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Magnetic Resonance Imaging Compatible Pneumatic Stepper Motor With Geneva Drive1

2016; ASM International; Volume: 10; Issue: 2 Linguagem: Inglês

10.1115/1.4033244

1932-619X

Adam Wineland, Yue Chen, Zion Tsz Ho Tse,

Iterative Learning Control Systems

Magnetic resonance imaging (MRI) is an imaging approach to acquire high-resolution images of soft target tissue particularly for medical applications in both preoperative and intraoperative procedures [1]. In addition, MRI can be used to enhance the understanding of anatomy and pathology due to its capability to detect subtle changes of the target of interest. MRI-guided robotic therapy has drawn increasing attention due to its ability to provide accurate, efficient, and safe operation inside the MRI room. However, the magnetic field precludes the use of traditional direct current (DC) motors fabricated with ferromagnetic or paramagnetic materials in the magnetic resonance (MR) room as they affect the homogeneity of both the static and gradient magnetic field in the MRI scanner. Therefore, MR-conditional actuators should be designed to provide the safe actuation and ensure the image quality at the same time.Unlike electric and electromagnetic actuators, pneumatic motors demonstrate a few advantages. MR-compatible materials, such as plastic, ensure easy fabrication and customized design; the air supply of the pneumatic actuators can be found in most MR rooms; the compressed air does not interface with the MR imaging physics; furthermore, pneumatic actuators can provide little to no signal-to-noise ratio (SNR) reduction in MR images. These advantages result in the fast development of pneumatic actuation for MR-guided robotic interventions [2,3].The device detailed in this paper is a pneumatic stepper motor. Previous stepper motor designs have generally relied on the intermeshing of different sets of "teeth" to create the rotation and stepwise effect. Chen et al. used a different approach creating a stepper motor composed of two cylinders and a supporting structure [4]. The two cylinders operated in sequence to rotate a shaft in discrete steps.The pneumatic stepper motor presented in this paper can rotate 3.6 deg for each step and generate torque up to 918.75 mN·m. The system setup, motor working principle, and calibration will be discussed in the Methods, and Results sections.Figure 1 shows the main components of the device, which include a continuous pneumatic motor, two pneumatic switches, two pneumatic cylinders, a Geneva drive, and a housing. All the components were fabricated from acrylonitrile butadiene styrene (ABS) to ensure the MR-compatibility of the motor. A planetary gearbox was attached to the continuous motor to increase torque and reduce the operation speed. In addition, the planetary gearbox is necessary to overcome the resistance force of the pneumatic switches inside the housing. Inside the housing are the pneumatic cylinders, pneumatic switches, and a Geneva drive. Attached to the output of the Geneva drive is a second series of planetary gears. The switches and cylinders inside the housing are made by LEGO (LEGO Group, Billund, Denmark). The continuous motor, housing, and Geneva drive are customized designs.The compressed air supply was split into two lines with one line directed to the control box and the other to the switches. Depending on the direction of rotation, a signal was sent to one of two pneumatic valves in the control box to open the pneumatic valve. Then, air flows to the corresponding cylinder, and the cylinder pulls its connected switch to the open position. During a digital low, the pneumatic valve closes and exhausts the air that is trapped in between the valve and cylinder. The supply of air attached to the pneumatic switch flows into the continuous motor. The continuous motor rotates the main axle inside the housing. Attached to the main axle is a cam which flips the currently open switch back to the closed position, stopping air flow to the continuous motor, and causing the motor to slow to a stop. Also, the corresponding cylinder was reset since it is attached to the switch. Essentially, the motor turns itself off. Figure 2 shows how the cam operates.After one rotation of the main shaft, the Geneva drive has advanced by one step. Since the Geneva drive has four slots, each step is equal to 90 deg of rotation. In order to increase the resolution of each step, a second series of planetary gears with a ratio of 25:1 was attached to the output of the Geneva drive resulting in 3.6 deg of rotation per step. After the completion of a step, the output shaft is locked in place due to the inherent design of the Geneva drive. In other words, when the pin of the driving gear is not interacting with a slot on the driven gear, the driven gear is held in place by the driving gear's stop disk.Figure 3 shows the interior view of the control electronics. The system uses a computer with labview installed, a data acquisition (DAQ) card, two three-way pneumatic solenoid valves, motor drivers to power the valves, and an air supply source. In clinical trials, the computer is located in the control room while the DAQ card, valves, and drivers are in the MRI room housed in a shielded enclosure. The pressurized air is supplied by the institution.At 50 psi, the motor is capable of operating at 5.76 deg per second with no load attached. With a maximum load of 100 N, the motor is able to turn at a maximum rate of 4.32 deg/s. The maximum torque produced is 245 mN·m.At 60 psi, maximum rotation speed is 6.12 deg per second with no load attached. With a maximum load of 375 N, the maximum speed is 5.4 deg/s. The maximum torque produced is 918.75 mN · m.Figure 4 shows the motor's angular error versus target angle at 50 psi with no load and at different speeds. The load is irrelevant to accuracy because, under proper operating conditions, the Geneva drive either completes one rotation or it fails. That is, if operated within its load limits, the Geneva drive will lock into place after each step. Thus, overshoot and undershoot of the output are negligible. The motor developed by Chen et al. is given for comparison [4].Figure 5 shows the relationship between load and rotational speed. The shaded area denotes acceptable operating conditions for 50 psi and 60 psi. For example, at 50 psi, the motor is capable of moving a load of 100 N at any rate from 0 deg/s to 4.32 deg/s. And if a speed of 2 deg/s is desired, the motor can handle any load from 0 N to 100 N.The stepper motor is relatively inexpensive and easy to fabricate. The Geneva drive allows for locked output during the dwell period. The optimal pressure for this design of the stepper motor is 60 psi, and the maximum operating speed is 5.4 deg/s with a maximum load of 375 N. The motor is capable of advancing 3.6 deg every step with an accuracy of less than half a degree while producing up to 918.75 mN·m of torque.