Is there a better way than relays ?

[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]

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

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.

[Jeroen3]

Just program a button on your transmitter for thrust reversal. Those RC transmitter are very fancy nowadays.
Or were you still using 27 Mhz?

[don_and_his_boats]

Thank you all for your replies

My transmitter is new to me and is a bit of an all singing and dancing 2.4GHz jpbbie with lots of settings, so after reading some of the replies I studied the manual a bit more closely

It seems I can set the throttle curve in 5 steps , so the idea came to me that if I set the ESC for forwards/reverse then the mid point 50% is off or stopped
so if I set the 5 throttle points as

1    40%  = 10% revese speed
2    50%  = stop
3    50%  = stop
4    75%  = 50% forwards speed
5   100% = 100% forward speed

So I get quite a big null band which is easier to find and even if I overshoot and hit the bottom end stop then its only going to be a slow reverse speed

So I think that should solve my problems

But again thanks to all that helped
Don

[Jeroen3]

You can probably also program a button that activates a different throttle curve.

[don_and_his_boats]

I finally found a great way to do it

I stumbled across an arduino library called ESC master

It caters for both forward only and fwd/rev,
When you use forward/reverse you still have full stick travel but you have to tell it which direction you want

so simply setting a switched channel for direction the arduino can make the decision
if the switch is not pressed go forwards at the speed set by the throttle stick
if it is pressed go backwards at  the speed set by the throttle stick

It does it all with less than 20 lines of code

But thanks to everyone for your sugestions

Don