Working on my RC mower, the cable pull that turns on the ignition circuit AND releases the brake, the one on the handle you have to pull before you can try and start it. It moves 22mm and takes about 30 pounds to pull the cable. I would love to be able to pull that and have it release if I turn it off or power is cut. Worried about the power consumption. I was thinking of a manual lever to pull the cable with a servo to hold that lever somehow, then if I turn it off or the power is lost it releases.
Anyone have any ideas on what type of servo I can use to accomplish or any designs where someone has solved this issue. I want to keep that heavy spring on there since it activates the brake on the engine to stop the blade.
If it is possible to wind the cable half a turn on a drum with a 20mm - 24mm diameter, you can get by with a 20 KG model building servo. If you want to work with a lever, and the lever (measured from the center of the servo axis, has to be longer than 12mm so that you can pull the 22mm), you need a stronger servo. Please note, however, that model making servos of this performance class and this one load at 6V need between 3 - 4.5 A. The second question that needs to be clarified is how long the servo has to maintain the 30 pounds of pulling force, because model building servos are seldom designed to apply high forces over several hours. If you have to maintain this force for a longer period of time, you either have to construct the mechanics in such a way that when the cable is fully pulled, the force curve runs straight across the center of the pivot point so that the servo does not have to constantly apply the force via the holding torque, or you you have to loosen this using a threaded spindle that places the load on the thread, because in continuous operation under full load, a model making servo becomes too hot and becomes defective.
What I would do is tape that bar down and use a relay in series or parallel with the ignition kill switch. Yes, the blade will take longer to coast to a stop.
MY old Craftsman mower is exactly the way the OP mower is described. The brake is on the flywheel to limit liability when someone gets cut by the mower blade. The OP is already beyond that point, so will have to concur with John's suggestion.
Paul
I am hoping to keep the brake, was going to also have a separate ignition. I could also just put a much lighter spring and a servo to turn on and off, but that would also lose the bake.
Let's see it I understand this project. You are making an RC lawn mower. That suggests it will be running some distance from where you are controlling it. No need for a brake as the motor will be stopped by the time you get to the machine.
If you have other reasons for the brake, make it a manual, lever operated device so when you are close by, you can use it to stop the motor(engine).
Paul
It should go without saying but I will anyway: In the US at least 16CFR §1205.5(a)(iii) reads "A walk-behind rotary power mower shall have a blade control system that will perform the following functions ... cause the blade motion in the normal direction of travel to come to a complete stop within 3.0 seconds after release of the control."
That rule is there to help prevent gruesome amputations and soft-tissue obliteration caused by a blade spinning at 3500RPM. The operator of the machine recognizes a hazard and releases the control, allowing the blade to stop in 3-sec.
Without any such intelligence on your mower and without you there to take that task on yourself, you need to understand the hazards to which you're exposing yourself and others if you disable the safety and the thing goes off on its own through a daycare.
You can reduce the risk by having a trip mechanism that releases the brake control in the event the MCU calls for it OR if an external "watchdog" circuit isn't reset periodically by the MCU. This could be a commercial quick-release gizmo triggered by a solenoid. The solenoid in turn is controlled by a circuit that fires it if the MCU sets a pin 'high' (e.g.) or if the MCU fails to send periodic (e.g. 10Hz) pulses to the circuit.
The mechanism would require you to "set" it manually before sending the machine on its way, the quick-release would hold it in that position (and consume no power while doing so). An "emergency stop" button on your remote would be used to send a message to the MCU on the mower to trip the solenoid. You'd want to structure your code so that you send periodic stop-status messages to the MCU from the remote and for each message the MCU receives it refreshes the auto-trip watchdog circuit.
If the MCU stops receiving those messages it can trip the solenoid but it will also stop refreshing that circuit so it should trip automatically. Similarly, if the MCU loses its mind and stops refreshing because it's stuck in a loop somewhere, the watchdog should trip the solenoid.
It's not failsafe and because you wouldn't be using safety-rated parts nor safety-rated code but it should be safer than nothing at all. And when you're facing down the parents of a now-foot-less kid in court it might make for a better outcome for you if you can demonstrate that you at least thought of risk management.
Here is what I have so far, its going to be a slope mower for my mountain property where it is too steep for the tractor and I am tired of trying to brush cut it. There is no chance at all it will make it off my property.
I am thinking my only cost effective choice is a lever I set by hand and then some sort of servo to hold the trigger. What type of servo will default to a certain position if all power is cut. I do not want to rely on code to stop it because with no power noting would turn it off. I plan one relay to turn on the ignition circuit, and some way to hold the brake open which also completes the ignition circuit. That way I can turn the motor off and on remotely and only release the brake if something fails or I hit the kill switch that kills the motor then requiring me to reset that lever. I am also putting in a tilt sensor and figure out how to turn it off if it flips over. So I am looking for quick release that will release if power is cut aslo.
The wheelchair motors still have their brakes, and I tested on a 45 degree ramp and they hold, nothing will turn those wheels if the controller is in neutral or you turn the controller off. While there is nothing that can get hurt by this on my property, I want it to be safe still.
That would be the best idea, manual activation. and the servo to lock the lever to automatically release it to disable the mower.
That way the servo does not have to be powerful, only strong enough to hold a trigger. OR
A holding solenoid on the trigger so that it is a "deadman" type, power stops, motor stops.
If you've assessed basically zero risk then just disable the brake by zip-tying the linkage to pull the pad away from the flywheel and use a relay to ground the ignition lead to stop the engine.
I thought about it last night, an electromagnet to hold the trip lever down, if power is cut it will release the lever. Went through my pile of wheelchair parts and I found a continuous duty 24 magnet used for brakes, and with just 6 volts to it it will be more then strong enough. So I can control it from the Arduino from input form the controller, tilt sensor or even distance sensor I have in case something gets in its path. So turn the power on, turn the magnet on and manually set the lever. Thanks for all the help "holding solenoid" was the concept I needed to get to.
As far as I know, all modern small engines generate about 12 volts from the flywheel magnet/coil. That charges a capacitor large enough to power a transistor ignition. Many also power several 12 volt lights, when running a riding mower.
So there should be enough more 12 volt power available to power a solenoid to hold the engine brake lever in the off position.
Paul
