Final Design

UCSD Center for Functional MRI needs a one-axis linear positioning system that can carry and move a sample in a MRI environment.

Functional Requirements

• • Able to transport sample mass up to 2 kg

  • The system must be able to do a linear movement of 762 mm from stop to stop within 10 seconds.

  • The system must be able to stop at an arbitrary point from a homing position with an accuracy of ±0.5 mm.

  • Every component except the motor must be non-magnetic.

  • Radio frequency noise that interferes with the MRI RF coil is unacceptable.

  • The system must be controlled from another room via fiber optics.

  • LabVIEW interface is preferred.

Description of Functionality

Final Design Schematic

The project required the system to be controlled from outside the MRI room. Because the MRI system is very sensitive to RF-noise, conductive wires cannot go through the walls and into the room. The reason for this is that the wires might bring noise from outside the room to the MRI room through the wires which could interfere with the MRI imaging. The project’s sponsor suggested fiber optics, since they already used this for monitoring equipment for living samples.

Since the project had a limited budget, and we wanted to minimize the amount of cables going in and out of the room, it was decided to use a stepper motor with open-loop control.

Mechanical Design

Overview of System

The final mechanical design is a rail system that with a carriage on chamfered nylon wheels, guided by a slotted aluminum rail. The carriage is pulled by a urethane timing belt, which is driven by a stepper motor. The timing belt is run through the center of the rail, which attaches with a belt clamp to the underside of the carriage. On either end the belt is turned with timing pulleys.

Both pulleys are rigidly connected to shafts, which are constrained on each side by ball bearings with glass bearings and plastic races. The driven pulley is connected to the stepper motor with a helical coupling to account for slight offset and angular misalignment.

The carriage rides on four nylon wheels, chamfered to allow for minor self-alignment. The wheels attach to the carriage body with plastic and glass ball bearings. The left pair (viewed from the motor along the main axis) is adjustable so that the pressure between the wheels and the rail can be adjusted. In the top of the carriage, a grid of 1-inch threaded holes was placed, for the versatile mounting of various, sample-holding hardware. This mounting plate is 457 mm (18 in) long and 203 mm (8 in) wide. The total range of motion of the carriage is 648 mm (25.51 in).

CAD of carriage

Motor, Coupling and Pulley Assembly

Electrical Design

The electronics are separated into two localities: the components inside the control room, and the components inside the scanner room. Identical fiber optics send/receive (FOSR) circuits are placed in both locations.

Control Room

Inside the control room, the FOSR circuit and Arduino UNO microcontroller are housed together in an acrylic case, which sits on a desk next to a computer with LabVIEW 2010 installed. A USB cable runs out of the box and connects the Arduino with the desktop computer. Protruding from the box are the eight LED/photodetector connectors, which attach to the eight respective fiber optic lines. The FOSR circuit in the control room both inputs and outputs signals exclusively to the Arduino.

Scanner Room

Inside the scanner room, the FOSR circuit is housed in a RF-shielded (copper-lined) acrylic case, along with the stepper motor driver. The shielding was requested by the sponsor as a further precaution to stop RF noise generated by the electronics from reaching the sensitive RF coil and distorting the scanned image. There is also cause for concern with the MRI dynamic field switching generating stray currents in the electronics; the shielding prevents interference both ways. The FOSR circuit in the scanner room collects inputs from the two limit switches and current sensor, and provides outputs to the motor driver as step, direction, and disable commands.

LabView-based Graphical User Interface

User Interface for Positioning System

LabVIEW was preferred by the sponsor because it allows more flexibility for the user interface than a physical control box. In the future, the LabVIEW GUI can easily be modified by the sponsor to add or remove features. The LabVIEW graphical user interface (GUI), as shown in Figure 7, has an intuitive design which allows the end user to control the position of the mechanical system with little instructions.

The GUI allows the user to move the carriage to several preset positions, input a desired distance to travel, as well as finely adjust positioning using incremental buttons. The GUI has indicators to inform the user of the current state of the system, such as “in motion,” “holding position,” and “motor disabled.” Position readouts, such as “desired position,” “current position,” and “saved position” are included to allow the user to know the position of the sample at all times. The GUI will also include a “Motor Disable” button which allows the user to turn off the motor from inside of the control room. A “Lock/Unlock” button allows the user to lock the front panel to prevent accidental input commands while using the control computer.

The sponsor expressed a desire to have a panel which allows the system to be fine tuned in the future. The developer panel was included to allow the user to configure key settings such as distance between landmark and isocenter, distance between home and landmark, carriage speed and acceleration. These controls can be activated when the user toggles from “Basic Mode” to “Expert Mode.” The sponsor confirmed that a simple button to toggle on expert mode will be more than sufficient to prevent users from accidentally changing settings.

Operating Procedure

Flowchart of Operating Procedure

The user must first disable the stepper motor, and physically move the sample in and out of the bore to verify that the system will not crash. Secondly, the user then positions the sample at the indicated position on the rail. After the sample is positioned on the rail, the user can “set landmark” to gain relative positioning. Once relative positioning is set, the graphic user interface can be used to command the system to move to the MRI scanner’s isocenter, incremental movement buttons to finely adjust the position to obtain the desired image, or return to landmark or home.

Safety

A final requirement for our system was one of general safety. This applied to both protection against operator injury and protection against damage to equipment or samples. A major concern was raised specifically on crashing, for example, the operator not properly aligning the tooling and driving the system into the scanner, or the fingers of the operator being pinched between the carriage and the rails. An attempt was made at detecting obstacles by way of current sensing, but the final design instead used current limiting to prevent the motor from producing harmful torque at all. The final prototype is capable of a maximum force of 12 N (2.7 lb force), which is not enough to injure the operator even at pinch points. The force required to damage the tooling, however, depends on the application, and so a manual test to check for obstacles is recommended before operation.