Reconfigurable 3D Magnetic Control System

Background:

Magnetic-responsive soft robots can be actuated in any direction solely by manipulating the external magnetic field lines the sample lies within. The chief benefits of such soft robots are found in the biomedical field for use in noninvasive surgical procedures. Small magnetic ‘microbots’ can efficiently navigate through arteries and veins, unclogging blocked pathways without requiring large actuators within the body. The Zhao Lab at MIT has demonstrated the control of soft microbots through model arteries using nano magnetic technology [8]. One of their primary goals was creating micro magnetic robots that can be controlled with handheld permanent magnets. The dynamic behavior of such robots can be studied and shown to have very precise movements.

The team’s sponsor, the CAI Research Group, is interested in studying the behavior of new nano magnetic responsive materials. Specifically, the research group is trying to find a way to research automating magnetic actuation through a control system. In order to do so, they require a system which can produce 3D magnetic field vectors of varying density. In this regard, the goals of the project are different from the Zhao Lab, as the research is focused on creating a closed control system instead of creating new small scale soft robots.

The goal of this project is to design a modular system permitting the sponsor to control the direction and density of magnetic fields within an adjustable working space, optimized for 50 x 50 x 50 mm. To achieve this, 3 electromagnet pairs are operated simultaneously: producing homogeneous field vectors to control the position and movement of the soft robot within the working space. To satisfy the goals of the Cai research group, this chamber will require strong magnetic actuation in all three directions, while allowing the user to adjust the required density to enable repetitive demonstrations of the magnetic chamber. Since the goal of this project is focused on creating a magnetic environment, the maximum field density is one of the most important qualities. The initial goal of the project was to achieve 1 Tesla (T) within the working space. For the resources and timeline allocated to the project, an updated goal was set to achieve 100mT maximum in the vertical (z) direction while 10mT in the x and y directions would be acceptable.

Objectives:

Statement of Requirements:

The team’s sponsor requested that the team meet the following requirements:

1. Primary

   1. Modularity:

      1. Slots for 3 pairs of removable solenoids to be positioned around a test area of 50x50x50mm

      2. any iron cores used should be individually removable from solenoid coils

   2. Magnetic Field Homogeneity

      1. Magnetic field produced by each coil pair must be homogeneous within 20x20x20mm space within test area

      2. Goal of ± 5% axial and radial homogeneity

      3. ± 10-20% lowest acceptable

   3. Magnetic Field Strength

      1. Magnetic field must be intense within 20x20x20mm space

      2. Initial goal of 1-1.5T max field density for each coil pair

      3. Updated goal of maximum 100mT field density in the vertical direction, 10mT density in the horizontal directions

   4. Safety

      1. Coils should operate for 3min at a time without overheating

      2. Max temperature: touchable without burning hand (130-140^oF)

      3. No magnetic responsive materials to be used near the electromagnets (iron)

      4. Ideally 5A or less current in coils, though more may be used if overheating shown not to be an issue.

      5. Structural integrity of frame should secure weights of solenoids +Magnetic force

   5. Space

      1. Entire system lies within 400x400x400mm working space

      2. Open, accessible, visible test area 50x50x50mm

2. Secondary

   1. Control System

      1. Has an on-off switch for each coil pair

      2. Ideally have integrated housing for wires

      3. Wow condition: Arduino controlled, bidirectional field

Deliverables

The team’s sponsor requested that the team provide the following deliverables:

1. Primary:

   1. Working space

      1. Modular frame: the sponsor should be able to reconfigure the working space to experiment on different sized samples

   2. Electromagnets

      1. 3 pairs of removable and replaceable electromagnet pairs

      2. Optimized for field density within the 50x50x50mm working space

      3. Simulations to predict magnetic field based on electromagnet physics

2. Secondary

   1. Integrated control system, based on current to the electromagnets and wiring

Final Design:

The final design consists of 3D actuation of the magnetic field density using three independent electromagnet pairs arranged around the testing area. The pairs magnify the field density in the test area relative to single electromagnets, and increase the homogeneity. The electromagnet pairs were constructed from machined pure iron cores and copper winding solenoids. The iron cores were machined using a CNC Mill to hollow a groove for the windings. The copper windings can be pre-wrapped and inserted into the iron using a custom jig and can use 16-20awg wire depending on the application.

These electromagnet pairs were able to reach a magnetic field density of up to 80 mT with a separation distance of 50mm during the testing phase while the air core solenoids were only capable of reaching a magnetic field density of 10 mT.

Based on the team’s sponsor’s request, the frame was designed to be reconfigurable. An aluminum base plate consisting of four grooves enables the horizontal mobility of the two pairs of horizontal solenoids (i.e. the solenoids in the x-y plane). In conjunction with the aluminum base plate, T-bars make up the backbone of the project’s frame by providing support and stability. In addition to giving the frame its stability, these T-bars also provided a space to mount the iron core solenoids. The sponsor can place the T-bars with solenoids as close as they desire to one another and the working space to optimize the magnetic field density of the magnetic elastomer (i.e. the soft robot) inside the working space.

Another important goal of the project was to develop a controls system and power variable power supply that can be used to accurately control the current powering the system’s electromagnets and therefore the magnetic field density. Two controls systems were developed and delivered to the project’s sponsor: (1) an Analog Arduino-based potentiometer control system, and (2) a MATLAB-based GUI. Both the MATLAB-based GUI and the Arduino-based potentiometer controller make use of ammeters and feedback loops to ensure that the desired current is provided to each solenoid.

The Analog system utilizes four potentiometers for input from the user: 1 for overall power as a safety feature and 3 to control each bidirectional channel running to X, Y, and Z directions. An LCD screen displays current to the user in real time to help key in on the desired current value. The current is measured, using ammeters, directly from each of the three channels and digitally filtered. An Arduino Mega 2560 and three Cytron 10A motor driver shields were used to limit output from Meanwell single output 12V, 30A and 24V, 15A power supplies. Currently, due to the 10A limit of the motor driver shields, a 9.8A limit is imposed in the arduino script for each channel. The Analog system runs independently from a computer and requires a power outlet and a 5V USB-A source. If the GUI is run, the code for the analog script must be flashed from the Arduino IDE through the USB-A cable in order to run it again.

The other system that was developed is a MATLAB-based GUI. This GUI enables the end user to attain high degrees of precision when working with the 3D magnetic control system. To use the GUI, the user starts by plugging the Arduino Mega 2560 USB cable into their laptop. Next, the user will launch the 3D Magnetic Control System GUI. The GUI will have sliders and switches that enable the user to control how much current is output to each pair of electromagnets and in which direction. There are also dropdown menus that enable the user to update the separation distance of the solenoid pairs in the X and Y directions (the Z direction solenoids are fixed in place). The user can then press the “Update Simulation” button to get an estimate of the magnetic field density vector and magnitude. Once the user is satisfied with the simulation results, they can press the “Send Signal to System” button which will power on the motor drivers and send current through the 10A Cytron motor drivers as specified in the prior steps. While current is being sent through the motor drivers, red LEDs will light up to indicate which pairs of electromagnets are being powered on. When the user is done with the system, they can press the “Power Off” red button to cut power to the motor drivers and turn off the system. Again, no software needs to be flashed to the Arduino Mega 2560 prior to using the MATLAB GUI. However, if the user wants to switch from the MATLAB-based control system to the Arduino-based control system, they will need to flash the Arduino Mega 2560 with the provided Arduino sketch.