MR60

How-to guide: Pixhawk with 6S batteries (> 4S)

3689574013?profile=originalHi,

1. Introduction:

Pixhawk 3DR kit is delivered by default with a 4S maximum power module. For those wanting to use 5S or 6S or higher voltage batteries there is currently, to my best knowledge, no “how-to” guide for the Pixhawk board. I therefore decided to document it for others who might need it too.

For those who would like the same “how-to guide” for APM 2.x , here is a link I wrote a while ago:

http://www.diydrones.com/profiles/blogs/powering-your-apm-drone-or-how-not-to-shutdown-apm-like-the-us

Pixhawk comes standard with three (redundant) ways to powered it up:

1-USB : not used to fly obviously; just useful on the ground for connection on a ground station software.

2-The power module port accepting a maximum input voltage of 5.7volts (and will not get destroyed up to 20 Volts input)

3-The RC input pins. Will accept a maximum voltage of 5.7volts also (and is also protected up to 20 Volts)

This guide assumes a use of Pixhawk’s power module port which provides not only a way to power the board but also the pins to measure current and voltage values of the main battery.

This guide assumes a use of a 6S battery in combination with a Attopilot current & Voltage sensor board. This Attopilot “power module” replaces the 3DR 4S limited power module. The Attopilot board comes in three flavors: 45 amps, 90 amps or 180 amps.

3689573992?profile=originalThe choice of the right Attopilot board (45A, 90A or 180 A) will depend on your motor/props combination: take the Attopilot version that has the smallest amps capacity above your max multicopter current consumption. However we will introduce in this guide a way to use the 90 amps Attopilot board to measure up to 150 amps, still using Pixhawk’s power module port.

 

2. Attopilot description:

An Attopilot board provides three wire soldering pads to solder : a current measurement wire, a voltage measurement wire and a ground wire. See picture below:

3689574026?profile=original

Attopilot 90A support up to 50Volts for a maximum of 90A. However the resistor specifications exceed the 90A limitation which makes it possible to use it for measuring 150 amps (we will take this as a assumed max current as our example for the rest of the explanation).

The datasheet of Attopilot specifies that the Voltage measurement wire outputs an analog voltage of 63,69 milliVolt per Volt. Similarly the current measurement wire outputs an analog voltage of 36,60 milliVolt per Volt.

So for a 6S battery the maximum analog voltage values will be:

-For voltage measurement: [min 0V -  max 1.6V]

-For current measurement: [min 0V – max 3,3 V]

 

3. Pixhawk power port description (pinout):

3689574113?profile=original

Reusing the excellent pixhawk infographics published in the wiki, the image shows circled in yellow where the power port is on the pixhawk board.

The power port is a so-called DF13 connector with 6 pins.

The six pins of this connector are assigned in the following order, starting by the red wire on the leftmost pin:

Power Port Pinout Description:

  • 1- Vcc (5V input)
  • 2- Vcc (5V input)
  • 3- I (Battery current measurement analog voltage input)
  • 4- V (Battery voltage measurement analog voltage input)
  • 5- Ground
  • 6- Ground

 

4. Wiring Case 1 : to measure up to a maximum of 90 amps

The connections between Attopilot and Pixhawk are shown in the illustration below:

3689574082?profile=original

We have added an optional BEC in the illustration that would be connected to the Vcc and Ground wires of the power module. It is optional as Pixhawk could alternatively be powered via the RC inputs.

4. Wiring Case 2 : to measure up to a maximum of 150 amps

The connections between Attopilot and Pixhawk will integrate resistors to be able to measure up to 150 amps.

Indeed the ADC of this power port on Pixhawk has a range of 0-3.3V. This means that for the maximum true current of 150 amps, we want the current analog wire of Attopilot to output maximum 3.3Volts (as it is the case in case1 for 90 amps max without additional resistors).

Note : this part has been updated with a resistor scheme simplification (only one resistor to add in parallel rather than the originally classical R1, R2 resistors divider) thanks to a contribution of Bo, a diydrones member who analyzed in depth the Attopilot circuitry.

So we will build a small resistors divider on wire 3 (current measurement) & wire 5 or 6 (Ground) as follows:

3689574212?profile=original

The Attopilot current measurement output masks a circuit that contains an existing output resistor called Rl. According to the Attopilot datasheet, the following equation links to measured current I (called MeasuredCurrent in the equation), the output analog voltage for current measurement Vout, and an existing Rs resistor in Attopilot:

3689574168?profile=original

 What we want is to get Vout = 3,3 when Current =150 amps. To do this we will add a new external resisto (Rx) in parallel with the existing Attopilot Rl resistor, so that the new resulting Rl resistor (called Rl’) must be (knowing that Rs = 0.5Mohm as per Attopilot specs):

3689574150?profile=original

Therefore we can calculate the Rx resistor value we need to add in parallel as follows:

 3689574221?profile=original3689574230?profile=original

So, Rx must be ~ 110kohm to have a Vout at 3.3V when the measured current is 150 amps.

You can choose another max amp output (but I would not advise higher than 150 amps with the 90 amps Attopilot, otherwise use the 180 amps version instead) and calculate the resulting Rx resistor

As a result, when the current is 150amps, Vout will have value of 3.3Volts.

 

5. Mission planner / parameters configuration in battery monitor screen:

In the battery monitor parameters screen, you can manually select which current and voltage sensor you are using. In the present case, you will select the power module and modify the following parameters to make the mission planner voltage and current display match the real values (measured using a wattmeter for example). The explanation below is an extract from the Arducopter parameters list.

Battery monitoring (BATT_MONITOR)

Controls enabling monitoring of the battery’s voltage and current

Value

Meaning

0

Disabled

3

Voltage Only

4

Voltage and Current

Battery Voltage sensing pin (BATT_VOLT_PIN)

Setting this to 0 ~ 13 will enable battery current sensing on pins A0 ~ A13. For the 3DR power brick on APM2.5 it should be set to 13. On the PX4 it should be set to 100. On the Pixhawk powered from the PM connector it should be set to 2.

Value

Meaning

-1

Disabled

0

A0

1

A1

2

Pixhawk

13

A13

100

PX4

Battery Current sensing pin (BATT_CURR_PIN)

Setting this to 0 ~ 13 will enable battery current sensing on pins A0 ~ A13. For the 3DR power brick on APM2.5 it should be set to 12. On the PX4 it should be set to 101. On the Pixhawk powered from the PM connector it should be set to 3.

Value

Meaning

-1

Disabled

1

A1

2

A2

3

Pixhawk

12

A12

101

PX4

Voltage Multiplier (BATT_VOLT_MULT)

Used to convert the voltage of the voltage sensing pin (BATT_VOLT_PIN) to the actual battery’s voltage (pin_voltage * VOLT_MULT). For the 3DR Power brick on APM2 or Pixhawk, this should be set to 10.1. For the Pixhawk with the 3DR 4in1 ESC this should be 12.02. For the PX4 using the PX4IO power supply this should be set to 1.

This is a parameter to adjust to match the real Voltage value with the displayed mission planner value.

Amps per volt (BATT_AMP_PERVOLT)

Number of amps that a 1V reading on the current sensor corresponds to. On the APM2 or Pixhawk using the 3DR Power brick this should be set to 17. For the Pixhawk with the 3DR 4in1 ESC this should be 17. Units: A/V.

This is a parameter to adjust to match the real Voltage value with the displayed mission planner value.

There you go! I hope this will help you configure your pixhawk with higher than 4S batteries.

Cheers,

Hugues

E-mail me when people leave their comments –

You need to be a member of diydrones to add comments!

Join diydrones

Comments

  • I believe the wait for a nice 6S sensor is over:

    http://www.mauch-electronic.com/apps/webstore/products/show/6418310

    I've always wanted to "steal" the V/A readings off my FrSky FAS-100 sensors, but I never knew how...

  • MR60

    You're welcome. Yes Dom, on the wiki the diagram they have put should be indeed corrected to have the BEC placed after the Attopilot in order for the Attopilot to also measure the amps consumption going through the BEC to power the Pixhawk.

  • Hi Hugues,
    Thanks for this clarification. I think I was getting confused about the diagrams at:
    http://copter.ardupilot.com/wiki/common-using-a-current-sensor/

    and at the top of your original post. On the wiki, the BEC is wired in *before* the Attopilot which, combined with your most recent post, has helped me understand that what we're doing here with the Attopilot only partially replaces the functionality of the 3DR Power Module, i.e: measure the voltage and *not* power the Pixhawk.

    The powering must be achieved elsewhere, i.e: dedicated 5v BEC.

    Again, thanks for the reply and hopefully I'm understanding correctly!
  • MR60

    @Dom, i do not know if my answer comes too late but : i understand you're asking about a configuration where you would use a single BEC to power both the power port of Pixhawk and servos. If so, you would not get a very stable power into the power port of Pixhawk. If the power port sees a bad power input (for example a voltage drop below 4,5V), it will switch to use the eventual backup BEC you would have on the servo rail. If you do not have any backup power on the servo rail, it will simply Brown out...(and reset)

    @Ron, I am not sure I understand your question as for me VCC is same as the + wire and GND is the same as - wire. Of course you need both connected (ground is ALWAYS needed).

  • I just finished wiring one of these up. Should I be using both +VCC and -GND wires in the power plug, or is a single + and - wire sufficient?

  • Hi Thomas,

    Thanks for your reply. I guess what I'm trying to ask is, if we're powering the Pixhawk as per the diagram in "4. Wiring Case 1 : to measure up to a maximum of 90 amps" in Hugues guide at the top of the page, and we have servos (gimbal motors let's say) attached to the *same* BEC that we are using to power the Pixhawk via a splice into the power port, do we need to take similar anti-voltage spike precautions?

    For example:

    Servos <---> BEC <---> Pixhawk's servo rail (Zener diode needed)
    Servos <---> BEC <---> Pixhawk's power port (Zener diode needed?)

    In both cases we are not using the 3DR power module as we are doing this 180A Attopilot business.

    Hopefully this makes my question clearer!

    Thanks,

    Dom
  • Admin

    @Dom,

    The Pixhawk uses the BEC power on its servo output power bus as a secondary power supply incase the PM fails.

    Based on my experience with my rovers, servo induced spikes on the Pixhawk servo output power bus can cause the Pixhawk to go into reset. That is why it is important to have a 5.6vdc 5watt zener diode installed across the Pixhawk servo output power bus to keep servo induced spikes from resetting the Pixhawk.

    Regards,

    TCIII AVD

  • Thanks for the reply Hugues. So let's say I had some servos that generated some voltage spikes and were attached to the BEC, and that same BEC (on a different output) was spliced into the Pixhawk power port, Pixhawk is happy?

    Is it a question now of if these spikes 'travel' from it's 12V side to it's 5V side?

    I guess what I'm asking is if I should have a little dedicated BEC just for the Pixhawk...

    Thanks!

    Dom
  • MR60

    Hi Dom,

    You would not need to use a zener diode for the power port. The need for this zener diode is to protect pixhawk from voltage spikes that could be generated by servos (so only when powering via the servo rail).

  • Hello chaps,

    I don't know if it has been linked in the comments yet, but there is another page on the arducopter wiki titled Attopilot 180A Voltage and Current Sensor:

    link: http://copter.ardupilot.com/wiki/common-optional-hardware/common-po...

    This seems to echo what Hugues comment on May 6th says. 180A is the way to go. I'll have to find another use for the 90A Attopilot I bought in earnest before reading *all* the comments!

    I did have a question regarding powering the Pixhawk when using the attopilot for voltage/current monitoring:

    If you are splicing in a BEC between the Attopilot and connecting it to the power port of the Pixhawk, is a zener diode necessary? It is recommended if powering the Pixhawk 'off the servo rail', but not clear if needed if powering through the power port. The BEC will be powering a gimbal and other things too.

    The BEC is this one:
    http://gryphondynamics.co.kr/product/dual-bec-for-gpd-1000-series-p...
    if it matters?

    Thanks!

    Dom
This reply was deleted.