Feedback and valving for very low speed applications

What if a system needs to move at extremely low speeds? What needs to be considered from a design standpoint, and what should we expect from the final system? Let’s take a look at the feedback and output to make a decision.

0.010 in/minute velocity
10" diameter bore

Position feedback consideration:
An RMCs standard loop time is 1 ms, and at the time of this writing, the finest magnetostrictive position sensor resolution on the market is 0.1µm. Since our speed is in inches, we’ll convert to µm (micrometer, also known as micron).

\left(0.010 \frac{in}{min}\right) \left(\frac{1 min}{60 sec}\right) \left( \frac{25,400 µm} {in} \right) = 4.23 \frac{µm}{sec} = 0.00423 \frac{µm}{millisecond}

If we assume best case 0.1 µm resolution, and 1 ms loop time, we can only see the position value change every 0.1/0.00423 ≈ 24 loop times! Even lengthening the RMC loop time to 4 ms means we get a position change about every 6 loops. Due to the very high resolution, the controller will likely see some bouncing around of the reported position between those updates. Because velocity is derived from position, that bouncing around causes some noise and uncertainty in the velocity measurement. The reported average velocity may match the desired velocity. But if it is required to very tightly control velocity as measured over a short time period, the resolution may be too low to actually measure a change. This is a practical limit to how precisely the velocity can be controlled.

Another problem is that if the position is heavily quantized, it will “stair-step” across the target position. While there is a difference between the Actual position and the Target position, the gains will cause the output to slowly increase (or decrease, depending on direction) until the next position update. So the control output will be an odd sawtooth waveform rather than smoothly following a profile to the final position.

Flow consideration:

\left(0.010 \frac{in}{min}\right) \left(\frac{10 in} {2}\right)² (π) \left(\frac{1 gal}{231 in^3}\right) = 0.0034 gpm = 0.012 lpm

It will be challenging to find a standard off-the-shelf valve with a flow rating this low. If the same system must also move quickly, the best plan may be to have two valves, one for very low speed control, and another for rapid moves.

Cylinder consideration:
The last suggestion is to specify a cylinder with the lowest friction seals possible. Very low speed cylinder movements are vulnerable to “slip-stick” conditions, where the static friction holds the cylinder stopped until the force builds up to break the cylinder loose and it jerks forward. (See Boundary friction.)