Motion Control Requirements for Medical Instruments
- Friday, 01 April 2011
The sample handling motion capabilities are defined in the selection of the mechanical design including its coupling to the motion components, the motion components including drivers and controllers, trajectory shaping, and the tuning of closed-loop control systems. Backlash should generally be minimized in the mechanical design. The use of direct-drive, single-stage pulleys or single-stage gearing helps to minimize backlash and compliance. High-ratio gears should be avoided to prevent damage in the case of back-driving; when needed, they should be rated to not fail when back-driven. Direct-drive systems not only need to provide the required torque, but also need to be able to handle higher inertial mismatches in order to provide smooth, reliable motion.
Stabilizing Motion Control
S-curve and raised sine trajectories are a starting point for configuring these motions, but improved results can be found in the tuning of the control loop. The addition of viscous inertial damping, either physical or emulated within the control system (or sometimes both), can be used to improve the phase margin of the system. The resulting additional phase margin available to the designer can be used to reduce overshoot and ringing. The reshaped phase response can allow the system gains to be increased for tighter control. The added effective inertia of the viscous inertial damper can also help damp out limit cycle oscillations that arise from backlash in the system. As the forces reverse across a gear having backlash, the teeth momentarily disengage and then reengage on the opposite side of the drive tooth. The uncoupling of the load during this period reduces the inertia reflected to the motor, allowing for higher acceleration for the same motor torque. This reduced load temporarily raises the gain of the control system as compared to when the teeth are engaged.
According to the gear material, there can be significant rebound at tooth engagement, causing the teeth to bounce several times before remaining in contact. Without care in the design of the control system, an oscillation can occur in the control loop in this transition time, causing noise and wear at the least, and damage and wrong volumes and answers at the worst. Viscous inertial damping effectively raises the (effective) inertia coupled to the motor, reducing the change in acceleration when decoupled. The viscous coupling operates differently from a pure inertia, as can be seen when the teeth again come into contact. The addition of a tightly coupled inertia would cause high forces as the teeth came into contact and the now larger inertia coupled to the driving gear is suddenly slowed to match the speed of the load gear (or visa versa). This is like ball-peen hammer to an anvil. The viscous inertial system has the mass less directly coupled — more like a rubber mallet to the anvil. The impact impulse is spread over a larger time period, lowering the forces.
To a significant degree, the actions of a physical viscous inertial damper can be simulated within the control system. This includes the phase boost improvements to the system and the reduced acceleration upon load decoupling. The simulated viscous inertial damper has advantages in cost and size over using a physical inertial damper. Further, motor torque is not wasted accelerating and decelerating a physical inertia.