学术英语国际会议发言稿——
The Heart of Robot——Motors
学生姓名：刘净川
学 号：S312070113
学 院：机电工程学院
专 业：机械设计及理论
2012.11.20
The Heart of Robot——Motors
Thank you very much, Miss. Zhao, for your kind introduction. Ladies and gentlemen, Good afternoon! I am honored to have been invited to speak at this conference. Today, my topic is about The heart of robot—Motor. As we know, almost all of the robots use electric motor to provide motive power, since its small size and high accuracy. I want to share my interesting research result on motors with you. Overview
The content of this presentation is divided into 4 parts: In session one, I will talk about how to choose a motor for a robot; In session two and three, I will introduce two kinds of motors which are most widely used on robot technology; and finally, I will make a conclusion.
1. Choosing a Motor
Electric motors, both ac motors and dc motors, come in many shapes and sizes. Some are standardized electric motors for general-purpose applications. Other electric motors are intended for specific tasks. In any case, electric motors should be selected to satisfy the dynamic requirements of the machines on which they are applied without exceeding rated electric motor temperature. Thus, the first and most important step in electric motor selection is determining load characteristics -- torque and speed versus time. Electric motor selection is also based on mission goals, power available, and cost. Above all, the type of motor chosen for an application depends on the characteristics needed in that application. These characteristics include: How fast you want the object to move,
The weight, size of the object to be moved,
The cost and size of the motor,
The accuracy of position or speed control needed.
2. DC Motors
Now, let`s come to the first part—DC motor
2.1 The working principle of brushless DC motor
D.C. machines are characterized by their versatility. By means of various combinations of shunt-, series-, and separately excited field windings they can be designed to display a wide variety of volt-ampere or speed-torque characteristics for both dynamic and steady state operation. Because of the ease with which they can be controlled, systems of D.C. machines are often used in applications requiring a wide range of motor speeds or precise control of motor output.
The essential features of a D.C. machine are shown schematically. The stator has salient poles and is excited by one or more field coils. The air-gap flux distribution created by the field winding is symmetrical about the centerline of the field poles. This is called the field axis or direct axis.
As we know, the A.C. voltage generated in each rotating armature coil is converted to D.C. in the external armature terminals by means of a rotating commutator and stationary brushes to which the armature leads are connected. The commutator-brush combination forms a mechanical rectifier, resulting in a D.C. armature voltage as well as an armature m.m.f. Wave then is 90 electrical degrees from the axis of the field poles, i.e. in the quadrature axis. In the schematic representation the brushes are shown in quadrature axis because this is the position of the coils to which they are connected. The armature m.m.f. Wave then is along the brush axis as shown. (The geometrical position of the brushes in an actual machine is approximately 90 electrical degrees from their position in the schematic diagram because of the shape of the end connections to the commutator.)
2.2 The advantages of brushless DC motor
DC motor have the performance with fast response , a large starting torque, it can provide rated torque from zero speed to rated speed . But advantages of the DC motor is what its shortcomings, because the DC motor produce constant rated load torque , then the armature magnetic field and rotor magnetic field must be constant maintenance of 90 °, which would produce sparks, toner form carbon brush and commutator in the motor rotation ,it will not only make group damaged, but also the using is limited. AC motor have no carbon brushes and commutator, it is
maintenance-free, strong, applied widely, but AC motor need to be achieved with sophisticated control technology if the characteristics of the AC motor is equal to the DC motor performance.
Today the rapid development of power semiconductor and more speed of components' switching frequency enhance the performance of the drive motor. Microprocessor speed is also faster and faster, enabling the AC motor control placed in a rotating two-axis orthogonal coordinate system. Proper control of AC motor in the two-axis current components make it achieve similar to the control of DC motor and have comparable performance with the DC motor. In addition, there are a lot of microprocessor controlling the motor's necessary functions made in the chip, and its volume is smaller and smaller, such as digital converter、pulse width modulation and so on. Brushless DC motor control AC motor commutation electronic way , it can get the characteristics similar to the DC motor and it have not a lack of DC motor's institutional.
2.3 Brushless DC motor's applications
The research of Variable Frequency Induction Motor in three decades , the final analysis, is looking for induction motors torque control method, rare earth permanent Magnetic brushless DC motor because of the characteristics that wide speed range, small size, high efficiency and of small steady-state speed error has advantages in areas of changing speed apparently.
Brushless DC motor have the characteristics of DC brush motor characteristics, but also it is frequency devices, they are called the DC conversion, international general terms for the BLDC.
Brushless DC motor because of operating efficiency, low torque, speed control of precision is better than any inverter technology , so it's worth to concern by the industry. This product has produced more than 55kW, can be designed to 400kW, can solve the saving industry and the demand for high-performance driving.
3. Stepper Motors
A stepper motor is a brushless, synchronous electric motor that converts digital
pulses into mechanical shaft rotation. Every revolution of the stepper motor is divided into a discrete number of steps, in many cases 200 steps, and the motor must be sent a separate pulse for each step. The stepper motor can only take one step at a time and each step is the same size. Since each pulse causes the motor to rotate a precise angle, typically 1.8°, the motor's position can be controlled without any feedback mechanism. As the digital pulses increase in frequency, the step movement changes into continuous rotation, with the speed of rotation directly proportional to the frequency of the pulses. Step motors are used every day in both industrial and commercial applications because of their low cost, high reliability, high torque at low speeds and a simple, rugged construction that operates in almost any environment.
3.1 Types of Step Motors
There are three basic types of step motors: variable reluctance, permanent magnet, and hybrid. This discussion will concentrate on the hybrid motor, since these step motors combine the best characteristics of the variable reluctance and permanent magnet motors. They are constructed with multi-toothed stator poles and a permanent magnet rotor (See figure A). Standard hybrid motors (such as the models offered by OmegamationTM) have 200 rotor teeth and rotate at 1.8? step angles. Because they exhibit high static and dynamic torque and run at very high step rates, hybrid step motors are used in a wide variety of commercial applications including computer disk drives, printers/plotters, and CD players. Some industrial and scientific applications of stepper motors include robotics, machine tools, pick and place machines, automated wire cutting and wire bonding machines, and even precise fluid control devices.
3.2 Step Modes
Stepper motor "step modes" include Full, Half and Microstep. The type of step mode output of any stepper motor is dependent on the design of the driver.
Omegamation? offers stepper motor drives with switch selectable full and half step modes, as well as microstepping drives with either switch-selectable or
software-selectable resolutions.
3.2.1 FULL STEP
Standard hybrid stepping motors have 200 rotor teeth, or 200 full steps per
revolution of the motor shaft. Dividing the 200 steps into the 360? of rotation equals a
1.8? full step angle. Normally, full step mode is achieved by energizing both windings while reversing the current alternately. Essentially one digital pulse from the driver is equivalent to one step.
3.2.2 HALF STEP
Half step simply means that the step motor is rotating at 400 steps per revolution. In this mode, one winding is energized and then two windings are energized
alternately, causing the rotor to rotate at half the distance, or 0.9?. Although it provides approximately 30% less torque, half-step mode produces a smoother motion than full-step mode.
3.2.3 MICROSTEP
Microstepping is a relatively new stepper motor technology that controls the current in the motor winding to a degree that further subdivides the number of
positions between poles. Omegamation microstepping drives are capable of dividing a full step (1.8?) into 256 microsteps, resulting in 51,200 steps per revolution
(.007?/step). Microstepping is typically used in applications that require accurate positioning and smoother motion over a wide range of speeds. Like the half-step mode, microstepping provides approximately 30% less torque than full-step mode.
3.3 Accuracy
The accuracy for can-stack style steppers is 6 - 7% per step, non-cumulative. A
7.5° stepper will be within 0.5° of theoretical position for every step, regardless of how many steps are taken. The incremental errors are non-cumulative because the mechanical design of the motor dictates a 360° movement for each full revolution. The physical position of the pole plates and magnetic pattern of the rotor result in a repeatable pattern through every 360° rotation (under no load conditions).
3.4 Resonance
Stepper motors have a natural resonant frequency as a result of the motor being a spring-mass system. When the step rate equals the motor’s natural frequency, there may be an audible change in noise made by the motor, as well as an increase in vibration. The resonant point will vary with the application and load, but typically occurs somewhere between 70 and 120 steps per second. In severe cases the motor may lose steps at the resonant frequency. Changing the step rate is the simplest means of avoiding many problems related to resonance in a system. Also, half stepping or micro stepping usually reduces resonance problems. When accelerating to speed, the resonance zone should be passed through as quickly as possible.
3.5 Stepper Motor Advantages and Disadvantages
3.5.1 ADVANTAGES
1. The rotation angle of the motor is proportional to the input pulse.
2. The motor has full torque at standstill (if the windings are energized).
3. Precise positioning and repeatability of movement since good stepper motors have an accuracy of 3 to 5% of a step and this error is non-cumulative from one step to the next.
4. Excellent response to starting/stopping/reversing.
5. Very reliable since there are no contact brushes in the motor. Therefore the life of the step motor is simply dependant on the life of the bearing.
6. The stepper motors response to digital input pulses provides open-loop control, making the motor simpler and less costly to control.
7. It is possible to achieve very low speed synchronous rotation with a load that is directly coupled to the shaft.
8. A wide range of rotational speeds can be realized as the speed is proportional to the frequency of the input pulses.
3.5.2 DISADVANTAGES
There are two main disadvantages of stepper motors:
1.Resonance can occur if not properly controlled.
This can be seen as a sudden loss or drop in torque at certain speeds which can result in missed steps or loss of synchronism. It occurs when the input step pulse rate coincides with the natural oscillation frequency of the rotor. Resonance can be minimised by using half stepping or micro stepping.
2.Not easy to operate at extremely high speeds
3.6 Applications
Stepper motors can be a good choice whenever controlled movement is required. They can be used to advantage in applications where you need to control rotation angle, speed, position and synchronism.
These include
printers
plotters
medical equipment
fax machines
automotive and scientific equipment etc.
4. Conclusion
A world without electric motors is difficult to imagine. From the tiniest motor found in a quartz watch to a million-plus horsepower motor powering a ship, motors are used in many diverse applications. And since motors have features as follow: Energy scales span 38 orders of magnitude
Currents in a magnetic field feel a force
Currents produce magnetic fields
Permanent magnets can spontaneously aligned moments
Motors are widely used in robot technology.