Brushless Gimbal with Arduino

Brushless DC motors are ideal for this project because they are efficient, precise, and have plenty of torque; the trick is actually controlling them. The most common hobby route is to buy programmable electronic speed controllers which can take throttle inputs and convert that to rotational speed in the brushless motors. However, these can cost $20 for the cheapest ones, so we decided to save costs and figure out how to control the motors without them.

Unlike brushed DC motors, which work by simply reversing the current in loops of wire in a permanent magnetic field via a commutator, brushless motors have no physical connection between the wire coils on the stator and the rotor. The most common brushless motors are known as three phase brushless motors. This means that they have three distinct sets of coils, often labelled A, B, and C. Surrounding these are permanent magnets with alternating orientations.

Our motors have twelve coils or electromagnets (3 sets of 4) and fourteen permanent magnets on the rotor. The idea is that we change the polarity of our electromagnets in a cycle and the permanent magnets follow the changing magnetic field. In the simplest form, we have three electromagnets (A, B, and C) and the rotor is just a single NS permanent magnet. If we have coil A creating a north magnetic field and coil B creating a south field and coil C off, the magnet will turn to align itself as closely as it can to the generated magnetic field. We can then rotate the polarities of our electromagnets by one position (A off, B North, and C South) and the magnet will follow. In this manner, we have a set of commutation phases that when we run them in order cause the rotor to turn, and we can reverse this cycle to run it in the opposite direction.

Watch these two videos for a clean and clear explanation with diagrams.

We did some research and found that sending Sine waves that are 120 degrees out of phase through the three sets of electromagnets, we can create a dynamic magnetic field that drives motor rotation. Introducing a sinusoidal signal smooths out the motion of the motor considerably by changing the polarity of the electromagnets gradually. This site was very useful in taking that theory and turning it into Arduino software. The trick to this approach is running our PWM signal from our Arduino through an H Bridge to allow negative polarity of one of the coils since PWM signals only output High (1) and Low (0).

One of the biggest challenges of brushless motors is that to properly control them you need information of where the rotor is in reference to the stator coils. That way the controller knows exactly where in the commutation cycle to begin or to jump to. We were hoping that the accelerometer may be able to give this feedback, which seemed to work with moderate success. More sophisticated methods include attaching hall effect sensors to detect the position of the rotor or even using the “back emf” induced in the coils.

Introducing the h-bridge to control the motors allows us the bi-directional motion that the gimbal requires, and also allows for very smooth stepping of the motor’s position. The H bridge essentially flips the sign on one of the electromagnets so that we can have a high, a middle and a low voltage. As mentioned in the introduction, we used L298 h-bridges, which have 15 pins. We wired each of them as seen in the image above. Here is the data sheet.

• Pin 1: Ground
• Pin 2: Motor Pin 1
• Pin 3: Motor Pin 2
• Pin 4: External Supply Voltage (put a 0.1 microFarad capacitor to ground)
• Pin 5: Arduino PWM Digital Output (pin 3 or 9)
• Pin 6: +5V from Arduino (put a 0.1 microFarad capacitor to ground)

• Pin 7: Arduino PWM Digital Output (pin 5 or 10)
• Pin 8: GND
• Pin 9: +5V from Arduino
• Pin 10: Arduino PWM Digital Output (pin 6 or 11)
• Pin 11: +5V from Arduino
• Pin 12: Not Connected
• Pin 13: Motor Pin 3
• Pin 14: Not Connected
• Pin 15: GND

Our source describes a clever scheme of approximating a sinusoidal signal to the motors. They construct an array of 48 values between 0 and 255 which represent the values of the sine function at equal increments. They begin each electromagnet state A, B, and C 16 values apart from one another, which divides the period of the function into thirds or, in other words, phase shifts the states by 120 degrees. Then, they simply increment each state’s value in the array so that the states cycle through the sine wave with each loop of the program. The result is very smooth rotation in the motor, although it is somewhat speed limited by the physical ability of the motor to keep up with the Arduino signals.

We use the same PWM sine-array scheme as our source, which allows to increment though a numerical approximation of the sine function over a given number of steps in the period. In our case, we doubled the number of values in the array from 48 to 96 so that we could more precisely control the motor. This is because our gimbal application does not need the motor to make full rotations, well, ever; we only need to adjust the camera’s position against the motion of the base. Quick observations suggest our scheme allows the motor to step in 0.2 degree steps at a time.

It’s important to note that this is mostly a makeshift way of controlling brushless motors. At any given moment, we want one electromagnet high current, one low, and one in a high-impedence state, which approximates to neither high or low. In our case, the “high-impedence” state actually lets current run off to ground, which generates a significant amount of heat in the motor. A definite place for improvement is to keep the motors from getting hot in the case of long-term (e.g. over a minute or two) operation.

Another hazard with our implementation is in the case that the motor physically misses or overshoots one of the steps, which can happen given the significant weight of the frame and camera relative to the motor’s torque. In this case, our program is unaware that the motor isn’t in sync with the signals, and cycles through the sine wave once before “catching” the motor and resuming normally.