Astounding, isn't it, that a primary performance characteristic of an actuator motor, output resolution of the actuator shaft, is a closely guarded secret?
There's a different class of electric actuators for the massive valves used in waste treatment plants, by vendors like Rotork, and I'll bet they're more than willing to define what sort of resolution their actuators provide.
I suspect the nmber of repositions steps doesn't matter a heck of lot when the motor drives a damper in an air handler. And since the major market for this category of actuator is HVAC, well, you take what you get . . .
But I've seen first hand the effect of an acuator that can not resolve sufficient positions to control a gas valve. While investigating a "poor control" complaint, I discovered that Barber Colman motors (this was 20 years ago) used the same die cast housing, same electric motor, same slidewire, but just changed the gear train.
The fast motors were geared lower for speed, the two trade-offs being
- the higher the speed, stop to stop, the lower the torque.
- the faster motors had fewer reposition steps, or further radial movement for each step.
I was never able to clarify why the apparent fewer reposition steps. It would seem that if the motor only runs when the input voltage is applied through the control circuit, the motor could run 0.1 seconds or 0.3 seconds or whatever, until the controller ceases its output, at which time the motor stops.
The complaint was hunting, where the controller couldn't maintain setpoint on a belt oven. The PV oscillation synched with the motor hunting. Backing off the tuning constants to slug the controller response down made no difference, the PV was clearly, to the eye, tracking the motor position and the desired motor position could not be achieved. The ideal cotnrol position was in between motor reposition-steps.
BC's support response was to replace the motor with a slower motor. I did it, it worked. The oven lined out nicely with a slower motor and the motor did not hunt.
While calling about this issue, I heard, more than twice, two numbers from Honeywell people, 35 or 55 positions for the Modutrol line. So bear in mind there's still some question on that value.
Honeywell has a higher torque motor, the Herculine 2000 series, in a different class.
It specs the reposition resolution. There is some interplay between a deadband adjustment in the motor and its resolution, but I can't recall the details.
But it's given its cost, a 2000 can only be justified where the torque and resolution performance is required, like ID or FD control dampers on boilers. This class of actuator can be had with Modbus, as can Rotork.
Dan