Industrial control theory: what does time-proportioning (time-proportional) control mean, and which statement correctly describes it in practice?

Introduction / Context:
Many industrial processes use actuators that can only be ON or OFF (e.g., relays driving heaters). To achieve intermediate “average” power without analog hardware, controllers use time-proportioning control (also called time-proportional or duty-cycle control). Understanding this technique is crucial when tuning temperature loops, ovens, or simple thermal systems where continuous analog outputs are not available or not necessary.

Given Data / Assumptions:

• Actuator is binary (ON/OFF), such as a contactor or SSR for a heater.
• The process has inertia (thermal mass), smoothing the pulsed output.
• Controller can define a time base or window and adjust ON time within that window.

Concept / Approach:
Time-proportioning control divides time into fixed windows (for example, 10 seconds). The controller outputs ON for a fraction of that window proportional to the control signal, and OFF for the rest. Average energy delivered over the window approximates an analog level when the process integrates the pulses. This method is widely implemented in PLCs and microcontrollers using timers and is compatible with P or PI algorithms; full PID may also be combined when dynamics warrant.

Step-by-Step Solution:
Identify that time-proportioning targets ON/OFF actuators needing “analog-like” control.Recognize that the mechanism is duty-cycle modulation within a fixed period.Select the option that states this explicitly.

Verification / Alternative check:
Controllers for ovens routinely specify “cycle time” settings; smaller cycle times yield smoother power delivery (subject to relay life). This matches the duty-cycle concept and validates the description.

Why Other Options Are Wrong:
Option A describes analog continuous output, not time-proportioning. Option C is false; PLCs and microcontrollers commonly implement it. Option D is over-generalized; PID vs duty-cycle efficiency depends on process and actuator.

Common Pitfalls:
Setting the cycle time too long (causing oscillations) or too short (stressing relays); using mechanical relays instead of SSRs for rapid cycling.

Final Answer:
It modulates an ON/OFF actuator by changing the duty cycle over a fixed period