等价

只是你必须了解，当你选择随波逐流时，麻烦或许减少许多，相对的，欢乐也是。
因为，这是等价交换。

Stepper Motor

1) Definition: A stepper motor is an electromechanical device which convert the electric pulse into discrete mechanical movement.

2) Types of stepper motor:

A) Variable Reluctance (VR)

-most easier to understand

-consist of a soft iron multi-toothed rotor and wound stator.

-when stator windings are energized with DC current the poles become magnetized.

-rotation occurs when the rotor teeth are attracted to the energized stator poles.

B) Permanent Magnet (PM)

-tin can/ canstock

-low cost and low resolution step (typical step angles 7.5-15 degree or 48-24 steps)

-does not have teeth.

-the rotor is magnetized with alternating north and south poles situated in a straight line parallel to the rotor shaft.

-provide an increased magnetic flux intensity and hence exhibits improved torque characteristics when compared to VR.

                     

C) Hybrid (HB)

-more expensive than PM.

-better step resolution, torque and speed.

-typical step angle: 3.6 to 0.9 degree ( 100-400 step).

-combines the best features of both VR and PM.

-multi-toothed like VR and contains an axially magnetized concentric magnet around its shaft.The teeth guide the magnetic flux to preferred locations in the airgap.

-increase the dentent, holding and dynamic torque

D) Disc Magnet Motor (special type)

-designed as a disc with rare earth magnet.

-low inertia and optimized magnetic flow path with no coupling between the two stator windings.

3) Most commonly used stepper motor: PM and HB

-if a designer is not sure which type best fit, he should first evaluate the PM type as i it is normally several time less expensive.

4) Size and Power:

-in addition to being classified by their step angle, stepper motor are also classified according to frame sizes (diameter of the body of the motor)

-size 11 (body diameter 1.1 inches)

-size 23 (body diameter 2.3 inches = 58mm)

-torque output from a motor of a particular frame size will increase with increased body length.

-power levels for IC driven stepper motors typically range for very small motors (10W) up to larger motors (20W).

-maximum power dissipation P = VI.

-eg: a size 23 stepper motor rated at 6V and 1A per phase. Therefore with two phases energized the motor has a rated power dissipated of 12W.


5) Application: A stepper motor can be a good choice whenever controlled movement is required or control rotation angle, speed, position and synchronism.
- eg: printer, plotters, highend office equipment, hard disk drivers, medical equipment, fax machine, automotive.


6) Advantages:
-the rotation angle of the motor is proportional to the input pulse.
-the motor has full torque at stand-still ( if the windings are energized.)
-precise positioning and repeatability of movement since good stepper motors have an accuracy of 3-5% of a step and this error is non cumulative from one to next.
-excellent response to start/ stop/ reverse.
-very reliable since there are no contact brushes in the motor. Therefore the life of the motor is simply dependent on the life of the bearing.
-response to digital input pulses and provides open-loop control, making the motor simplet and less costly to control.
-low speed synchoronous rotation with a load that is directly coupled to the shaft.
-wide range of rotational speeds. ( v proportional to f)


7) Disadvantages:
-resonances can occur if not properly controlled.
-not easy to operate at extremely high speeds.


8) Operation of steeper motor:
-open load control.No feedback information about position is needed.
-eliminates the need for expensive sensing and feedback devices such as optical encoders.
-position simply known by keeping track of the input step pulses. 

多重标准

在辩论场上，一旦被对方质疑采用了双重标准，紧咬不放时，己方队伍的印象将大跌。
但是，在生活上，原来我们一直持着双重，更甚至是多重标准。而我一直没发现，直到一位学长这样说：“世界上的人事物形形色色，如何能够用同样的标准去评断每件事物？”
当时我嗤之以鼻，这不就是见人讲人话，见鬼讲鬼话嘛~~ 墙头草一个。
过了不久，才发现他说的话不无道理。不谈那些专打小报告，只为自己利益着想而持多重标准的人，这无意义。
我想谈的是，有些励志、规劝的道理，乃至社会上的秩序维持其实都存在双重标准。

先从个人修养看起，“严以律己，宽以待人”。对待自己必须严格，对待别人则要宽容。这话是双重标准，却是赞美人的一句话。如果标准统一，理应宽己宽人或严己严人。

再如之前看到一段话，很有道理，其实也是双重标准。“生气的时候，先想想是谁错了。若是自己错了，有什么资格生气？若是别人错了，又为什么生气？”前一句采用的生气标准是看谁做错了，谁没资格生气。那么理应，谁做对，谁有资格生气。但是后一句采用的生气标准却是，谁做对了，谁没有理由生气。
而若把标准调换，“若是自己错了，为什么不生气？若是别人错了，他有什么资格生气？”同样是双重标准，却是截然不同的效果。

来到学校，若是老师采取同样的标准要求每一个学生，不管学生的资质、性格、问题等，这时会弄巧反拙。

再看向整个社会，给予富人与穷人同等的机会，这本身就是不公平。

另外，法律面前人人平等，犯同样的罪，刑罚的轻严却会依据犯罪的动机、文化、知情与否而改变。这时，硬要一种犯案、一种刑罚的统一标准，其实最不尽人情。

（续）