Greenhouse roof vent: actuator torque and fail-safe control
A school greenhouse has six roof vents, each 900 mm × 600 mm, hinged along the top edge and opening outward. Each vent has a mass of 4.5 kg and its centre of mass is 300 mm from the hinge. A linear actuator pushes at a point 250 mm from the hinge, perpendicular to the vent when closed. The actuator is rated at 150 N maximum thrust and 50 mm/s, powered by a 12 V supply. The existing controller simply runs the motor for 20 s to open and 20 s to close. On hot, windy days the vents sometimes jam, the fuse blows, and the motor is found stalled. The greenhouse also needs the vents to fail safe: if power is lost, they should open at least 100 mm to prevent overheating. The technician proposes fitting limit switches and a current-sensing relay.
1. Calculate the moments acting on the vent when closed and decide whether the 150 N actuator can open it in still air. Show your reasoning and state one assumption.
Read the worked answer
Assume the actuator force is perpendicular to the vent at the attachment point and hinge friction is negligible. Weight W = 4.5 × 9.81 = 44.1 N. Moment of the weight about the hinge = 44.1 × 0.300 = 13.2 N·m. Actuator moment = 150 × 0.250 = 37.5 N·m. In still air the net opening moment is 24.3 N·m, so the actuator can open the vent.
The worst case is near the closed position, where the effective lever arm is greatest. A wind load of 60 N at the centre would add 60 × 0.300 = 18 N·m, making the total resisting moment 31.2 N·m. That leaves only 6.3 N·m margin, about 17%. Ice, dirt in the hinge or slight misalignment could therefore stall the motor and blow the fuse.
2. The motor stalls and blows a fuse. Explain why the time-based controller is inadequate and propose a control strategy with sensors and fail-safe.
Read the worked answer
A time-based controller assumes the vent always moves at 50 mm/s and never meets resistance. Wind, dirt and ice change the load, so 20 s may be too long and stall the motor, or too short and fail to open the vent. Fit limit switches at fully closed and fully open to stop the motor at the correct positions. Add current sensing to detect a stall, for example current above 4 A for 0.5 s, and cut power or reverse briefly. A microcontroller can provide PWM soft-start and a temperature sensor input.
For fail-safe operation, do not rely on the motor. Fit a spring or counterweight that opens the vent at least 100 mm when power is removed, or use a wax actuator as a backup. Test by removing power under load and measuring the opening. This addresses the safety requirement independently of the control electronics.
3. Why might a wax actuator be better than a motor for fail-safe operation? Give one limitation.
Read the worked answer
A wax actuator contains wax that expands when heated, producing a slow, powerful push without electricity. It is inherently fail-safe: on a hot day it opens the vent even if power fails, which is exactly the greenhouse requirement. It is also quiet and cheap compared with a motor and controller. Its limitation is that it responds to its own temperature rather than the greenhouse air, has hysteresis, and cannot be controlled precisely or forced closed in cool weather. It is best used as a backup vent opener or on a separate vent, not as the only controllable actuator.