between the rotor and the output shaft. This eliminates the backlash and cogging behaviors found ins steppers, where there is period of slop between the gear teeth before movement actually begins, and where the shaft continues to move after the motor has stopped. This can lead to jerky starts and stops, as well as a time delay in movement. This does not impede static positioning performance markedly, but it presents major issues when on-the-fly velocity changes or hard starts/stops are needed.

A model of a typical radial brushless DC servo motor is shown before in figure 1.1 For a long time, servo motors used brushes to transfer current from the static winding to the rotor, but this would lead to wear on the brushes, in turn shortening the lifespan of the motor. With the advent of electronic motor controllers, the brusheless design was adopted, which uses control electronics to vary the currents phases to the motor's windings in the same way the brushes do. For the rest of this paper, all mention of servo motors will be of the brusheless type.

Looking at figure 1.1 below, there are several objects of interest. First are the armature windings (held by the stator), which create a magnetic field that travels through the air gap to the permanent magnets on the rotor. Even though there are normally no gears in a servo motor, cogging can still exist, as there are gaps between the magnets on the rotor where the flux decrease, though this only becomes noticeable at low speeds. This type of congging in servos is perhaps more accurately termed "detent torque." There are two ways to minimize this type of cogging, the most common being the addition of some gearing to the drive shaft. This allow the motor to run at a higher speed out of its cogging region, but does not compromise power output or precision, thought it can induce some backlash. The other way of minimizing cogging is to skew the magnets on the rotor so that a radial line from the center of the rotor always intersects a magnet at least once. When using a motor without gearing, it is known as a direct drive motor. This allows for the best transfer of power to the load, and avoids any of the negative aspects of gearing previously mentioned. A feature in newer servo motors (including the Bodine models used in this thesis) is the use of an ironless stator, which eliminates iron saturation, a situation where the magnetic properities of the iron limit how much current can be applied to the windings. Inducing iron saturation too ofen will cause overheating and possibly damage the winding or magnets. With an ironless stator, rotor magnet skewing is not necessary, as the magnetic fields aren't influenced by the material of the stator. Also, since the only mechanical connection between the shaft and the body is through the bearings, friction is very low (especialy when using ball bearings).

In high torque motors such as the ones used in this thesis, the rotor actually consists of two plates of permanet magnets sandwiching the stator, which allows for a major increase in torque. This feature only exists in axial flux motors, due to the design where the stator lies in between the rotors, whereas in radial flux servos, the rotor is completely enclosed by the stator. The majority of the heat dissipated from a servo motor comes from the stator, so its outside location adis in cooling. In fact, the main limiting fator in the power of a servo motor is the heat capacity of the stator and the armature windings.

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The Advantages and Working of a Servo Motor

Servo control, which is known as "motion control" or "robotics" is used in industrial processes to move a specific load in a controlled fashion. These systems can utilize either pneumatic, hydraulic, or electromechanical actuation technology. The choice of the actuator type (i.e. the device that provides the power to move the load) is based on power, speed, precision, and cost requirements. Electromechanical systems are typically used in high precision, low to medium power, and high-speed applications. These systems are flexible, efficient, and cost-effective. Servo Motors are the actuators used in electromechanical systems. Through the interaction of electromagnetic fields, they generate power.

Servo Motor Advantages

Servo motors with related controls provide very precise and repeatable control of both position and velocity, and its various feedback parameters allow users to closely monitor the dispensing process and detect any abnormalities before they can become major problems. There are three main advantages to servo drive technology in adhesive and sealant dispensing equipment:

1. Servo drive motors allows you to have preset, multi-segment shot profiles, with each segment having its own material volume and flow rate, and with the ability to smoothly blend the motion of each segment into the next one in the profile.  The user can then select from among these preset profiles before initiating the dispense cycle.

2. Servo technology also allows you to continuously vary the material flow rate during the dispense cycle based upon a command reference from the process control.  This allows the user to either apply a continuous bead of material with varying bead widths, or conversely, to maintain the same bead width despite changes in the applicator's linear speed, for example when a robot slows down to negotiate a complex curve.

3.  A major advantage to servo control is its ability to reliably maintain the commanded volumetric flow rate of material despite changes in the physical conditions of the dispensing system or its environment.  Examples of these types of changes are variations in material viscosity, and therefore in the back pressure it generates during the dispense cycle, due to such things as variations in ambient temperature or differences in batches of material; variations in the plant utilities supplied to the dispensing equipment, such as air pressure or electrical voltage; and load changes due to physical wear on the dispensing equipment as it ages.  The servo drive simply increases or decreases the amount of current it supplies to the motor as required to maintain the commanded material flow rate, up to the current limits of the drive.  If those limits are ever exceeded, the drive generates a fault and stops the cycle, which prevents the customer from unknowingly making out of spec parts.

Working of a Servo Motor

Servo motors are utilized to control position and speed very precisely, but in an easy case, only position may be controlled. Mechanical position of the shaft can be sensed by using a potentiometer, which is coupled with the motor shaft through gears. The current position of the shaft is converted into electrical signal by the potentiometer, and the compared with the command input signal. In modern servo motors, electronic encoders or sensors are used to sense the position of the shaft.

Command input is given according to the required position of the shaft. If the feedback signal differs from the given input, an error signal is generated. This error signal is then amplified and applied as the input to the motor, which causes the motor to rotate. And when the shaft reaches to the required position, error signal becomes zero, and hence the motor stays standstill holding the position.

The command input is given in the form of electrical pulses. As the actual input applied to the motor is the difference between feedback signal (current position) and applied signal (required position), speed of the motor is proportional to the difference between the current position and the required position. The amount of power required by the motor is proportional to the distance it needs to travel.

Connecting a Servo Motor

Servo motors typically have three wires. The power wire, usually red, is connected to teh power rail. The ground wire, ususally black or brown, is connected to the ground rail. The third wire, usually yellow or oragne, is the signal wire and is connected directly to a digital pin on the Arduino. The Arduino can normally directly supply power to a servo motor, but when using serveral servo motors, you ned to separate the Arduino power supply to the servo power supply to avoid brown outs. Servo motors, even if they do not always act like typical motors, still have a small motor inside and can draw large amounts of current, far more than what the ATmega can deliver.



Before using servo motors, you must import the Servo library. You can do this either by importing the library. You can do this either by importing the library through the Arduino IDE menu (Sketch Import Library servo) or by manually typing:

#include (servo.h)

In your software, you must first creat a new servo object before issuing instructions. You must create one object per servo motor (or group of servo motors) to contorl.

Servo frontWheels;
Servo rearWheels;

To tell the Arduino which pins the servo motors are connected to, call attach(), specifying the pin, and optionally, specifying the minimum and maximum pulse size.

servo attach (pin)
servo attach (pin, min, max)

By default, Arduino uses 544 microseconds as the minimum pulse length (equivalent to 0 degrees) and 2,400 microseconds as the maximum pulse width (equivalent to 180 degrees.) If your servo motor has different settings for a maximum and minimum pulse, you can change the values in attch () by specifying the duration in microseconds. For example, a servo motor that uses a 1 millisecond minimum and 2 millisecond maximum can be configured like this:

servo.attach (pin, 1000, 2000);

From then on, the Arduino automatically calculates the length of the pulse according to the wanted angle but will not issue commands until a function specifically orders the servo motor to move.