ThunderBorg High and Low Battery Voltage Questions

Submitted by jjmellon on Thu, 04/12/2018 - 04:27

Forums:

I'm trying to understand how the ThunderBorg works and how to program it, and I have the following questions:

1. When my NiMH batteries are fully charged, they read 13.6V. The MonsterBorg motors are rated at 12V. Does the ThunderBorg reduce the voltage? What's the maximum voltage I can supply (e.g. 4 Li-Ion cells at 14.8V) for these motors?

2. As the batteries are drained, they get below 12V. How low a voltage can the MonsterBorg motors tolerate, and does the ThunderBorg shut down at some lower limit?

3. Can/should the PWM rate sent to the motors increase as the battery voltage decreases, to maintain nominal power levels (e.g. 40% or 75%) that are the same at all voltages?

Thanks.

piborg's picture

piborg

Thu, 04/12/2018 - 12:34

The ThunderBorg works using a technique called Pulse Width Modulation (PWM). In simple terms PWM works by turning the output on and off at a fast rate, simulating a reduced voltage.

If you want a better explanation of PWM check out this video: Circuit Skills: PWM. For a detailed explanation see the Pulse Width Modulation Wikipedia page.

Generally speaking the motors will naturally smooth out the voltage to get a steady power flow from the batteries. This allows us to drop the battery voltage down to both stay within the limits of the motor and reduce it further to control the speed the motor runs at.

In answer to your specific questions:

1. The ThunderBorg reduces the battery input voltage to the percentage set by the power level set by code. It does not automatically adjust the output based on the battery voltage. Instead our example code has settings for the battery and motor voltages and works out the correct maximum power output for the motors.

A good rule of thumb for the upper limit for the battery under this type of control is double the motor voltage, in this case 24V. You can typically go higher than that, but we would not recommend it.

You can run the motors with a slightly higher voltage than designed for short periods of time without causing any damage. The NiMHs will not maintain the full 13.6V for very long (see graph below).

If you were to use a 4S LiPo (14.8V) then you need to look for these lines in our examples:

# Power settings
voltageIn = 1.2 * 10            # Total battery voltage to the ThunderBorg
voltageOut = 12.0 * 0.95        # Maximum motor voltage, we limit it to 95% ...

And change the voltageIn value to match the LiPo. As a bonus you can remove the 95% output as the battery voltage is now over 12V, like this:

# Power settings
voltageIn = 3.7 * 4             # Total battery voltage to the ThunderBorg
voltageOut = 12.0               # Maximum motor voltage

2. The ThunderBorg will be operational all the way down to 7V, below this and it cannot supply the 5V to power the Raspberry Pi.

The motors will work fine at 7V as well, but they will move at a significantly slower speed. The ThunderBorg will shutdown around 6V to protect itself from damage

3. It is possible to change the code so that it will adjust the power output as the battery voltage falls, but it is typically not required. Typically the voltage curve from NiMH batteries looks like this:

Image from Mad Scientist Hunt.

This means that the voltage is a fairly stable 1.2 per AA battery for most of the run, with a slight speed boost with a freshly charged set and a drop in speed as they start to get flat.

Bear in mind that the ThunderBorg is not able to increase the voltage above the battery output, only decrease it.

In summary:

  • ThunderBorg needs at least 7V to work fully
  • The MonsterBorg motors will work fine at lower voltages (e.g 6V), but they will move slower
  • Assuming the ThunderBorg is set to an appropriate power limit the MonsterBorg motors will be fine with a battery voltage up to 24V.
  • The examples are set for a 12V battery, the voltageIn value needs to be changed if you use a different battery voltage.
  • It is possible to change our examples to tune the power output for the current battery voltage, but it is generally not required for most uses
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