Torque, Power, Speed Considerations

Some learning experiences from the designs so far:

  • peak static torque has been a limitation for some designs
  • some designs have managed to use mechanical advantage (lever, gear, screw, pulley) mechanisms to over come this
  • stepper motors seem to be the weapon-of-choice (precision, control, holding-power)

Mechanical advantage can be used to trade off rotor speed for torque, and this relationship is linear, and this relationship is linear, however stepper motors have a characteristic torque drop off at high step-rates.

It might be worth collating some specs for torque vs. step-rate or RPM for some common NEMA steppers, at various voltages etc. This could be used to inform the design of both the power-supply, and the mechanical apparatus.

Specifically:

  • at a given voltage, what max rotor RPM can we expect while staying in the 'high-torque' band of the stepper
  • for a given required RPM (within the high torque band), what supply voltage will we need

This will allow the mechanical guys to gear everything appropriately and avoid slipping off the torque 'cliff' by needing to drive the motor too fast. It will also make sure we don't paint ourselves into a corner by not being able to boost to 24V or 48V if that's what we need.

I guess the most basic questions is how much airflow we will need:

  • what is the maximum breathing rate we will need to support:
  • what is the stroke-volume at that breathing rate (assuming high breathing rate means lower stroke length)

These boundary parameters should probably be known up front, as some of the designs might end up out of spec.

From a mechanical perspective:

  • what is the maximum linear speed at which we would need to depress the surface of the AMBU?
  • what is the maximum linear distance we would need to depress it for this max speed?

From an electromechanical perspective:

  • what is the max stepper RPM we can theoretically support (within, or near, the high-torque band)?
  • does this satisfy RPM required by the mechanical design?
  • what supply voltage is needed to support this?

These seem to be the intersection points between the mechanical and electrical guys, maybe a well-defined set of 'interface specs' will help. Can we focus on this a little?