Servo actuator reverser  - Page 1
Forum section: electronics / projects

author: Connecteur
author_rank: Frequent Contributor
timestamp: February 02, 2024, 01:27:16 pm
content: I'm designing an actuator that extends when power is applied, and retracts when reverse polarity is applied. It has only 2 leads. The actuator disconnects from power at each end of the stroke. The operating voltage is 12V (automotive application. This is the linear actuator:https://www.amazon.ca/dp/B0BCDCTD3N?ref=ppx_yo2ov_dt_b_product_details&th=1The actuator will be activated by the vehicle's backup light circuit; when the car is in reverse, the actuator retracts to uncover the backup camera. When reverse is deactivated, the actuator is to extend. I will need a live 12V+ lead for the actuator (both directions) and the backup light circuit will activate a relay. I am uncertain which type of relay to use and how it is to be wired. Any help appreciated.

author: Ian.M
author_rank: Super Contributor
timestamp: February 02, 2024, 01:50:00 pm
content: Although its tempting to simply use a single DPDT relay with power and ground cross-wired to its fixed contacts to reverse the polarity to the actuator whenever the backup light is on, if anything jams the mechanism so the end of stroke limit switch in the actuator doesn't disconnect the power, the actuator will burn up. As its body is plastic this may cause a fire hazard.To prevent this you need to only apply a pulse of power for a few seconds whenever the backup light switches. You may be able to find a single shot time delay relay capable of triggering on both edges, to feed the polarity reversing relay. If not you'll need two SPDT single shot time delay relays, one rising edge triggered and the other falling edge triggered, which will provide both the polarity reversal and the pulse timeout functions. When not triggered they connect both actuator wires to ground. When one its triggered it connects its actuator wire to 12V.Also consider the all solid state solution of a low pin count MCU monitoring the backup light circuit, fed by a HV tolerant regulator, driving a H-bridge.

author: Connecteur
author_rank: Frequent Contributor
timestamp: February 02, 2024, 08:27:52 pm
content: Quote from: Ian.M on February 02, 2024, 01:50:00 pmI appreciate your safety concerns, but I don't think over-engineering is the answer. I'll install a fuse to belay that fear.I'd like to think that a simple DPDT relay will work. It needs to extend the linear actuator in the open position, and retract it in the closed position. I hope to accomplish this with one live wire and the trigger from the backup lamps. If there's a better way, I'm open to it.

author: Ian.M
author_rank: Super Contributor
timestamp: February 02, 2024, 11:33:30 pm
content: A fuse probably wont help! It will pull its stall current (briefly) every time it starts moving, and *if* you manage to find a slow blow fuse capable of distinguishing between its running current and its stall current the startup surges will tend to fatigue it resulting in short fuse life. Look at the actuator's rated duty cycle, and you'll see its quite capable of burning out just due to normal operation!QuoteIt *claims* to be overload protected but as it's from Shenzhen, you cant trust that unless you do a full teardown, and it may not be reassembleable.You do however need a fuse to protect against relay contact failure resulting in a three terminal short.

author: Connecteur
author_rank: Frequent Contributor
timestamp: February 03, 2024, 04:40:48 pm
content: Quote from: Ian.M on February 02, 2024, 11:33:30 pmWell, I got 2 actuators. If one burns up, it's on the outside of the car, so it might burn some paint.