Rc motor control

[bristol188]

Help please, am I missing something here. I built this component board (attach. A) to
drive a small motor (1 amp. max.) I thought it would run directly off the rc receiver PWM. All I get is slow rotation in one direction only, there is a slight change in sound
from the motor when I use the controls (switch or stick). Do you think I may need a driver. The reverse (Astern) is to get out of the reeds etc. main motor could be fouled.
Please don’t chide me on the rough model - I’ve got a rough head to match.
Attach. circet, model.

 

[hjames]

The PWM *control* signal varies between 5% and 10% duty cycle, and is intended to go to a *control* circuit that then creates a PWM signal that can be used to control a motor. The control signal just doesn't work like that.

What do you want the motor to do? continuous rotations if the stick is moved past a certain point?

 

[dknguyen]

A microchip can produce a 5V signal but at very low current. The 5V might be enough for the motor, but the current is much too low. The low current 5V signal is enough to switch a transistor on and off though. The transistor acts like a switch which can control the much larger current straight from the battery- and it is this current that powers the motor.

That is how a microcontroller uses it's very low current PWM signal to control the high current motor. So you need add a transistor (or a group of transistors in an H-bridge if you also need to reverse the motor) to control motor speed.

A transistor has 3 terminals. One terminal is called a gate or base (depending on the type of transistor) and you connect the microcontroller to this. THis terminal can switch on and off current flowing between the other two terminals which you can connect directly to the battery to produce much larger currents to drive the motor.

 

[bristol188]

Yes

I would like the motor to run for short bursts forard & reverse.using what ever controls I can from the transmitter.
Could I use the small board from a servo as a driver to the H bridge?
Thank's:

 

[dknguyen]

Yes, only if:

-the motor currents will not exceed the PCB current (the original servo motor current)
-control it exactly the way you would a servo (modify it for continuous rotation which would cause the PWM to control the speed)
-you know how to get rid of (or use) the potentiometer (the variable resistor)

Is this for continuous rotation? Or "elbow" rotation? For continuous rotation, you would basically do the exact same thing you would do to the PCB in order to modify a servo for continuous rotation except that you would remove the PCB and connect it to your own motor. The PWM would pretty much control the speed (with 1500ms being stop, and either side of that being progessively faster forward or reverse).

I don't have to explain how if how the servo will travel faster when trying to move to a point farther away right? When you modify the servo for continuous rotation it makes this point "run away" and stay a set distance ahead of the servo. The farther the point stays ahead of the servo, the faster the servo runs.

It may appear to behave the same, but in this case the PWM isn't directly controlling the speed of the motor and the mechanisms behind it are not the same as using an H-bridge (as described in paragraph above this one).

 

[bristol188]

Yes, again. I understand what you are saying. The motor needs to turn the prop. continuously but just short bursts to quick turn or back up. The IC will
take the motor current, it will drive the motor at full speed if I connect the
black wire (channel 2) to recv. neg. this turns on the IC full, it hardly gets warm in say 30sec. I just thought the TC4424a had inbuilt control, I was wrong it seems.
I do concide that the IC will get hoter at intermedate speeds.

 

[dknguyen]

With the servo PCB modified for contious rotation, the PWM pulse you send is NOT the one bursting the motor. Rather, the PCB translates the PWM to a position, and depending on whether how far away that position is, the PCB will generate ANOTHER PWM signal to drive the motor as fast as possible to reach that position (which continuously runs away, so the motor spins continuously). This PWM signal generated by the PCB is (because of the way the servo chooses it's travel speed based on position) will certainly be nowhere near the PWM you applied to the motor. It will be very hard to find a correlation between the PWM to the PCB and PWM that is being sent to the motor.

It you need to just spin the motor in one direction (I believe that is all you need for this propeller, but you said something about reeds), all you need is a single transistor AND the % duty cycle of the PWM signal you apply directly translates to the % of battery voltage that the motor effectively sees (and there it is much easier to choose the PWM duty cycle required to drive the motor at a particular speed. No need for a servo PCB in this case, and much better control. So maybe you should weigh the much easier method with more direct control against the ability to reverse the motor to get rid of reeds.

I didn't know boats could back up...I thought the props were unidirecitonal.

 

[bristol188]

It's the main motor that can be not operational when the thing runs into reeds or simular (main motor is 30amp esc controlled brushless) The little motor at
the back turns with the rudder, I just want to switch it on momentarily manualy with the 5 & 6 channel switches. You could say just put a boat on the pond! Yes but!

 

[dknguyen]

I'm still confused about which motor you are mucking with. Is it the main propeller motor? Or the rudder motor? Or is there a secondary propeller motor?

 

[hjames]

It's the main motor that can be not operational when the thing runs into reeds or simular (main motor is 30amp esc controlled brushless) The little motor at
the back turns with the rudder, I just want to switch it on momentarily manualy with the 5 & 6 channel switches. You could say just put a boat on the pond! Yes but!

So would it be easier to add some sort of reverse to the main motor? A relay which switches two phases of the brushless will cause the motor to run in reverse - just hook it up to the control board from a scrapped servo and you'll be able to change the prop direction from another channel.

James

 

[bristol188]

Proplem solved fo now

Thanks to all for your intrest I will drop the Hbridge & go with a servo board.
The main motor is an air prop and somtimes cannot be reversed, The model
will take off from the water so if I get it wrong taking off or landing it can be lodged in reeds or any other lakeside stuff.