Navpilot 700 Dynamic breaking and pulse width modulation

[aznarepse]

Hi,
I am trying to find out a definitive information confirming whether the NavPilot 700 series features dynamic braking and pulse width modulation.

Basically:

• Will the autopilot brake a DC rotary unit when it is not driving it (by short circuiting the unit)?
  Will the autopilot vary the speed at which actuates the rotary unit as needed?

If the answer to the above is negative, does someone know whether the autopilot would work with a ComNav CT2 or similar in order to drive a DC rotary unit?


Many thank in advance.

 

[Johnny Electron]

No the system won't support those types of drive systems. For sailboats we recommend Liner Drive units or units that work from a solenoid or clutched based hydraulic system. We have recently stated in a similar posting that Jafa drives are not supported at this time.

 

[aznarepse]

Hi Johnny,
thank you for your reply. Sorry, I did not see such statement (clearly) elsewhere in this forum.
I currently have a NavPilot 700 system with a rotary unit and it works. The rotary unit has a worm drive and that acts as a brake when the clutch is engaged. It is very power inefficient though.
I am looking for a replacement unit (not necessarily Jafa), since the one I have is very old (+25 years) and tired. For easiness of replacement, I was looking to sprocket driven rotary units so I can just switch one for the other.

 

[aznarepse]

On a more specific scope, please, could you advise if the autopilot uses pulse width modulation and or dynamic braking?
As mentioned, I am not specifically looking at the Jafa units, I just need to know the features so that I can make an informed decision.
I am for example considering using a middleware between the autopilot and the drive as well. (like the solenoids system but electronic).

 

[Johnny Electron]

Our units are Pulse Width Modulated but currently we don't have have Dynamic Braking.

 

[aznarepse]

Thank you very much again for your response. It has been very helpful.
I will continue my research on available options. Might even consider building a simple dynamic braking middleware (using MOSFETs for example, short circuiting the drive when no voltage is delivered).

 

[Taniwani]

Two comments to your questions.

It is quite likely that you do not have to short the motor to prevent the drive unit from turning under load. Even without a worm drive, the gear reduction is usually so big that you cannot overcome friction in reverse direction. I have a Whitlock Mamba drive and it incorporates a 3-stage planetary gear. There is no way tu turn the rudder with the motor disconnected and the clutch engaged.

Second, it is also quite likely that your NavPilot actually shorts the motor. Most common H-bridges energise both low-sides when both inputs are low. You can certainly program it to float, but that would be an explicit design decision to not brake the motor. You can try buy taking one lead off the motor and connect it to Bat+ when the unit is engaged but not driving. If it starts turning, you can assume that the other end is pulled to the low side.

(Most H-bridges also allow braking to the high side, I think that's quite unlikely used, but you could try, just to make sure.)

Johnny Electron said the units do not have 'dynamic breaking', he didn't say they were explicitly not braking at all. Dynamic breaking would PWM modulate the braking (float/brake) until it eventually applies full brake. That way the breaking current through the motor is reduced, which may help the life of the motor. But note, that a small pause before applying brake would pretty much do the same thing since the motor cannot continue to spin complete free, with the fly-back diodes limiting its output voltage to battery voltage (charging).

 

[aznarepse]

Hi Taniwani,
thank you very much for your input.
When I bought the NavPilot, I was assured by Furuno UK support that it will drive any rotary unit: and it does drive mine.
It is just now that I need to replace the unit that I am finding contradictory information about it.

I believe you are right. H-bridges are designed for such operations. My concern is that there is so little technical documentation available about the hardware and also that a new rotary drive is a significant investment and there is not a 'try before you buy' option... I wish I could borrow one

I understand Furuno's position in this matter (not officially supporting or endorsing a unit that the manufacturer clearly discourages its use with the brand), since we are obviously talking about some liability. But still, it just places the risk (and confusion) in the user.

Thank you very much again. At least we now know that it 'certainly works with Whitlock Mamba and with an old Cetrek (my case).