ONE STEP AHEAD -
WITHOUT STEPPER MOTORS
(A cautionary fairy-tale, but true)
(A cautionary fairy-tale, but true)
Back to the site Home Page Back to "About Autoscan Systems" Back to the Main Autoscan Systems PageINTRODUCTION : THE STORY SO FAR
Once upon a time, in 1979, a company which was to be incorporated in 1984 as Autoscan Systems Pty. Ltd. built its first prototype motorised microscope stage. The clever engineering lads who designed the stage worked out that dc motors with shaft encoders will always be far ahead of stepper motors in performance. Today, many years later, their wisdom still holds and their vision has proven to be true. Here's why :
CHAPTER ONE - BIG BOOTS, BUT LITTLE RESULT
To start with, commonly available stepper motors have a 3.6 degree "step angle". That means that every pulse applied to such a stepper motor will rotate the stepper through 3.6 degree, or to put it another way, one revolution consists of 100 steps. It is possible to obtain stepper motors with finer step angles (1.8 degree or even 0.90 degree), but usually at a premium price, with the accompanying problem of ensuring guaranteed supply. By contrast, our dc motors with shaft encoders generate 500 pulses per revolution, allowing much finer control of motor angle (and therefore stage position).
CHAPTER TWO - MICROSCOPE STAGE, WHERE ARE YOU ?
Next, it is usually impractical for stepper motors to carry shaft encoders, for two reasons : the first is price, and the second is that it is difficult to synchronise the resting positions of the stepper motor and the shaft encoder. The problem is that, without a shaft encoder or similar means of position feedback, the electronic control system relies on "trust" to ensure that the stage has arrived at the desired location. In other words, the controller sends a number of pulses to the stepper motor and "assumes" that the motor has moved the given number of steps. That is not necessarily a safe assumption. If, for instance, there is excessive friction (which will make the stepper motor stall), or the operator manipulates the manual stage movement control, the electronics immediately becomes "lost in space". In addition, minor positioning errors accumulate. By contrast, our systems always "know" exactly where the stage is, because the shaft encoder always "reports" back to the electronics.
CHAPTER THREE - STEPS THAT ARE NOT REALLY THERE
Many stepper motor stage manufacturers claim incredible accuracies for their stages. Such accuracies are actually not achievable in practise. What is often done is that stepper motor stages are controlled by means of a technique known as "microstepping". This technique involves the artificial division, by means of sophisticated electronics, of each step into a number of "microsteps". The fly in the ointment is that such division results in loss of torque. In relatively plain English, this is the inability of the stepper motor to hold its position in the presence of external vibrations. There is also a limit to the number of microsteps per real step, which should not be exceeded, but often is. For instance, one stage we tested had a real motor step size of 20 micrometers, but claimed a sub-micrometer resolution. In fact, step division ratios beyond 10 are inaccurate unless shaft encoder feedback is used. By contrast, our stages have real sub-micrometer position resolution.
CHAPTER FOUR - SHAKE, BABY, SHAKE
Stepper motors have another limitation : speed. If steppers are driven at a certain (slow) speed, they exhibit what is known as a resonant oscillation. This is a vibration superimposed on the motor movement - a most undesirable characteristic. The vibrations are caused by the fact that stepping motors, by definition, move in discrete angular increments. Steppers also have an upper speed limit. Microstepping results in additional undesirable resonant vibrations at higher speeds, and actually reduce the maximum achievable speed of the system even further. These vibrations further impair positioning accuracy. By contrast, dc motors with shaft encoders are able to move quickly, smoothly and accurately to the required position.
CHAPTER FIVE - HOW GOOD IS IT, REALLY ?
Many manufacturers confuse (often in ignorance, sometimes deliberately) the terms resolution, accuracy and repeatability. Just for the record : Resolution is simply the smallest graduated motion increment. It has nothing to do with accuracy. For example, a steel ruler could be ruled with a resolution of 1/1000 inch increments. That does not mean that anyone using it could achieve accuracies of that order ! Accuracy is the ability of the system to "hit the nail on the head", ie. to position the stage at a given location. Repeatability, or precision is the ability of the stage to consistently achieve accuracy on multiple occasions. This characteristic is usually expressed statistically, as a plus-and-minus measure or percentage. Repeatability has nothing to do with accuracy, because the location may be repeatable, or precise, but actually wrong. It's like not hitting the bulls-eye every time ! It is true that microstepping gives greater resolution, but this is usually obtained at the expense of accuracy and repeatability.
CHAPTER SIX - FOCUS ON THE END RESULT
Our stages actually have the focus mechanism built right in - we do not disable the microscope's focus mechanism, nor do we rely on it in any way. Systems which make use of a "rim-drive" or "rubber-band drive" on the focus mechanism must pay the penalty of an accuracy which rapidly deteriorates with use. The microscope focus mechanism is not intended for heavy-duty "Rambo" driving.
CHAPTER SEVEN - OH, WHAT A JOY !
Most stepper motor stage systems can only be moved by using the electronic controller. Our stages can be moved in three ways :
1. With the computer, from one of our programs which, apart from the special-purpose application programs, also include general-purpose stage movement routines and "libraries" of routines, so that users can write their own applications and call on a long list of stage control commands.
2. In what we call "dumb mode" (ie. without the need for a computer), using the system joystick to move the stage in all three axes.
3. In manual mode, by using the manual motion control knobs provided.
CHAPTER EIGHT - BUT WHAT CAN IT DO ?
We are always amazed by, and sorry for, people who will buy a stepper motor stage without software in the mistaken assumption that they can save money by doing their own programming. They do not realise what they are letting themselves in for. Using the most modern software development tools, our new AutoScope program has taken us 4 years of solid work to develop - it currently contains about 24,000 lines of code, and we are still adding new features. (Its DOS predecessor, Alfascan, contained 150,000 lines of code [modern code is more efficient] and took three years to develop). Our systems are no more expensive than the stepper-motor systems, but with the added attraction of being "turnkey" systems - specifically tailored for particular applications. Our customers in various disciplines, located in 20 countries to date, can testify to the fact that our after-sales support and willingness to provide special features are unparalleled. That is why Autoscan Systems Pty. Ltd. is today the world's leading manufacturer of motorised microscope stages. Why not contact us NOW and allow us to suggest a solution for YOUR requirements ?
| Street Address : | 4/293 Bay Street, Brighton 3186 Victoria Australia |
| Postal Address : | PO Box 112, Ormond 3204 Victoria Australia |
| Telephone : | +61 3 9596 8065 / +61 3 9596 8092 |
| Facsimile : | +61 3 9596 8369 |
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| Geographic coordinates : | Lat. 37 deg. 52 min. South, 145 deg. East |
| Melway streetmap reference : | 67 F9 |
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| NEW URL ! : | http://www.autoscan.com.au/ |
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