MICROMECH LTD working with supply partner PM Bearings B.V. Holland

Compact and High Precision XYZ positioning stages for Light Source and Laser Research applications.

Introduction and overview:

In most Beam-Line and Laser Target sample manipulation arrangements the installation volume available is normally very challenging as more data capture and monitoring equipment is required in one specific location. For IN-VAC research it is even more restricted in order to keep the evacuated volume as small as possible.

The current trends towards sub-micron and nanometric resolutions also establish a requirement for very solid and stiff mounting arrangements in all three axes, such that meaningful and accurate data will be retrieved.

Sub-miniature mechanisms with moving parts are by nature quite delicate and often flimsy, so difficult to ensure the degree of mechanical stiffness needed for the above resolution and good repeatability.

Positioning Stage Technologies:

The most common motion-control technologies being employed for linear transport stages can be listed as follows:-
a) Stepper motor driven lead screw.
b) Stepper motor driven ball screw.
c) DC brushed motor driven lead screw
d) DC brushed motor driven ball screw
e) Brushless motor driven lead screw
f) Brushless motor driven ball screw
g) DC Linear motor
h) Ceramic Piezo Servo 40Khz ( tuned oscillatory)
i) Piezo Legs Stepper 3Khz ( walking beam)
j) Piezo Flex
k) Piezo Saw-tooth stepper.
l) Voice Coil DC Servo.

Comparative cost:

All of the above work well in principle and can be designed to achieve 100 nm resolution or ‘move sensitivity’…. ..or better in the case of Piezo technology.

Apart from the difficulty of installation volume, the other significant challenge in most research projects is budget.

So all of the above can be assigned a ‘typical’ price category as follows:-

Items a) to d) Lowest cost.

Items e) and f) Medium cost.

Item g) High cost.

Items h) to i) Highest Cost.

Comparative size:

The following categorises the installation size from ‘most compact’ to ‘largest’ with some indicators on the aspect of the installation volume required:-

j) Piezo Flex . Most compact but very small linear range. Microns.

i) Piezo Legs. Very compact with linear range up to 150mm. 10 to 15 mm /sec

h) Ceramic Piezo Servo 40Khz . Quite compact with linear range 200mm.
200mm /sec.

l) Voice Coil DC Servo. Varies with linear force and stroke required.

k) Piezo Sawtooth stepper. Quite narrow, but long in aspect. Low force and slow. 10mm per sec.

g) DC Linear motor. Quite compact . Travel unlimited. 7000mm per sec.

a) Stepper motor driven lead screw. Short motor body and nut. 20 to 50 mm per sec depending on screw pitch.

e) Brushless motor driven lead screw. Short motor body and nut. 50 to 100 mm per sec depending on screw pitch.

b) Stepper motor driven ball screw. Short motor body but longer ball-nut. 100 to 200mm per sec. Depending on screw pitch.

f) Brushless motor driven ball screw. Short motor body but longer ball-nut. 200 to 500mm per sec. Depending on screw pitch.

c) DC brushed motor driven lead screw. Long motor body but short nut. 50 to 100mm per sec. Depending on screw pitch.

d) DC brushed motor driven ball screw. Long motor body but short nut. 200 to 500mm per sec. Depending on screw pitch.

Notes:
1) All piezo and voice-coil driven systems require a linear encoder for position control.

2) All DC motor driven systems require a linear or rotary encoder for position control.

3) Stepper systems do not require any encoder unless resolution and linear precision required dictates the need.

Electrical Drive systems and Motion Control interfaces.

All of the above ‘motion providing’ devices will require a power drive plus a position controller interface to effect precision positioning.

a) Stepper Motors.
These are the most popular devices used in research applications as they are stable when the supply is off or there is no command pulse train. The stepper drives are also lowest cost.

b) DC Brushed Motors.
These are most popular in miniature devices as the motor body size can be as small as 8mm Dia. The drives are relatively low cost. A rotary encoder is required for position control.

c) DC Brushless Motors.
The drives required for these are more expensive and normally require an Encoder feedback for position control applications.

d) DC Linear Motor.
The drives required for these are virtually the same as for the DC Brushless rotary motors. An Encoder is always built into the assembly for phase commutation but a more precise linear encoder may be required for a particular application.

e) Voice-Coil Linear servo.
A special power amplifier is required and a feedback encoder.

f) Piezo Technology.
All piezo technology motors require a specific type of drive amplifier for their individual design. The simplest is the Piezo Legs as it is very similar to a stepper motor drive technology and low cost to produce.

g) Motion Controllers.
Most Research establishments use versatile multi-axis ( 4, 8, 12 or 16 axis) controllers which can be software configured to drive Stepper Motors, Brushed or Brushless servos. They cannot drive Piezo devices though so an intermediate piezo amplifier or controller is required to be commanded by step and direction or an analogue set point form the motion controller. So this extra cost makes Piezo systems the most expensive to use.

Innovative compact XYZ Stage products from Micromech Ltd.

From the information above it can be seen that the ‘lowest cost’ product is given by the Stepper Motor driven lead-screw stages or the DC Brushed motor lead screw stages. In the case of the DC motor types an additional high ratio gearhead is normally included to increase the torque and reduce the output shaft speed to a more useful level. This just makes the motor assembly much longer. So although the linear stages themselves may be quite compact the length of the overall assembly is often doubled by the long, in-line motor assembly with gearhead, coupling and encoder, extending from each axis element.

In collaboration with our Dutch supply partner PM Bearings we have devised the compact XYZ stepper stage using an integral ‘Yoke Thrust’ arrangement which is mounted in parallel to the stage footprint. A precision linear encoder is also integrated plus small ‘end of travel’ limit switches.
The illustrations below show a 50mm x 50mm x 50mm travel XYZ stage and the smaller 10mm x 10mm x 10 mm travel XYZ stage.

Both these designs take up the smallest cube volume possible as there are no motor bodies extending from the stages.

The thrust elements are spring-preloaded so there is no backlash and the linear encoder can give a move resolution or sensitivity of 0.1 micron. So these will be good for achieving reliable 1micron integrity over the travel range.

If specific accuracy is not required the stepper motor open loop resolution without linear encoder can be as small as 1.5 micron per full step ( 0.3 mm pitch) with more subdivision at half step or micro-stepping mode although it must be emphasised at this point that micro-stepping is not a reliable position division process beyond one full step ( 1.8 degrees).

This open loop performance is still very impressive.

Fig A. Partial Solid view of XYZ Stage with 50mm x 50mm x 50mm Travel.

PM Bearings XYZ Stages

The linear stages used in this XYZ assembly are high performance cross – roller tables produced by PM Bearings with payload capability far exceeding the linear thrust of the stepper motor. These small thrust motors can provide up to 100 N force at slow speeds with phase current levels of 0.49 Amps Bipolar from a 24V DC powered Stepper Drive. A Numerik Jena linear encoder is used in this assembly from Heidenhain.
Vacuum compatible versions can be built up to 10^-7 Torr or millibar by selection of suitable encoders and thrust motors.

Fig B. GA of XYZ Stage with 50mm x 50mm x 50mm travel.

PM Bearings XYZ

Fig C. GA of XYZ Stage with 10 x 10 x 10 mm travel.

PM Bearing XYZ

The same 20mm square frame ( Nema 8 ) Hybrid Stepper Thrust motors are used in this smaller assembly but the linear encoder is a Micro-E type due to the miniature integration requirement.

Connectors and cable management.

Encoder cables are brought out independently as they may be routed directly to the motion controller. Each axis has a sub D connector on a short cable for handling the Stepper Motor and Travel End Switch connections.

As these stage assemblies are custom built they can be supplied in 1 axis 2-axis or 3-axis versions. Larger footprint X-Y stages can be designed having a common centre plate for lower profile and increased lateral stiffness , but still combining a thrust stepper yoke assembly to specifically meet the application loading requirements.

For all enquiries about these special units please contact: MICROMECH SALES OFFICE on 01376 333333

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