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| Is there a better way than relays ? |
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| don_and_his_boats:
Hi I build R/C model boats . The speed controller in the boat ( ESC ) can be made to run as forward only or as fwd/rev with center stick as off however my transmitter doesnt have a self centering stick so finding the middle quickly when in a panic is not easy and a crash is the likely outcome , At 70 years old I dont think so quick as I did With the ESC set for forwards only using the throttle its easy , I just pull the stick right back , no fidling to find ther middle and the boat stops but then you get the problem of if you are in a corner you have no reverse to back out The motor in the boat is a brushless one ( 3 leads) . I know I can switch any two leads and the motor will reverse so 2 changeover relays , one for each of the two motor leads on another channel makes for a simple reverse switch , but the motor has a maximum current draw of 40Amps and if I forget to shut the throttle first before activating the relays I fear the contacts would soon weld together, , and what happens if one relay is a bit sluggish, wont I get a dead short? I am thinking I could maybe do the job with two powerfull mosfets but how to wire them I am not sure , or even maybe an H bridge could it be done that way? Any circuit ideas would be great either for mosfets or H bridge , or if you know an even better way then great Regards Don |
| JuanGg:
I understand you want throttle control with one stick and an additional channel to change between forward and backwards. I personally would throw in a micro that monitors both channels, and drives the ESC appropriately, on the upper end to go forward or on the lower end to go back as needed. Its just a matter of measuring the pulse width and should be relatively simple to code. It could even be programmed so as to ramp up the speed in case you flip the switch at full throttle. Juan |
| rstofer:
Reversing a motor with a relay is usually done with a single double pole-double throw (DPDT) configuration. You can Google for 'reversing a DC motor with a relay'. There's nothing wrong with the relay approach and it really comes down to deciding which is the bigger problem: Having a relay to mount and wire or replacing the control box with a joystick with centering. I don't know that I would have a preference as long as I had a channel to control the relay. I would wire the relay such the Forward was the default position. |
| MarkF:
--- Quote from: don_and_his_boats on July 15, 2019, 06:41:04 pm ---Hi I build R/C model boats . The speed controller in the boat ( ESC ) can be made to run as forward only or as fwd/rev with center stick as off however my transmitter doesn't have a self centering stick so finding the middle quickly when in a panic is not easy and a crash is the likely outcome , At 70 years old I don't think so quick as I did --- End quote --- If you are in a panic and want to stop quickly, isn't switching into the reverse a good thing to help you stop. It doesn't seem to me that being at an exact center for quick stops as being a problem. Being able to go beyond center may be beneficial and desirable. I agree that switching a double pole relay at medium to full load will burn the contacts in short order. I'll have to study a little to see if there is simple way to lock-out relay switching until the throttle is at zero. Maybe with a latching relay and disrupting the signal to the coils. Are there MOSFETs to control the speed now? I assume you are using a typical RC Radio as used in RC planes and cars? There is no way to increase a null either in the radio or receiver? |
| Ian.M:
Reversing the motor with a relay between the ESC and a three wire motor would be a really bad idea, and switching while running risks blowing the ESC. You *NEED* to intercept the control signal. At its simplest you could use another channel to drive a small relay (simple On/Off) to disconnect the input servo pulse from the ESC and ground its input, which should then go into its fail-safe 'Loss of Signal' mode. Another approach would be to add a wider deadband to the control so its easier to find the off position. It *MAY* be possible to program the ESC for this, otherwise a MCU receiving and regenerating the servo pulse would be the way to go. You could also do stuff like rapid transition through center in either direction latches the OFF state till the control returns to center but slow transition gives Foward/Reverse control with a moderate deadband. |
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