Wide-Range Reconfigurable DC Motor Microcontroller
Minimally invasive surgery (MIS) of the throat is characterized by the insertion of endoscopes and multiple long tools through a narrow tube (the laryngoscope) into the patients mouth. Current manual instrumentation is awkward, hard to manipulate precisely, and lacks sufficient dexterity to permit common surgical subtasks such as suturing vocal fold tissue. This clinical problem motivated the development of a novel system for MIS of the upper airway including the throat and larynx. This system has a master/slave design similar to other telesurgical robots. A brief overview of the slave unit, focusing on its snake-like mechanism because it placed the most demands on the design of a micro-controller, is presented.
The design of the 3-armed slave robot that works through a laryngoscope is shown below. The design includes a laryngoscope, a base link, two-similar Distal Dexterity Units (DDU) for tool/tissue manipulation, and another DDU for suction.
Each DDU is a 7 axis robot, which is manipulated by a 4 axis Tool Manipulation Unit (TMU). The total robot system consists of three DDU/TMU arms (each with 11 axes) a nd a rotating base unit (RBU) for a total of 34 axes. The snake-like unit of the DDU is composed of a base disk, an end disk, several spacer disks and four super-elastic tubes (backbones) arranged as shown in the figure. The central tube is the primary backbone while the remaining three tubes are the secondary backbones. These secondary backbones actuate the snake in both push and pull modes, which make it possible to satisfy the static of the structure while preventing buckling of the backbones an important feature for the successful reduction of diameter to 4mm or less. This makes the design of the microcontroller challenging because it must prevent buckling by ensuring that the backbones do not get overloaded. Also, because the surgical robot is a small-scale design and does not require high joint speeds, it was possible to use small, low-power motors with high gear reductions.
|Distal Dexterity Unit
Based on the aforementioned characteristics of the surgical robot, the following requirements for a micro-controller that would control it are identified:
- Control the three 7-axis DDUs, three 4-axis TMUs and one RBU, for a total of 34 actuated joints. The controller should be software-selectable between position, speed or torque control.
- Avoid buckling the snake portion of the DDU by preventing the motors from exceeding the strain limits of the secondary backbones.
- Provide precise feedback of the motor currents so that interaction forces with the environment can be estimated (assuming accurate models of joint friction).
Chip Design and Architecture
To realize a chip with the capacity to meet the system requirements, we propose an architecture that combines both on-chip and off-chip circuitries with a supervisory microprocessor control via a digital interface. Fig. 2 shows the design with the on-chip circuits indicated. G1 is the gain of the power op that is used to drive the motor. Because the chip will be limited to 5V signals, G1 will scale the chip output to the maximum motor voltage. For example, by setting G1=10, the chip can be used to drive motors rated at 48V. Vcontrol is the motor speed control voltage which is connected to the inverting input of the power op amp with gain, G1= -R2/R1. The block, Digital Position & Speed (DPS), estimates the motor speed and position by using the quadrature output of the encoder. In addition, the CurLimit signal specifies the motor current limit for protection while the Pos2Meas proves a position reference from a potentiometer. The main contribution of this chip, which incorporates both armature and encoder feedbacks, is precision control (velocity, torque, position) of low power motors at both high and low speeds while offering features for re-configurability and environmental operation adaptation.
|Architecture of the motorcontroller
The chip, a mixed-signal system, should resolve as low as 0.8N torque and precisely estimates the robotic interaction forces with the tissue environment and compensate any change in real time. With SPI interface and on-chip programmability, it can find applications in toys, video-game hardware, other dexterity surgical robot, disk controllers etc.
|Motor Controller Chip
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