In building automation and HVAC systems, actuators respond to different types of control
signals that determine how they move or position a valve or damper.
These control types define how an actuator receives, interprets, and reacts to electrical signals
from a controller.
Below is an overview of the most common control signal types and what each one means.
On/Off (Open/Close) Control
The actuator moves fully open or fully closed—there are no intermediate positions.
Commonly used for simple isolation or shutoff applications.
Spring return actuators use the same logic:
o When power is applied, the actuator drives fully open.
o When power is removed, it springs back to the closed (zero) position.
o This action can also be reversed (spring-open or spring-close).
3-Point / Tri-State / Floating Point Control
The actuator has two separate control inputs:
o One input drives the actuator clockwise (CW) to open.
o The other drives it counterclockwise (CCW) to close.
When no signal is applied to either input, the actuator holds its last position.
This type of control allows for intermediate positioning without the need for a
proportional signal.
Proportional Control
The actuator’s position is directly proportional to the input signal it receives.
This allows smooth and continuous modulation throughout its entire range of motion.
Typical control signal ranges include:
o 0–10 VDC
o 2–10 VDC
Some controllers output current signals such as 0–20 mA or 4–20 mA, which can be
easily converted to 0–10 VDC or 2–10 VDC using a 500 Ω resistor.
Pulse Width Modulation (PWM)
The actuator’s position is determined by the duration (width) of the control pulse.
The signal may come from a dry contact closure or a triac-based controller.
Example:
o Time base: 0–10 seconds
o Output pulse: 5 seconds
o Actuator position: 50% open
This control method is often used when proportional feedback is not available.
Phase-Cut Control
The actuator responds to the portion of the AC waveform that remains after the signal
“cuts” part of the wave.
The actuator interprets this average voltage or power as a proportional signal.
Commonly used in lighting dimmers and certain proportional motor controls.
Programmable Control
Developed by Siral, programmable actuator technology allows users to customize the behavior
of the actuator to match application needs.
Key adjustable features include:
Control Input Type
Selectable between On/Off, VDC (proportional), or Floating Point control.
Motion and Valve Settings
Adjustable running time and travel limits.
Signal Rescaling
Adjust input signal ranges to match customized travel ranges.
Input/Feedback Options
Selectable values for control and feedback signals.
Compatibility
Siral actuators are designed for wide compatibility with most control signal types and Direct
Digital Control (DDC) systems.
Modern controllers support a variety of input and output options, and Siral actuators are
engineered to integrate easily with these systems for dependable and precise operation.
Control Signal Quick Reference Chart
| Signal Type | Description | Typical Input Range | Actuator Response | Common Applications |
| On/Off (Open/Close) | Drives fully open or fully closed; no intermediate positions. Often includes spring return to default position. | 24 VAC / VDC (2- position) | Fully opens when powered; closes when de-energized (or reversed). | Isolation valves, dampers, safety shutoff. |
| 3-Point / Tri- State / Floating Point | Two directional control signals: one opens, one closes. Actuator holds position when no signal is present. | 24 VAC / VDC, two binary signals | Moves CW or CCW depending on active input; holds last position when idle. | Mixing valves, balancing dampers, slow- acting control loops. |
| Proportional (Analog VDC/mA) | Actuator position is proportional to input signal voltage or current. Provides continuous modulation. | 0–10 VDC, 2–10 VDC, 4–20 mA | Modulates smoothly between fully closed and fully open based on signal level. | Modulating valves, VAV dampers, temperature control loops. |
| Pulse Width Modulation (PWM) | Position determined by signal pulse duration within a fixed time base. | Pulse signal (e.g., 0–10 sec base) | Moves to a position proportional to pulse width (e.g., 5 sec = 50%). | Low-cost controllers, simple modulating systems. |
| Phase-Cut Control | Uses chopped AC waveform; actuator interprets average voltage as position signal. | Phase- controlled AC (varies) | Proportional movement based on remaining wave amplitude. | Lighting dimmers, certain proportional motor actuators. |
| Programmable | Actuator parameters can be customized: input type, running time, travel range, and feedback. | User- selectable | Behaves according to programmed settings (On/Off, Proportional, or Floating Point). | Multi-application actuators, adaptable control systems. |