Radio receiver controlled enable switch

[CraftyHobo]

Hi all!

I want to make a Signal Interrupt Module for testing motor failure scenarios. The motor is controlled by an ESC that receives signal from a Futaba radio receiver.
What I want to do is simply have an enable signal from the receiver either pass through the ESC control signal unaffected or blank it out completely depending on the state of a two position switch on the radio transmitter. Does anyone know a lightweight way to achieve this when my enable signal from the radio receiver is in PWM format?

 

[Gryd3]

Hi all!

I want to make a Signal Interrupt Module for testing motor failure scenarios. The motor is controlled by an ESC that receives signal from a Futaba radio receiver.
What I want to do is simply have an enable signal from the receiver either pass through the ESC control signal unaffected or blank it out completely depending on the state of a two position switch on the radio transmitter. Does anyone know a lightweight way to achieve this when my enable signal from the radio receiver is in PWM format?

The enable signal you stated is in PWM format.. can you clarify what the signal is in the on and off position?
Reason I ask, is PWM is of course a signal that varies it's on/off time. If at any time this signal stays 100% on, or 100% off (or very close to) you could use a couple small simple parts to complete your project.
If you want to test, you can get a diode, capacitor, and resistor. The diode will allow the signal from the radio receiver to pass to the capacitor, but not allow the capacitor to back-feed to the receiver. The resistor will simply sit across the capacitor. You can use a multi-meter now to roughly determine the % of which your PWM signal is high or low based on the voltage you measure. (Note Analogue meters are great for this! You do not need these components with an analogue meter )

If you can get me those details about the exact output when enable is on/off we can move forward.

 

[CraftyHobo]

Thanks for the prompt reply! Luckily this is conveniently provided by Futaba in the following format:
Frame rate: 20ms
Minimum position occurs with a pulse of 1ms
Centre position occurs with a pulse of 1.52ms
Maximum position occurs with a pulse of 2ms

My control scheme will operate with a low signal (motor disabled) at 1.3ms, and high signal (motor enabled) at 1.8ms. These states are to be controlled by a two-position switch.

 

[BobK]

That is the control signal for a servo. You could use a servo to flip a switch!

Bob

 

[CraftyHobo]

Haha Bob, that is true, but I need this to be a small device without any moving parts.
I guess my biggest question is how can I easily interpret a servo control signal as an enable (sustained high or low)?

 

[Gryd3]

Haha Bob, that is true, but I need this to be a small device without any moving parts.
I guess my biggest question is how can I easily interpret a servo control signal as an enable (sustained high or low)?

I don't know enough about the inner workings of a servo... but would it be possible to simply rip apart a tiny servo, and use it's control board for what you want? (It's an H-bridge with feedback... but if the feedback is gone, won't is simply try to keep turning in one direction? This would result in the H-Bridge making one of the motor wires high which you could use for 'enable')

 

[CraftyHobo]

That sounds like it could be a clever idea, Gryd3!
I've had an alternative idea as well, just want to run it by you guys to check its feasibility. Could I introduce a low pass filter to convert the PWM to an analog voltage, then use a comparator with a reference voltage chosen to distinguish between the radio receiver ON/OFF position? Since the duty cycle for OFF is only 0.10 and ON is 0.05, would this be too close together to get a reliable cutoff?

 

[Gryd3]

That sounds like it could be a clever idea, Gryd3!
I've had an alternative idea as well, just want to run it by you guys to check its feasibility. Could I introduce a low pass filter to convert the PWM to an analog voltage, then use a comparator with a reference voltage chosen to distinguish between the radio receiver ON/OFF position? Since the duty cycle for OFF is only 0.10 and ON is 0.05, would this be too close together to get a reliable cutoff?

I thought of this as well... but am unsure of the exact behavior... If I had one I would test it
In any case, if you have a multi-meter and can build a simple low pass filter to test, this would be a viable solution.
The trickiest part would most likely be getting a stable reference voltage for the comparitor.
Additionally... I am certain there has got to be a much simpler method... RC has been around for years!

 

[BobK]

The RC time constant would have to be much longer than the period between pulses (which I assume is 20mS) in order to come to 2 distinguishable voltages. This means you would have a response time of maybe 20mS x 10 = 200mS. Would that be too slow?

Bob

 

[CraftyHobo]

The RC time constant would have to be much longer than the period between pulses (which I assume is 20mS) in order to come to 2 distinguishable voltages. This means you would have a response time of maybe 20mS x 10 = 200mS. Would that be too slow?

Not at all. This is one of the circuits I've needed where speed is not a constraint at all!