[PSUBS-MAILIST] Speed controllers

Sun Apr 24 15:37:30 EDT 2016

Good points, all.  Psubs for me is a hobby.  $1500 for a pressure
transmitter is low likelihood given oil price of $40/bbl.  When the oil
price goes back up, my psub budget will go up as well.  As to error for
typical altitude measurement, I use a sounder, and it seem to be quite
accurate but my assessment is purely subjective as I don't have hard data
on the accuracy.  As a starting point I may give your suggestion on the PID
setup (underdamped system with perceptible but slow integral correction,
and forget about derivative entirely) a try.

cliff

On Sat, Apr 23, 2016 at 3:50 PM, Sean T. Stevenson via
Personal_Submersibles <personal_submersibles at psubs.org> wrote:

> Regarding transducer accuracy, rarely does the accuracy specification
> correspond to variable short term drift. Usually it is the worst case
> scenario combining absolute accuracy, nonlinearity and hysteresis, but the
> transducer reading tends to be stable at a given temperature and pressure.
> In a depth keeping application, it actually isn't critical what the actual
> depth is unless you're making bathymetric charts or operating close to
> crush depth. You don't care what the absolute output of the transducer is -
> only that it is properly representative of changes so that a PID controller
> can respond appropriately to the error signal when excursions from the
> setpoint occur. That said, I'd still look at what the transducer cost is
> for such a critical measurement. Your 0.25% transducer may be fine if the
> drift is acceptable, and you can also lowpass filter the measurement and/or
> PID output signal to improve stability (at the expense of response sp!
> eed). You don't necessarily have to respond to errors rapidly, and your
> momentum will limit that anyway.  I would probably run a depth control loop
> at something like 20 Hz, versus the 300ish range that I use for things like
> proportional valve PWM.  Integrator resolution is less critical in sluggish
> systems. In fact, you could possibly get away with an even cheaper
> transducer if you can accept the response lag.
>
> I bought a Honeywell STJE (0.05%) last year for about $1500 CAD.
> Expensive, but that was for an application where absolute accuracy was
> important, and you can't do much better in an industrially robust
> transducer. The standard TJE's are a bit less expensive, and of course
> there are all sorts of lower cost options available. Your 0.25% transducer
> might be a good tradeoff. I'd probably be hesitant to go much looser on
> accuracy though. 1% is probably not accurate enough.
>
> Auto altitude is not something I know much about, other than operating by
> means of a sonar transducer or array / DVL. What sort of error is typical
> of such measurements? Can a siltout or fish swimming under the transducer
> cause an erroneous measurement?  Filtering could help with that too. I
> never considered auto altitude because it requires a flat-ish bottom to
> work - good for seafloor pipeline inspections, but not so much for flying
> over dropoffs and shipwrecks.  Large step changes in your process variable
> measurement are not good for PID loops.
>
> As for tuning, you are correct that it wouldn't be easy. I'd probably
> shoot for an underdamped system with perceptible but slow integral
> correction, and forget about derivative entirely.
>
> Sean
>
>
> On April 23, 2016 12:19:54 PM MDT, Cliff Redus via Personal_Submersibles <
> personal_submersibles at psubs.org> wrote:
>>
>> Sean, I have also been toying with the idea of using a PID loop not for
>> all my maneuvering but just for depth station keeping.  I may give this a
>> try as an experiment.  My PLC has PID loops built in so it would be easy to
>> try this with minimal ladder logic programming.  Initially I was thinking
>> about establishing the depth set point via a user input through my HMI but
>> I kind of like your idea of using joystick to drive to desired depth then
>> taking this depth as the setpoint when the pilot lets the joystick reach
>> its neutral position via spring return. With my two MK101s vertical
>> thrusters, PID controller would pretty easily be able to automatically zero
>> out difference between setpoint depth and measured depth.  One other point,
>> if would be very in the PLC code to switch between depth stations keeping
>> and altitude station keeping if bottom was not flat.  Two issues come to
>> mind, first is it would not be easy to tune the PID controller and the
>> seco! nd is if my ambient pressure gage is accurate enough.  I have a 0.25%
>> gage now.   I might need a more accurate gage.  For altitude PID control,
>> the transmitter I have should work nicely.
>>
>> Cliff
>>
>>
>>
>> On Sat, Apr 23, 2016 at 8:00 AM, Sean T. Stevenson via
>> Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>>
>>> Part safety, part allowing for future upgrades. In my mind, if you let
>>> go of the controls, the vessel should stop, period. If you have an alarm,
>>> leak, fire or something else that demands your immediate attention, you
>>> don't want to waste precious time having to null the thruster output before
>>> dealing with the other problem. Hav! ing the stick(s) spring return to zero
>>> output when you let go is just prudent, so you (hopefully) don't crash into
>>> anything when you have to let go in an emergency, or when you drop your
>>> pencil on the floor and throw your back out when you bend over to retrieve
>>> it.  I would employ self nulling controls regardless of whether I was using
>>> direct or indirect control.
>>>
>>> With the indirect scheme I proposed, there is an additional advantage to
>>> be gained in the presence of sensing mechanisms for vessel motion (surge,
>>> sway, heave, yaw, roll and pitch) such as the ubiquitous pressure
>>> transducer for depth, gyro/fluxgate compass for heading, or e.g. Doppler
>>> velocity log for over bottom motion. In these cases, a control loop
>>> provides the ability to null vessel motion, as opposed to simply nulling
>>> thruster output, so that if you let go of the controls, the system can
>>> automatically apply reverse thrust to cancel headway or compensate for
>>> slight currents etc. to keep the vessel where it was when you let go.
>>>
>>> This is particularly useful in the case of vertical motion. I intend to
>>> implement such a depth controller so that I drive up / down with the stick,
>>> with full range on the stick corresponding to 100% thruster output, but
>>> when I let go, the current depth becomes the setpoint and the controller
>>> takes over, commanding the vertical thrusters as appropriate to maintain
>>> that depth. Furthermore, in the event that maintaining that depth then
>>> requires a sustained thruster output in either direction, the variable
>>> ballast system will automatically adjust in order to bring that necessary
>>> thruster output down to zero and thus conserve power.
>>>
>>> Sean
>>>
>>>
>>> On April 22, 2016 11:13:19 PM MDT, Alan James via Personal_Submersibles <
>>> personal_submersibles at psubs.org> wrote:
>>>>
>>>> Not quite following that Sean,
>>>> why not have a joystick without return to center function &
>>>> leave it on that setting? I can see the sense in running the joystick
>>>> through
>>>> the PLC with an over-ride on the vertical thrusters when on the depth
>>>> limit,
>>>> I have seen commercial psubs with this feature.
>>>> Alan
>>>>
>>>>
>>>>
>>>> ------------------------------
>>>> *From:* Sean T. St! evenson via Personal_Submersibles <
>>>> personal_submersibles at psubs.org>
>>>> *To:* Personal Submersibles General Discussion <
>>>> personal_submersibles at psubs.org>
>>>> *Sent:* Saturday, April 23, 2016 4:31 PM
>>>> *Subject:* Re: [PSUBS-MAILIST] Speed controllers
>>>>
>>>> Late to this thread, but I'll throw in my $0.02:
>>>> I had envisioned a control scheme whereby the joystick inputs are
>>>> decoupled from direct thruster / ballast control output. Instead, the PAC
>>>> runs the thruster outputs on the basis of PID control loops, where the
>>>> setpoints are adjusted by the pilot controls. Thus, the ramp rate of the
>>>> target setpoint is dependent on how far e.g. the joystick is pushed or
>>>> rotated, but on letting go, the stick springs back to center, and at that
>>>> point the setpoint is overwritten with the current depth, heading or what
>>>> have you, and the system automatically maintains that setting until you
>>>> touch the controls again.  Manually commanded fully automatic.
>>>> Sean
>>>>
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>>>>
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