Team Tiny Tools have specific requests from their sponsor, Aerotech Inc.  The following are the intended parameters that the meso-scale linear motion stage is expected to deliver.

·  The total travel of the carriage structure is to be 25mm, 12.5mm in either direction from the home position over the base structure.
·  The resolution (positioning accuracy) of the carriage is to be on the order of microns.
·  The overall dimensions of the stage structure are to be 45mm x 31.5mm x 12mm.
·  The optimal requested speed of the carriage is to be 5mm/s.
·  The carriage is to be able to support a load of 100g.

Base and Carriage

The final dimensions for the stage are to be 45mm long, 31.5mm wide and 12mm tall.  The stage housing will be comprised of a stationary base and a moveable carriage which will slide linearly along two railway bearing systems.   

The base of the stage provides stability for the linear motion stage and will provide reference shoulders for mounting the linear bearing railways and a platform for installation of the motor.  The quadrature encoder for position feedback will also be mounted to one side of the base.  The footprint of the base is 45mm by 31.5mm, roughly the size of a typical 9 volt battery.

The two bearing systems are composed of a 1mm wide rail that will be permanently mounted to the floor of the base structure, against reference shoulders 0.30mm tall.  On each rail will be two trucks which will translate the carriage effortlessly along the rail.  The truck contains many miniature ball bearings which alleviate friction and facilitate the fluid linear motion of the trucks.  A model of a bearing system is represented below.  On the top side of each of the trucks are threaded taps which will allow the carriage structure to be firmly mounted with size M1 screws.  Railstops will be installed to prevent the trucks of the linear bearing railways from extending past the end of the railway.

The carriage is a rigid platform for mounting auxiliary components.  Whatever the application may be: machining spindles, focusing lenses, sensitive measurement equipment or even another linear motion stage to provide bi- or tri-axial positioning.  The carriage can be fabricated with tapped, threaded holes similar to those on the trucks of the railway system for the mounting of any auxiliary equipment.  Endcaps will be mounted with M1 screws to either side of the carriage and will contain a cavity on the inside facing the motor for insertion of a radial bearing to facilitate in negating the rotational motion of the motor screw and allow the translational motion to drive the carriage on one linear axis.  An encoder strip cap will also be mounted on one side of the top of the carriage center to clamp down on the plastic strip that the quadrature encoder must read.  The shoulder which the strip is mounted to provides the correct elevation for aligning the strip with the groove in the encoder.  The strip will be cut down to size and have holes put in it to allow for installation.  The strip cap will sit on top of the plastic strip and will be bolted down with M1 screws.

Upon full extension along the motion axis, the linear motion stage will be capable of delivering 25mm of total travel.  The carriage is capable of extending 12.5mm in either direction of the stationary base structure.  This method of translation allows for the greatest distribution of the weight over the permanent base.  The stage will be designed to support a relatively large payload for its size, over 100 grams.  The maximum weight which the stage will be able to translate will be governed by the output force exerted by the internal motor.

Solidworks is the 3D modeling program that will be used by Team Tiny Tools.  In addition to 3D visual renditions, prototypes of the base, carriage, and auxiliary structures have been fabricated in the Learning Factory.  Utilization of the Stratasys FDM-2000 Rapid Prototyping machine allowed the opportunity to physically assess the design of the housing structures, allowing for insight into the actual fabrication of the stage out of aluminum.

 

     

Motor

There are a number of possible design options to consider regarding the motor used in the linear motion stage.  Because of the size constraints of the entire package, it is essential that the motor be as small and simple as possible while performing all of the necessary actions.  Each motor option has its advantages and disadvantages, offering a number of considerations for the application. 

            

One option available for the linear stage is worm screw type motor called a "squiggle" from New Scale Technologies.  The squiggle motor is a very precise and powerful motor that is compressed into an extremely small package.  This motor option will allow the stage to travel up to 30mm with a velocity of 5mm/s and a precision of 0.5μm.  This specific size of motor can handle a load of over 200g while experiencing virtually no backlash.  The squiggle motor offers greater power efficiency, higher reliability and ten times better precision than that of an electromagnetic motor of comparable size. 

Another motor option for the linear stage is the use of a linear shaft motor offered by Nippon Pulse America.  This type of motor operates on the same principle as a rotational electromagnetic motor.  The linear shaft motor uses a shaft consisting of a number of reversed polarity magnets.  The induction coil or forcer is the actual motor body itself.  The shaft slides through the motor housing as voltages are applied to the coil.  This motor offers 20mm of total travel at a precision of up to 0.14nm.  The linear shaft motor is larger than the “squiggle” and can only handle a maximum load of 125grams.

A third drive option for the linear stage is the use of a rotational electromagnetic motor coupled with a rack and pinion drive mechanism.  The electromagnetic motor can be found in a package comparable in size to each of the motors discussed above.  The motor provides rapid movements with moderate precision when compared to the screw type motor.  Some disadvantages to this type of system are the shaft play in the motor, the play between the pinion gear and the rack and the friction between the moving parts in the system.  When compounded, the efficiency and precision of the system decreases to an unacceptable level for the system. 

The final motor choice decision can be seen in the concept selection matrix displayed below.  Motor selection was based on a number of factors including: resolution, size, velocity, price, availability, load rating, ease of use, and total travel.  Since the linear stage is greatly constrained by size, the size of the motor held more weight than any other factor in the matrix.

As shown above, the best motor choice for the linear stage is the “Squiggle” from New Scale Technologies.  The squiggle offers a number of important qualities not currently offered by the other motor manufacturers.  The squiggle is the most compact of the three motors, yet still offers more linear travel than any of the others.  The motor can also handle a larger load and is fairly easy to operate in comparison with other motors of its size. 

The motor will be center mounted within the base of the linear stage using a machined mounting block and a strong adhesive.  The radial bearings, which are pressed into the endcaps of the carriage, will allow the screw of the motor to turn freely while moving the stage in a linear direction.  The motor has been ordered and is in route to Team Tiny Tools, but may take until the last week of November to arrive.  When the motor arrives, final fitment of each component into the stage will begin.  This will leave the team time to troubleshoot any problems that may be found in the system.

Position Sensor

In order to control the motor’s output and the carriage’s linear motion in a precise manner, a position sensor must be selected.  This position sensor will output a voltage that is proportional to the distance traveled.  There are many ways to perform this position analysis, however, due to the miniature enclosure of the mechanical and electronic components, this position sensor must fit in a tight space while achieving the desired stroke of 10-25 mm.  Another important design criterion for this selection is resolution.  This, however, is less important due to the fact that we are trying to get precise movement with low cost.  Adding money to the project is a quick fix for coarse resolution.  The physical dimensions will most likely dictate what sensor is used in the final prototype.

Team Tiny Tools has narrowed the search to 3 types of linear position sensors.  The first type that was considered was the LVDT (linear variable differential transformer).  It is the most broadly used variable inductance transducer in industry. It is an electro-mechanical device designed to produce an AC voltage output proportional to the relative displacement of the transformer and the armature.

The second type of sensor that seemed feasible was the optical encoder module and linear strip.  US Digital manufactures sensors that would work in our project.  The EM1 module, along with an encoder strip has been ordered for testing.  These modules are designed to detect rotary or linear position when used together with a codewheel or linear strip. The EM1 module consists of a lensed LED source and a monolithic detector IC enclosed in a small polymer package.  The EM1 module provides digital quadrature outputs.

Another option for the position sensor is a product from PC Midwest.  It is the LD50 Non-Contact Linear Displacement Sensor.  This sensor is comprised of a rail that is dynamic and attached to the carriage along with a stationary sensor.  This type of sensor uses the Hall Effect for its sensing technology.  Measuring the strength of a magnetic field is the key fundamental in Hall Effect sensing.  The dimensions of this device are currently being pursued and will be a deciding factor in what sensor is used in this project. 

After completing a weighted selection matrix for the 3 possible options of position sensing it is seen that quadrature encoding is the best option for this project.  Resolution, size, available examples of usage, price, and availability were considered in a weighted manner, and each concept was given a score between 1 and 3, with 1 being the best.

The EM1 transmissive optical encoder module was chosen for implementation in this design. 

The EM1 provides two quadrature signal outputs as well as an index pulse.  Along with the linear 500 count per inch strip, two square waves 90 degrees out of phase will be output from the A and B channels of the encoder.  The reason for the two different waves out of phase is for determining whether the strip is moving through the encoder to the left or to the right.  The figure below shows the strip and how the channel A and B waves differ with left or right motion.  The encoder outputs a 5 V (high) signal when viewing a white section and a 0 V (low) signal otherwise.  The first wave is the channel A output and the second is the channel B output.  This setup is a movement to the left.  As channel A switches from low to high, channel B is observed.  If B is low at this point the movement is to the left and the one is added to the total count.

An oscilloscope and required power supply was connected to this encoder and the output voltage was viewed on the screen.  A manual stage was implemented in this test for smooth controlled movement to monitor.  Developing code for this counting process with the Basic Stamp 2 along with Proportional Derivative control will be the control feedback system that control the Squiggle motor.

Microcontroller

A microcontroller is a type of microprocessor which can be used to control electronic devices through a user defined interface.  It is a single integrated circuit with a central processing unit, input/output interfaces, peripherals (timers), RAM, ROM, and a clock generator.  The integration reduces the amount of space needed as compared to an equivalent system using separate chips.  They generally run one program to achieve a specific task, are relatively low-cost, and provide low-power performance.  For this project, an algorithm will be written in order to control a motor (output) based on information delivered by the linear motion feedback sensor (input).  This motion control will result in moving a small stage according to user-defined parameters.

Several companies offer microcontrollers and respective environments to run them with.  The choice is between the “Arduino” and “BASIC Stamp” series of chips.  The Arduino is new and in the experimental stage.  The input/output printed circuit board is a simple circuit which uses the “Atmega8” processor from Atmel.  BASIC Stamp uses a printed circuit board that runs the PBASIC language developed by Parallax.  The stamps are then loaded onto another board to interface between devices in a system.  It is also possible to network between chips if necessary.  These chips also need a code developed in a compatible programming language.  The user’s code is stored in EEPROM which is also used to store data.  It is also more established as it has been used longer than the Arduino.  Below are BASIC Stamp chips that may be used in this project.

   

We have ultimately decided on using the BASIC Stamp microcontroller system software.  Because the Arduino is still developing, there is much less literature and help available for our project.  Due to time constraints, it will be much easier to use the established BASIC package when trying to read the feedback sensor and output and amplify input motor voltage.  Preliminary research has already led us to a solution including hardware and PBASIC code to read the feedback sensor and store the position of the stage.  Shown below is an example of how to read the signal from a quadrature encoder with the BASIC Stamp using a few auxiliary pieces of hardware.

The motor chosen for this project will operate on a voltage signal sent in from the BASIC Stamp.  The auxiliary equipment required for this include an amplifier and an enable line which dictates when the voltage is sent to the motor.  This voltage signal will be proportional to the linear distance which the stage moves.

In the system, the microcontroller will connect directly into a computer through a USB or serial port.  This will enable the code to run an algorithm from the development environment and control when and how the system reacts.

Testing of the system will commence once all of the ordered parts arrive.  Until then preliminary testing can be done on individual components. 

To prepare for the arrival of these parts, experimental testing will be done with the BASIC Stamp Board of Education Kit to further knowledge of the programming language and capabilities of the microprocessor.  Also, a definite solution must be reached on how to connect the motor to our system.