Proceedings of the Multi-Disciplinary Senior Design Conference Page 3 s1

Proceedings of the Multi-Disciplinary Senior Design Conference Page 3

Project Number: 11565

Proceedings of the Multi-Disciplinary Senior Design Conference Page 3

ITT Mirror Steering System

Andrew Bishop
Electrical Engineer / Matthew Manelis
Mechanical Engineer
Ben Geiger
Electrical Engineer / Nurkanat Suttibayev Industrial Engineer / Katherine Hall Mechanical Engineer

Proceedings of the Multi-Disciplinary Senior Design Conference Page 3

Abstract

The primary goal of the project is to deliver a device that redirects laser beams with high precision, accuracy, and speed using a mirror. The customer, ITT Geospatial Systems, will use the product on satellites and other aircrafts for pointing lasers or tracking targets.

The mirrors used in the system are interchangeable. However, a 3-inch round mirror is to be used primarily in the final assembly by ITT. This waffle mirror is specifically designed to be extremely light in weight and stable in structure, and is provided by the customer. This mirror rests in an elevated horizontal position by an aluminum dowel rod with a flexure portion that allows for slight movement of the mirror without material failure. Movement of the mirror is achieved by placing four current driven force actuators underneath the mirror. There are two axes of motion with a range of 5 degrees on each axis. Capacitive position sensors are each axis for proper feedback.

Through proper design, assembly and testing, this lightweight mirror pointing assembly will meet as many of the required performance specifications as possible.

Nomenclature

FEA Finite Element Analysis

FOS Factor of Safety

FSM Fast Steering Mirror

LIDAR Light Detection and Ranging

PID Proportional Integral Derivative

PCB Printed Circuit Board

RIT Rochester Institute of Technology

Vdc Direct current voltage

INTRODUCTION

The prototype proposed is a beam steering device using a rapidly slewed mirror. At this time, a number of similar optical systems are currently available on the market by suppliers such as Optics in Motion and Newport. The FSMs are primarily much smaller in size (one inch mirror on average), are not designed to perform in a vacuum. They are also associated with an exorbitant cost vary on technical specifications. In fact, a system that closely matches this project’s target specifications is offered for $15,000. [1] The main intention of this project is to build a system that can compete with or outperform similar systems on the market while maintaining a budget of $2,000. The final model will have optimal functioning capacity and will meet preferably all of the high performance parameters specified by ITT.

Aerospace applications include optical sensing of scattered light wherein a variety of parameters of interest can be measured from very far distances with this technology such as temperature, pressure, position, or vibration. Rapid motion of the mirror is required in order to collect the massive amount of data usually needed. Accuracy, precision, and efficient interaction are the most essential attributes of the system to optimize. This specification is monitored through the move time, the settling time and the slew rate (velocity) of the mirror once it is moved to a particular position at a specified angle of tilt. Furthermore, minimal overshoot and oscillation once the desired position has been attained is essential for accuracy. Seeing as fast time, quick speed, and accuracy are so critical, ITT did advise that their slew rate and settling time specifications were high goal to reach intentionally.

Once our product is supplied to ITT with our design specifications, ITT will alter the system adding their proprietary features for its confidential application. The product for our purposes is therefore supplied ±24V via a common power supply since it will first be used in laboratory applications by ITT. The system will be used to gather information at very long distances and, consequently, a small tilt range of movement of the mirror is sufficient, broad coverage. The entire system including the PCBs, the actuators, sensors and mirror should be safely contained in a cylindrical ~ 3-inch diameter enclosure for protection. All of the specifications fully detailed in Table 1 are important parameters to consider and aim to reach when developing the beam steering device.

Table 1: Performance Specifications

In space applications, it is not really practical to have a system that requires an excess amount of power to run the unit. An aircraft or satellite, for example, will have limited power from the engine or power source that is available for our unit to consume, hence the power consumption limitations. High voltage across the actuators has the potential of inducing high energy arcs in a vacuum, which would be very hazardous. There is also the issue of heat generated from the unit that should be monitored.

Various risks were considered when developing the architecture. Electrical noise would dilute input and feedback signals. This significantly reduces accuracy and is an anticipated very high risk. The signal to noise ratio should be monitored and minimized, as this is one way to measure hysteresis, creep, and sensitivity over life. Staying within the linear range of the voice coil would reduce this greatly as well as feedback and reduced friction.

The components that we have with our system are the mirror, the actuator, and the sensor.

Round shaped mirrors are common for FSM systems because of its evenly distributed mass and mass center location. The flexure type often depends on the application and manufacturer preferences. Furthermore, systems might have additional mirror damping features integrated for precision and accuracy purposes.

Actuators are used to change electrical energy into mechanical energy. There are various types that could be used, but the common application is to provide movement to the mirror with a given controlled amount of electrical input.

Voice coil and piezoelectric actuators are the most practical devices considered as options to use to move the position of the mirror. Voice coil actuators use wire or coil, a permanent magnet, and electric current for propulsion, whereas piezoelectric actuation is based on piezoelectric material that transforms its shape in linear or rotary directions when an electric field is applied. Piezoelectric devices, however, are problematic when supplied high voltage in a vacuum environment, and voice coils have linear behavior. Conclusively, it is extremely crucial that the prototype/device is fast, accurate and precise and therefore the voice coil design would be the best actuator device for its attributes.

The sensors are an essential component of the system; they provide the feedback of the system to the input. It is important to first note that the system is driven by constantly monitoring the position of the mirror. The desired angle of each axis as a function of time in essence is an input providing the position of the mirror mount, which is the vertical distance the actuator moves.

Options of sensors include four main types: memsgyroscope, inductive, capacitive, and optical sensors. The memsgyroscope is not accurate when measuring position, as the integrated signal does not come out clear. Implementation of the inductive is difficult to define, and optical sensors are expensive. Capacitive sensors are used since they are all desired qualities: accurate, very feasible, and reasonably priced. Capacitive sensors work by monitoring the position of the mirror through the voltage difference between two small conductive plates.

The electrical components include the power supply as well as two actuators and two sensors per axis. The circuit boards include the drive PCB which serves both axes, the PID controller PCB, sensor PCB, and power regulators per axis.

The most important considerations in the mechanical realm are the spring constant of the flexure on the dowel rod holding up the mirror as well as protection or containment of the mirror and all components.

DESIGN PROCESS

To clearly define the problem, it was important to first identify the customer needs and specifications. After meeting with Michael O’Brien, our representative member from ITT, a list of required performance specifications were established, as seen in Table 1.

From the customer interview, it was determined that rapid movement with minimal overshoot and high precision and accuracy to each position is essential. More specifically, the quantitative goal is to obtain a slew rate greater than 50 degrees/sec, a settling time of better than 90% in 80 milliseconds, and total power dissipation of less than 5 watts per axis. The system includes optical sensors to provide feedback, and four specifically fabricated actuators to reach all of the desired parameters. These actuators are to be voice coil driven, not piezoelectric, as requested by the customer.

All components of the system will be contained in a 3-inch diameter cylinder, including the actuators that will provide the mechanical force to move the mirror position. Each axis contains two actuators and two sensors in series attached to the drive PCB and sensor PCB.

Once these parameters were established, a house of quality was created in order to determine which specifications were most important to consider during the design process. A number of concepts were then generated, and a concept selection matrix was used to determine which design best met the performance specifications. The final upper half design is shown in Fig. 1, which includes the mirror, mirror mount, flexure spring, and actuators.

Figure 1: Upper Half Assembly

Each system component was carefully designed in order to be integrated into the system. Various computer models were created to simulate each part’s functionality and performance. This was especially crucial, as many components of the system rely on its adjacent counterpart.

The most important components of our assembly are the actuators, as this is what drives the system. Therefore, the design process began with this piece.

Two different types of voice coils were considered for our project. An under hung voice coil is designed so that the coil is larger than the field area in order to preserve linearity. The other type is an over hung voice coil, which has a limited range of motion, but produces more force per unit of current. Because the total vertical motion of the voice coil is only about 1 mm, it was decided that an under hung voice coil would be the best choice, as it would help keep power consumption to a minimum.

The final design of each actuator is a current controlled force device. It consists of an iron outer shell to generate the magnetic field into the right space, a magnet to produce the magnetic field, and a coil of wire to produce an opposing magnetic field which in turn produces a vertical force.

A neodymium magnet and iron core are stacked on top of each other and placed in a larger iron core bored cylinder. In this space, coils of wire are wrapped around the neodymium magnet. As current is induced through the wire, a magnetic field is produced due to the strength and proximity of the magnets. A rigid object is attached between the coils and the mirror mount to transfer this vertical force. The position of the mirror can consequently be manipulated by altering the current through the wire.

The voice coil actuators were modeled using COMSOL Multiphysics, a computer program that has the ability to model magnetic fields. Specific dimensions, such as wall thickness, magnet diameter, and overall height, were altered in order to create the greatest magnetic field possible. The final design has 300 windings and a coil radius of 6.5 mm. The selected design and its resulting magnetic field can be seen in Fig. 2.

Figure 2: Voice Coil Magnetic Strength

From COMSOL, it was determined that a 0.38 Tesla field exists when a 100 mA current is induced. Using Eq. (1), where I is the current induced, B is the magnetic field, N is the number of windings, and Rc is the voice coil radius, a vertical force of 0.466 N is calculated. With this information, all remaining system components could be finalized.

(1)

Once the force output was determined, the flexure spring could be designed to translate this force into an angular deflection. A simple flexure design was drawn in SolidWorks, and with FEA software, the spring rate was calculated using SolidWorks Simulation. Various forces were applied to the flexure spring, and the resulting displacements were documented. An Excel graph was generated, plotting the displacements on the x-axis, and the corresponding forces on the y-axis. A trend line was added to the plot, and using Hooke’s Law, the spring rate is equal to the slope of this line. As seen in Fig. 3, the spring rate was calculated to equal 626.8 N/m.

Figure 3 – Force vs. Displacement Plot of Flexure Spring

With the flexure spring rate determined, a system of equations was created to represent our model [2]. This model simulates how our system will behave when operating. The mechanical model of the system in one axis can be seen in Fig. 4. The two springs represent the left half and right half of the flexure spring, and the two dampers represent any damping that may be present in the system. It is later determined that little damping exists, so it is assumed that these values are approximately zero. The simplified equations of the model are documented below (Eq. 2 and 3).

Figure 4 – Mechanical model of system

mẍ + 2kx = 2F1 (2)

J Ӫ + 2kL22ө = 2F1L1 (3)

The next components designed for our system are the sensors. Different options were available when designing these parts, such as capacitive, inductive, and a MEMs accelerometer.

Ultimately, a capacitive design was selected. The idea behind this design is to have two plates on the same axis per voice coil. The plates, which are approximately 5x7 mm in size, are conductive pieces through which a change in voltage is measured when movement occurs. While one plate is stationary, the other moves with respect to the mirror. Once the desired position is reached, the sensor will be able to detect this correct position based on the change in voltage between the two plates.