Motorized Microscope Stages

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1 Welcome to the first in a series of training papers for the Linear Shaft Motor. This paper will discuss Motorized Microscope Stages. We will discuss what they are, and what the current methods are for driving them. We will also discuss the subject of piezoelectric motors. This is intended as an overview so you can talk to customers intelligently about this subject. The principles discussed also apply to many other types of applications. While this article is not intended to be sent to customers, we have made available excerpts that can be used to generate interest. To the best of our knowledge, everything in this document is true and accurate. Accuracy vs. Precision Accuracy is not the same thing as precision. "Accuracy" is the ability to get closest to the true point you are expecting. "Precision" measures the ability to repeat a move and get back to the same point (regardless of accuracy). There are many ways to explain this. To the right you will see a graph that helps illustrate this. A watch dial is graduated in l/5th of a second interval between each minute mark. Thus the watch is precise, but unbeknownst to the person using the watch to observe the time, the watch is five minutes slow. Reading a time to the nearest 1/5th second with this watch, while being precise, is ridiculous because the time reading is five whole minutes away from the true time - the watch is inaccurate. Motorized Microscope Stages Introduction Microscopes have been around since the 17th century. Since then, there have been continuous changes to make them better. Some of the ways have been increase the viewable area, increase speed at which objects are located, and increase throughput. The microscope stage has undoubtedly been a major player in making this possible. With the advent of the motorized microscope stage a number of things became possible that were impossible, or extremely time consuming, before. This includes eliminating the possibility of lateral stage drift, gathering sequential images of two or more view fields during timelapse sequence acquisition, and large scale microphotography. The drive mechanisms for microscope stages can be divided into three basic categories: manual, stepper motor, and piezoelectric. Manual drives and stepper motor drives have been viewed as the gold standard. They are able to move over long distances in contrast to piezoelectric Figure 1 transducers which move over very short distances, but with much higher resolution. The travel range of a typical motorized stage is 2 to 4 inches in both the x and y directions.

2 The ideal results for a microscope stage would be a command movement line as shown by the graph in Figure 1. Let s look at the basic make up of each of these standard drive systems, what are their advantages and disadvantages. Stepper Motors Stepper motors have been used on motorized stages for a long time. At one time, they were considered to be the gold standard in the motorized stage industry. Most stepper motor stages are targeting resolutions of 1µm or larger, and only a few systems using microstepping claim 10nm. These systems are typically limited to 100mm/sec or slower for speed, while a few low resolution systems can get as fast as 250mm/sec. Below is a picture of a standard stage. Steppers offer a number of advantages - there is no drift or jitter with a stepper, they are very durable and very precise in full step mode, and there is no need for an external feedback system. When deenergized a stepper stage, if set up properly, can be operated as a manual stage. Figure 2 shown by the graph in Figure 2. Some of the disadvantages of a stepper stage include the noise issue inherent to stepper motors. A stepper stage can be set-up for speed or resolution, but not both. If an encoder is not used, it is possible for the system to become highly inaccurate in its movement due to the system loosing steps as Another common practice is to make use of high rates of microstepping. This is done for one of two reasons: 1. The user feels that they can reduce the cost of the system by using a less precise leadscrew and keeping the same resolution, or 2. Increase the resolution of the system by a factor of 256. While this may give the false sense of Figure 3

3 higher resolution, it is highly inaccurate. The main reason is that microstepping, while very accurate at the full step of the motor, is very imprecise for the 254 in-between steps. This is shown by the graph in Figure 3. When you add this with the fact of loosing steps, you can possibly end up with a very imprecise and inaccurate system. Current draw on a stepper system is quite high. A two stepper motor stage can draw 4 amps or more, and 100W Piezoelectric Piezo motor is a generic term that refers to any motor that operates on the Piezoelectric Effect. Piezoelectric effect states that when a material is compressed, it will produce a voltage proportional to the applied pressure. The converse is also true, when an electric field is applied across the material, there ia a change of shape proportional to the applied electric field. This effect is extremely small in naturally occurring minerals, but newer manmade materials can expand up to 1%. Typically, linear extensions of up to 200 µm are obtained when suitable voltages are applied to the appropriate ceramic geometries. Several families of ceramics and types of devices have been developed. Piezoelectric ceramics (the most common material used) fall into two categories, hard and soft. Hard piezoelectrics have curie temperatures above 300 C and limited dimensional changes; while soft piezoelectrics have lower curie temperatures and greater dimensional changes but can depole (or loose their piezo effect) easily. Traditionally, high voltages (up to 2000 Vdc) have been applied to stacks of thin slices of piezoelectric materials to produce the required extensions. Note that the increase in length is approximately linear with the applied field and that there is some saturation at higher voltages. Also there is pronounced hysteresis which is greater with the soft piezoelectric material. Although high voltages are used, power consumption is low and almost no energy is consumed in maintaining a fixed position with a fixed load. Piezoceramics can respond rapidly to changing input voltages (microsecond time constants), and the positional resolution is limited only by the noise of the power supply. The need for voltages in the 1 to 2 kv range has restricted their utilization because of the cost, electronic noise, reliability, and safety issues involved. Piezoelectric devices are very prone to hysteresis. As shown in the chart in Figure 4 if the voltage applied to a piezoelectric device is increased from zero, the expansion vs voltage curve follows path 1. If the voltage is decreased, path 2 is followed. Path 3 is followed when the voltage is increased again. If the voltage is cycled between two fixed levels, the extension follows the closed loop defined by paths 2 and 3. Figure 4 The generally applicable technique to correct for this is to use piezoelectric actuators with closed-loop feedback control of their extension. Hysteresis and creep are then of no importance. More recently, there has been a move to Piezoelectric motors (sometimes called piezo linear motor or Ceramic Servomotor). Piezoelectric motors can be divided into two groups: ultrasonic motors, also referred to as resonant motors, and step / walk motors. Both can, in principle, attain unlimited travel, yet they are very different in their design, specifications, and performance.

4 Piezoelectric motors use a piezoelectric ceramic element to produce ultrasonic vibrations of an appropriate type in a stator structure. The elliptical movements of the stator are converted into the movement of a slider pressed into frictional contact with the stator. The consequent movement may either be rotational or linear depending on the design of the structure. Linear piezoelectric motors typically offer one degree of freedom, such as in linear stages. Rotating piezoelectric motors are commonly used in sub-micrometric positioning devices. These motors produce a force of 4N or less, depending on the model. Larger mechanical force can be achieved by combining many motors. Piezoelectric motors have a very linear force velocity curve, as shown in Figure 5 below. Their force velocity curve is very similar to that of a stepper or DC motor. Also, their max speed is between 235 to 300 mm/sec, and Piezoelectric motors cannot deliver 100% of the rated performance at 100% duty cycle. (See chart below (Figure 5) which is a published by Nanomotion for their HR1.) Figure 5 Piezoelectric motors have a number of potential advantages over conventional electromagnetic motors. They are generally small and compact when compared with their power output, and provide greater force and torque than their dimensions would seem to indicate. In addition to a very positive size to power ratio, piezoelectric motors have high holding torque maintained at zero input power, typically 80% of their driving force, and they offer low inertia from their rotors, providing rapid start and stop characteristics. Piezoelectric motors usually do not produce magnetic fields, and they are not affected by external magnetic fields. Because they operate at ultrasonic frequencies, these motors do not produce audible sound during operation. Piezoelectric motors do have some disadvantages. These disadvantages include the need for high voltage, high frequency power sources, the possibility of wear at the rotor / stator interface

5 (which tends to shorten service life), and they also introduce a large amount of side loading (typically 5 times the linear force produced) due to the fact that they drive the load from one side and the loading to make the ceramic on the ceramic drive work. Also, there is no standard drive electronics for piezoelectric motors and cost of the electronics is quite high. They are also somewhat sensitive to thermal transients. Piezoelectric motors require a system burn-in of 4 hours before it can be operated or anytime after a motor has been moved, replaced, or serviced. The manufactures of these motors state that the ceramic tips require a four (4) hour burn-in to wear to the point where there is full contact between the drive tip and the running surface. Nanomotion states that after this four hour burn-in period the motors will last 5 years. Linear Shaft Motor The Linear Shaft Motor is a very good option for the microscope stage world. It offers a number of advantages. These include cost, size, speed, high precision, and accuracy. Advantages include: Cost 1. The ability to use commercially available servo drivers, and not being tied into just one type or manufacturer. If they have controls designed for stepper motors when they use a digital servo driver, then there is no need to change their controls because these digital servos will accept step and direction. 2. Higher speeds are able to be achieved while retaining high precision. At the same time, extremely high precision low speed uniformity and high repeatability are possible. 3. Because of the non-contact design, no lubrication or adjustment is necessary. They are Eco-Friendly - no noise, no dust. 4. The setup and operation time is extremely short and very simple compared to either of the other technologies that we have discussed. This is due to the non-critical alignment properties of the shaft and forcer, and the fact that there is no required burn in period. 5. There is a much simpler alignment and QC period. The power requirements are much lower than that of ball screw systems. When talking with a customer, educate them in the true cost of building a microscope stage and explain that by using the Linear Shaft Motor, it is possible to lower cost over the entire process. True cost We have all been there, we are making the sale and the customer asks: How much does this cost? I only pay $X for my ball screw or the Piezo motor will cost $X. Explain their true costs. The component cost is only one part of the total cost of the system. There are also costs of design, procurement, machining, quality control, building, bill of material, support/maintenance, and life-time of the product. Let s look at each of these. Design cost Designing in the Linear Shaft Motor is very simple. The calculations are fairly simple, there is no need to take into consideration the mass ratio or inertia of the motor. There is no need to do calculations to figure out support length or how much shaft you need to get a section of usable stroke.

6 The published specs are True specs, no need to read all the small print to see why the specs will not work for you. There are no special considerations that need to be taken when designing in the Linear Shaft Motor. The tolerances of mounting are very loose when compared to other types of drive systems. There are no long discussions about whether to give up speed or performance or resolution in the design of the product. Also, the total number of supporting components that also require engineering time to select are much lower than with other types of drive systems. Thus the amount of engineering time required to design in a Linear Shaft Motor is less than other types of drive systems. Procurement cost, Quality control cost, and pre-build cost The savings really comes down to the number of parts needed to build the system and the complexities of these parts. A ball screw, lead screw, or belt type of drive system requires many parts while a Linear Shaft Motor system only requires two. Each part requires a PO before the system can be purchased or built, every part must be inspected to verify its quality, and all the parts need to be arranged and stocked to allow for an efficient build of the final product. Since the Linear Shaft Motor consists of only a magnetic shaft and a forcer, all three processes are simplified. Machining cost The savings here comes from the fact that the Linear Shaft Motors have such a large air gap (0.5 to 1.75mm), and that this air gap is not critical. All the other types of drive systems we have discussed require a lot of machining to maintain the tolerances that they need to operate. Ball and lead screws require true concentricity between the screw and the nut, and they require high levels of parallelism between these and the linear guide. Piezoelectric motors also require high levels of parallelism between the linear guides and the alumina plate. There is also a savings in that the number of supporting parts that must be machined is much lower for the Linear Shaft Motor. Build cost The savings here comes from two things: (1) the total number of parts being put together is less and (2) the amount of time required to do alignments is greatly reduced or eliminated. Also, there is no required burn-in period for the Linear Shaft Motor as there is for the piezoelectric motors. Bill of material cost (BOM) In some cases it is even possible to have a savings in the BOM cost. What needs to be looked at is the total cost of all the parts that will be drive system specific. For example, there is no need to look at the linear guide since it will be required no matter which drive system is used, but we would add in the cost of the driver since each motor will require a different type of driver. Below is a sample of such a comparison.

7 LSM vs Ball Screw This is for a system with a target of 50nm, 100mm usable stroke, and required continous force is 2N. Note: Prices Based on Misumi website as of 11/17/06 Ball Screw LSM Description Part Number Cost Description Part Number Cost Precision Ball Screw BSX $ Shaft Motor S160D-100 $ Support Unit Fixed side BSWR8 $ Shaft Support SHA16 $ Support Unit Support side BUNR8 $ Driver HAR2/100I $ Stopper BSTP8 $ Encoder, Analog RGH24B15L00A $ Bracket for Ball Screw BNFB304 $ Total $ 1, Screws CB4-8 $ 0.60 Coupling MCGS16 $ Motor PJB42S41B14 $ Driver $ Sub-total $ Encoder 50nm RGH24H15D30A $ Total $ 1, LSM vs Piezo This is for a system with a target of 50nm, 100mm usable stroke, and requared force is 2N cont. Note: Prices Based on ED0 Price List as of 11/17/06 Piezoelectric LSM Description Part Number Cost Description Part Number Cost Piezoelectric Motor EP $ Shaft Motor S160D-100 $ Piezoelectric Driver JEP 400A $ 1, Shaft Support SHA16 $ Encoder 50nm RGH24H15D30A $ Driver ZDR300EE12A8LDC $ Total $ 2, Encoder 50nm RGH24H15D30A $ Total $ 1, If you have any questions, please contact Jeramé.