Education · Commanding Actuators

Commanding Actuators Signals

A controller's analog output is a promise, not a position. The AO drives a voltage or a current down the wire; the actuator on the other end turns that signal into stroke — degrees of damper rotation, millimeters of valve stem. Between the two sit a handful of translation rules: which span the signal speaks, which direction the actuator runs, and where it goes when the signal or the power dies — and every one of them can be set wrong while all of the wiring is right. This page is about that translation. One question, start to finish: what happens between an AO commanding 50% and the damper actually sitting at 50% — and why do the two disagree? (How the conductors themselves land — the three-wire power + signal + common landing, and the triac pair that drives a floating actuator — is Controller Wiring's story. Here the wire is assumed right, and the trouble is still there.)

The Command Spans — 0–10 V, 2–10 V, 4–20 mA

A modulating actuator listens in one of three common command languages: 0–10 V, 2–10 V, or 4–20 mA. All three are linear spans meaning 0–100% of stroke; the difference is the endpoints. The two offset spans carry a live zero — the bottom of the range is a real, non-zero signal, so a dead wire (0 V, 0 mA) is distinguishable from an honest "fully closed" command — the same fault-flag logic Analog Sensing walks on the input side, running the other direction here. And they're close cousins: drive a 4–20 mA loop through a 500 Ω resistor and the voltage across it is exactly 2–10 V, which is why an actuator spec sheet so often says its 2–10 V input "accepts 4–20 mA with a 500 Ω resistor." (Drives take the same 0–10 / 4–20 command languages — a VFD's speed reference is usually one of them — but the run-command and speed-reference story belongs to the VFD lesson.)

Here is the whole page's problem in one number. The controller's AO is scaled 0–100% → 0–10 V, so a 50% command puts 5.0 V on the wire. The actuator, though, is configured for a 2–10 V input — its 0% lives at 2 V and its 100% at 10 V. It does the only math it knows: stroke = (5.0 − 2) ÷ (10 − 2) = 37.5%. The graphics say 50; the damper sits at 37.5; both devices are working perfectly. You can replicate this in the Signal Scaling tool: signal type 2–10 V, signal value 5, engineering range 0 to 100 — it answers 37.50, and the %-of-span readout says the same 37.5%.

One 5-volt command read against the 0–10 V and 2–10 V spans Two horizontal number lines. The top rail is the controller's AO command span, 0 to 10 volts; a dot marks 5.0 volts, which the controller calls 50 percent. The bottom rail is the actuator's 2-to-10-volt input span, labeled 0 to 100 percent of stroke, with the region from 0 to 2 volts hatched as a dead zone where the actuator cannot move. A dashed vertical line drops from the 5-volt dot straight down to the actuator rail, where the same voltage lands at 37.5 percent stroke — the 2-volt offset the actuator never counts. AO COMMAND — 0–10 V 5.0 V — the controller calls it 50% 0 V 2 4 6 8 10 ACTUATOR STROKE — 2–10 V INPUT dead zone no motion below 2 V 0% 25% 50% 75% 100% 37.5% stroke — the actuator's answer same 5 volts, two answers — the 2 V offset never cancels

The field signature of a span mismatch is its consistency. Everything tracks — command up, damper opens further — but every position sits offset, all day, on every unit that shares the configuration. One damper stuck low is a broken actuator; a whole floor of them sitting low is a setting. And near the bottom of the range the offset becomes a dead zone: every command below 20% (0–2 V from this AO) means the same thing to a 2–10 V actuator — nothing — so a loop trying to hold a small opening gets no response, winds up its integral through the silence, then overshoots when the signal finally climbs past 2 V. That's the hunting near the end of travel that mismatched spans are famous for. Run the mismatch the other way — an AO scaled 2–10 V into an actuator configured 0–10 V — and everything sits high instead: the AO's "fully closed" is 2 V, the actuator reads 2 V as 20%, and the damper never fully closes no matter what the graphics claim.

The usual cause is not a failed actuator — it's a switch. Most modulating actuators carry a DIP switch or jumper on the body selecting the input span (0–10 / 2–10 / 4–20 mA with the resistor) and often the direction, and the factory default doesn't care what your controller is scaled for. The fix is agreement, not replacement: check the actuator's span switch against the AO's scaling before touching the wire, and check the wire before condemning the motor.

Direction and Fail Posture

The span says how much; the direction setting says which way. A direct-acting actuator increases stroke as the signal rises — 2 V is 0%, 10 V is 100%. A reverse-acting one runs the same span backwards — full signal, zero stroke — usually the same DIP block or, on damper actuators, a rotation switch (CW/CCW). One disambiguation, because the words collide: this is direction at the actuator, the mapping from signal to stroke. A control loop's direct/reverse action — which way the controller's output moves as the measurement rises — is a different setting solving a different problem, and it belongs to PID Basics. Flip either one and motion inverts; flip both and the two inversions cancel, so the system even seems to work — until someone reads a position. Fix the mapping at the layer that's actually wrong. The chart below puts the whole section on one plot: the ideal 1:1 line, the 2–10 V line from the worked example, and a reverse-acting line — three configurations, one glance at what the same 5 V command gets you.

Stroke versus command for three actuator configurations A chart with AO command voltage from 0 to 10 volts on the horizontal axis and actuator stroke from 0 to 100 percent on the vertical axis. Three lines: a dashed 1-to-1 reference line for a 0-to-10-volt direct-acting actuator, rising from the origin to full stroke at 10 volts; a solid line for a 2-to-10-volt direct-acting actuator, flat at zero until 2 volts and then rising more steeply to full stroke at 10 volts — sitting below the reference everywhere; and a falling line for a 0-to-10-volt reverse-acting actuator, from full stroke at 0 volts down to zero at 10 volts. A dashed vertical marker at 5 volts crosses the two 0-to-10-volt lines at 50 percent and the 2-to-10-volt line at 37.5 percent. at 5 V: a 0–10 V actuator answers 50% — a 2–10 V actuator answers 37.5% AO command (V) → actuator stroke → 0 2 4 6 8 10 0% 25% 50% 75% 100% 5 V 50% 37.5%

0–10 V direct — the 1:1 reference 2–10 V direct — sits low on a 0–10 V command 0–10 V reverse acting

Direction answers "which way does it run." The next question is where does it go when everything dies? A spring-return actuator winds a spring as it strokes; kill its power and the spring drives it — at its own pace, with no controller involved — to one defined end of travel. A non-spring (fail-in-place) actuator just stops wherever it was. (A third family stores energy in a capacitor and drives itself to a configured position on power loss — electronic fail-safe — but the design question is the same.) Which end the spring owns is not an afterthought: it's decided when the hardware is picked, and it's the one behavior no software can override, because it happens precisely when the software is gone.

Valve people name that resting end normally open / normally closed, and the word normally is the trap: it means the position with no power and no signal — the spring position — not the position the valve usually operates at. A normally-open hot-water valve may spend nearly all of its life pinched almost shut; it's still NO, because open is where the spring takes it the moment control disappears. Dampers tell the same story in different words — fail open / fail closed, spring open / spring closed.

The classic example is freeze protection, and it's worth spelling out because the convention surprises people: the hot-water coil's valve is specified normally open. Picture an air handler in a Northeast January — power fails, a freeze-stat trips, a controller dies. The outdoor-air damper's spring drives it closed, so the cabinet stops swallowing air below 32 °F; the heating valve's spring drives it open, flooding the coil with hot water that the cold can't freeze. The building might overheat a floor; the alternative is a burst coil and a flooded mechanical room. Fail posture is a design decision made for the worst five minutes of the building's year, not for the normal ones — and when you find a valve specified "backwards," that's usually the reason.

Position Feedback — and Commanding Without It

Everything so far assumes the actuator heard the command. The only way to know is position feedback: many modulating actuators offer a feedback output — commonly 2–10 V across the same stroke — that lands on an AI. Now the graphics can show two numbers: what was asked, and what the actuator says it did. The command is a hope; the feedback is a measurement.

So the AO commands 50% and the feedback AI reads 34% — now what? Work the fork in order of cheapness. Time first: actuators are slow on purpose — 90 to 150 seconds for full stroke is ordinary — so a freshly written command and its feedback disagree for a while by design. If the feedback is still climbing, that isn't disagreement, it's transit. Configuration second: the feedback signal has a span and the AI reading it has a scaling, and every trap from the command side applies again on the way back — a 2–10 V feedback landed on an AI scaled 0–10 V misreports a perfectly healthy actuator. Mechanical last: feedback that stops short and holds while the command keeps climbing is the actuator telling you it physically can't get there — a jammed damper blade, a seized valve stem, a motor stalled at its torque limit. And watch for the inverse tell: command and feedback agreeing beautifully while the temperature never moves. The actuator is turning; a slipped linkage or a loose hub means the shaft isn't.

Floating (tri-state) actuators skip the analog conversation entirely. The controller drives two binary outputs — one inches the actuator open, the other inches it closed, neither on means hold — and the actuator carries no position input and, typically, no feedback. (The two-triac landing itself is on the wiring page.) The controller estimates position instead: it knows the actuator's stroke time and integrates its own pulses — "I drove open for 12 seconds of a 120-second stroke, so I'm 10% further open than I was." That is command without position: the number on the graphics is arithmetic, not measurement.

And that estimate drifts. Pulses round off, a power blip restarts the count mid-stroke, and somebody's crank on the manual override never reports itself. Left alone, assumed and actual position walk apart, and the loop starts producing quiet mysteries — setpoint held on the graphics, room complaining. The designed cure is re-synchronization at the ends of travel: periodically — commonly at schedule start or after a power-up — the controller overdrives the actuator toward one end for longer than the full stroke time, so wherever it actually was, it lands on a hard stop the controller can trust as 0% (or 100%), and the count starts clean. Which answers a very common service call: the damper drives fully closed every morning and then reopens — is it failing? No — that's the controller re-zeroing the only position reference a floating actuator has. Drift is the disease, the morning full-travel drive is the medicine, and neither one is a broken actuator.

That's the whole distance between commanded and actual: the AO speaks a span, the actuator answers in stroke, direction decides which way, and the spring decides where it rests when everything fails — with feedback, or a hard stop, as the only truth. When the two numbers disagree, the fault list is short and ordered: transit time, span configuration, direction, mechanics — and the wiring you already proved. One thing this page deliberately leaves alone: how much flow a given valve stroke actually buys. Fifty percent stroke is rarely fifty percent flow — that's valve sizing and authority, its own topic.

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