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A thruster testing tool.

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haggisuk

New Member
Hi Folks,

My name is Derek and I work offshore operating ROV's which have DC thruster motor's that fail from time to time.

I was wondering how feasible it would be to build a small testing unit to speed up the fault finding process.

I have a idea of what I would like it to look like and have some of the spec's of the system that it needs to connect into.

I also have access to an electronics workshop with the tools required to make such a thing, it's just my knowledge that's lacking :confused:

My interpretation of the finished tester would be something that would plug in-line between the existing connections and show that the Voltage was present and show direction of rotation and what percentage.

I'm not entirely sure that this may be the best way to test this so please read the techie bit that follows later. You may have a better approach to the needs.

haggisuk-albums-thruster-stuff-picture378-tester.jpg


Spec's of the thruster that would be attached to it.

SM4M Thruster Specification
Nominal Voltage: 250 VDC
Nominal Current: 4.5 A (normal running)
Thrust @ 2000RPM: 25 kgf
Weight in Air: 3.7 kg (including propeller and nozzle)
Weight in Seawater: 2.2 kg (including propeller and nozzle)
Depth Rating: Up to 2500 m
Control Signal: PWM for Speed signal
Electronics 24 VDC @ 200 mA reversible for direction control
Connector: 7-Way Connector

Conection details

haggisuk-albums-thruster-stuff-picture377-thruster-connection.jpg


Text from the system manual explain the control of the thrusters.

"The digital data containing the speed
and direction signals are passed to the Thruster Control PCB (6034P). The speed signal
is converted into a TTL pulse width modulated signal and applied to the required
thruster to control its speed. The thrusters are controlled by a direction DC voltage
signal, whose polarity dictates thruster shaft direction"


"Thruster Motor Control

Thruster demands from the surface are transmitted to the two thruster drive PCBs
(6034P) via the backplane under control of the CPU. Each thruster control PCB has
four channels, each capable of producing a speed and direction signal.

The speed signal to the thruster control PCB is a 12 bit digital signal, which is converted
into a TTL 50 Hz base frequency pulse width modulated (PWM) signal of which its
duty cycle is proportional to the required speed. For example a 100% duty cycle
represents a continuous 0 V signal causing the motor to run at full speed, no duty cycle
causes the motor to stop (10-12 Volt signal) and 50% duty cycle the motor runs at half
speed. The duty cycle is referenced to the direction 0 volt line. The speed signal is fed
to the thruster via the bulkhead connector pin 5.

Direction is controlled by a 24 VDC signal (derived from 6051AP, PSU1-1), DIR+ &
DIR-, of which the polarity decides thruster direction. The 24 VDC is routed to the
thruster motor via the motor bulkhead, pins 4 & 6 providing a supply to the motor
electronics as well as thruster direction."

If you need any other info such as drawings of the PCB's etc please let me know.

Thanks


Additional info from the manual that may better describe the control process...

Thruster Control PCB (Part No. 6034P)

Function

The Thruster Control PCB provides the electronics power supply, speed and direction
controls for up to four thruster motors.

Technical Description

The outputs are isolated from the control electronics to prevent a fault on the load
affecting the rest of the system.

Speed

Two counter/timer IC’s each produce a TTL pulse width modulated (50Hz) signal for
two thrusters. The signal is amplified and converted from TTL to +12 V (10-12V) for
output to the thrusters. The speed is referenced to the 0 Volt direction line. Fuses
protect against excess load on each speed output and a zener diode connected across the
signals output provides over-voltage protection.

Direction

Thruster direction is controlled by two digital bits per thruster. The four pairs of bits
are simultaneously latched and used by a direction control circuit to produce TTL signal
outputs. The outputs are used by a differential driver circuit to provide the required
thruster direction as follows:
• DIR+ High & DIR- Low give clockwise direction.
• DIR+ Low & DIR- High give anticlockwise direction.
The direction voltage range is 21-24 VDC.
Fuses protect each direction output and a zener diode provides over-voltage protection.

Fuses

Direction and speed fuses F1 to F12 – wire ended 250 mA.
24 VDC input supply fuse, F13 – wire ended 1.5 A.
 
Last edited:

user_88

Member
What motor control components are actually onboard the thruster unit .... at depth?

It looks like the cable assembly would consist of the 250V DC cables, + and -, ground, and also a wire carrying the speed control signal.

Is the cpu that provides the PWM actually onboard the thruster, or is it at the surface?

Is the main problem the fact that you need to simulate a speed control signal, when the the thruster is not actually connected to the diving Pod, so that you can conveniently energize the thruster and see if it is working correctly?
 
Last edited:

haggisuk

New Member
What motor control components are actually onboard the thruster unit .... at depth?

It looks like the cable assembly would consist of the 250V DC cables, + and -, ground, and also a wire carrying the speed control signal.

Is the cpu that provides the PWM actually on-board the thruster, or is it at the surface?

Is the main problem the fact that you need to simulate a speed control signal, when the the thruster is not actually connected to the diving Pod, so that you can conveniently energize the thruster and see if it is working correctly?

The Thruster is an external component fitted to the frame which is connected to an electronics pod by means of a water proof cable and connector that has various pcb's fitted, one of which is the thruster control pcb which is the one that I have shown extracts of.

So what I was hoping to do was fit this test tool in between the thruster and the electronics pod to see if the supplies and drive electronics are functioning correctly when trying to determine if its a fault with the motor or internal fault in the electronics pod.

The faults would be as follows from most common to rarest.

  1. Insulation breakdown in the motors windings causing DC fuses to blow on fuse and filter card
  2. Insulation breakdown between drive electronics and earth causing thruster drive pcb fuses to blow
  3. Insulation breakdown between drive electronics and motor windings causing all the fuses to blow
  4. Motor drive card failure either no direction signal or speed signal
  5. Fuses and filter card failure resulting in loss of 250VDC to motors

So this test box in my minds eye would be in the middle of the circuit and just show if the voltages are present and the command signals that are being sent to the thruster if any.
 

user_88

Member
It looks like the only signal of interest ... the one requiring a little more clarification... is the speed control.

Is the speed control signal at 5 volts, or 12 volts, or 24 volts, when it goes from the thruster control pcb to the DC motor?

Is it correct to say that the speed control signal has a 50 Hz base frequency, and is modulated ... 0 to 100% duty cycle, as it inputs to the DC motor?

Does the polarity of the speed control signal change .... or reverse ... according to the direction control input ... between the pcb control board and the DC motor?
 

haggisuk

New Member
It looks like the only signal of interest ... the one requiring a little more clarification... is the speed control.

Is the speed control signal at 5 volts, or 12 volts, or 24 volts, when it goes from the thruster control pcb to the DC motor?

Is it correct to say that the speed control signal has a 50 Hz base frequency, and is modulated ... 0 to 100% duty cycle, as it inputs to the DC motor?

Does the polarity of the speed control signal change .... or reverse ... according to the direction control input ... between the pcb control board and the DC motor?

Thanks for your replies,

It would appear from the speed signal description that it would be 0 - 12v, but I shall confirm this today when I go to work.

As far as i know the speed signal does not change polarity its only the direction lines on pins 4 and 6 that swap polarity.

Please see attachments for drawings of the motor drive pcb which may provide more useful info.
 

Attachments

  • thruster drive circuit.pdf
    1.7 MB · Views: 179

user_88

Member
Yes .... speed control circuit pdf is very helpful.
It looks like the output of the speed control board is a variable pulse width signal, with a magnitude in the range of + 12 volts to 0 volts.

It may be possible to devise a simple test/diagnostic circuit to provide a led readout that is proportional to the duty cycle of the speed control signal. .... ... Will have to try this out and see if it actually works at the stated operating frequency.

Is there a circuit diagram available for anything related to the DC motor itself?
... Trying to get some idea regarding the main DC power signal waveform .... That would be the 250 volt DC/4.5 amp circuit.
 

haggisuk

New Member
Unfortunatley there is no drawings of the motor assembies as the company don't want people to be able to service them in field.

The motors have quite a complex set of circuits that provided braking etc for the motors as the have to change direction rather quickly.

As far as I am aware the power side of the motor is just plus and minus DC which is permanently on and all the speed and direction is done inside the motor from the speed and direction signals fed to it from
the motor drive card which is external to the motor.
 

user_88

Member
proto-circuit for PWM evaluation ....

The principle of this circuit is to create a voltage that is proportional in magnitude to the input pulse width modulated signal ... according to the duty cycle that is present.

The circuit will require some adjustment/tweaking. However, the main principle seems to work..

With a small fraction of the 50 Hz time period turned on, Vout is about 500 mV or so.
With a 50% duty cycle, Vout is about 5 volts. With a 95% duty cycle, Vout is about 10 Volts.

... Would eventually like to get a little smoother waveform.

The circuit here is only a simulation, and the it will have to be constructed on a breadboard in order to determine if Vout gives a reasonable response as the duty cycle of the input signal is varied. ... Something that has not yet been observed ....

If there are no major flaws or critical problems, then it should be possible to use a comparator circuit to set one or more limit levels .... and turn on appropriate leds to display the PWM output level.

.... Another possibility for utilizing the voltage output of the circuit would be to use an LM3915, which would supply 10 output leds, turning on at the appropriate voltage levels. ... This is similar to using individual comparators though.
 

Attachments

  • Thruster Control 1.jpg
    Thruster Control 1.jpg
    38.3 KB · Views: 151
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