Gimbal Motor Technology: A Complete Guide to Precision Control

What is a Gimbal Motor?

A gimbal motor is the type of motor used in gimbal applications, devices that stabilize and control the orientation of a payload, like a camera, a sensor, a tool, etc. Such stabilization can be controlled across one or several axes. Typically, stabilization is happening in 2 axes, azimuthal and elevation, but sometimes it can be up to 5 or 6 different axes.

What is a Brushless Gimbal Motor?

A brushless gimbal motor is a BLDC that uses an electric controller to alternate the phase currents, instead of the physical alternation used in traditional brushed motors. This results in a motor which is quieter and runs more smoothly, as well as having an increased lifespan.

How Does a Gimbal Motor Work?

Gimbal motors are usually brushless motors (BLDC) that are optimized for precise, smooth, and low-speed rotation. They're often paired with sensors, like gyroscopes, accelerometers or encoders, and work together with a motor controller to create a closed-loop system that constantly adjusts the payload's position to keep it stable.

Gimbal Motor Types

Most electrical BLDC motors could be used in gimbal applications, but there are some key elements to consider when choosing the most suitable one. The options available are many. Most gimbal motors are typically slotted or slotless motors which can be used either as a direct drive or in combination with a gear/transmission.

Cogging Torque

An important aspect to consider in gimbal applications is the cogging torque. Cogging torque is the unwanted jerky motion or resistance that is detected when turning a slotted brushless motor. It happens because the motor’s permanent magnets are attracted to the steel teeth (slots) in the stator, creating a kind of magnetic “snap” between positions. This effect is most noticeable at low speeds and affects the precision of gimbals. Slotless motors show virtually no cogging torque.

Gears

Another aspect that affects the precision of Gimbals is the use of gears. Gears are used to increase the torque capabilities with a limited motor size. By implementing a gear, a possibility for backlash is introduced. Backlash causes uneven positioning, which affects the application’s precision. Therefore, a direct drive solution is often preferred, if it can handle the torque requirements. This is why a high torque density slotless BLDC motor is a great fit for a gimbal motor design.

Gimbal Motor Applications

Typical gimbal applications include:

• Electro-optics
• Lidar and laser pointing
• Satellite communication
• Telescope positioning
• Camera stabilizers 
• Remote weapon systems

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Most of these applications need a very precise positioning and/or a fast response to small angular changes. 

Gimbal Motor Benefits

The benefit of choosing a motor specifically designed and optimized for gimbals is that it limits the number of trade-offs that need to be considered in the design process. Many applications benefit from being compact and lightweight as this brings opportunities for using smaller and cheaper components, as well as increasing the ability of reaching high dynamic operation points.

In addition, many gimbal applications benefit from using a precise motor with high resolution to position with high accuracy, and speed. Finally, most gimbals use absolute encoders, which need to be fitted together with the motors, and a larger ID of the motor helps to make space for a larger encoder without increasing the overall space required for the system.

What Does Gimbal Motor Overload Mean?

An overload of a gimbal motor happens when it is run faster or with more load than it is designed for. This could either be a unique event, or because of frequent cyclic loading. For slotted motors, the saturation level of the iron core is reached at a certain supplied current, where the motor needs a bigger current increase than the increase of torque output it can normally provide. Simply put, this means the relationship is not linear at that level. For slotless motors this is a linear behaviour throughout the full capacity of that motor and it is typically why a slotless motor is a better choice when overload is expected to happen.

How to Fix Gimbal Motor Overload?

The most structured way would be to find out which of the motor's limiting factor has been reached (torque, speed, current draw, temperature) and then find a way of improving the use of the motor to avoid reaching that level of overload. If the design stage allows it,  changing the motor to one that can reach that operating point for that system, without being overloaded would be the right choice. Over dimensioning of motors is a strategy well adopted by many engineers. This strategy comes with a few disadvantages, the most obvious one is that the size and weight of the system will not be optimal.

How to Choose the Right Gimbal Motor

The right gimbal motor would be the one that fits the specification for that application best. 

The usual things to consider are:

• Size
• Weight
• Peak and continuous torque
• Speed demand
• Power consumption (supplied voltage and current) 
• Precision tolerance

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The right motor will fit all these specifications with the highest level of margin. 

It’s not only important to consider the motor, but the motor in combination with encoders, sensors and the motor controller. For example, both the absolute encoder resolution and the motor controller pwm-step resolution determine the precision for the complete system. A larger ID of the motor fits a larger encoder in the center (higher resolution) and a higher number of poles decreases the need for a high resolution on the motor controller (resolution is per pole). 

Maintenance and Troubleshooting Tips for Gimbal Motors

Most gimbal projects can be designed in a way that limits the amount of maintenance needed. For example, a brushless BLDC motor does not have any contact between the stator and the rotor, which limits the wear over time to virtually zero. The same goes for troubleshooting. By designing the system to accommodate for the right specifications, less trouble is to be expected, and no troubleshooting will be needed. Once it runs it may run forever, or at least until other components start to fail.

However, typical problems that can occur in a gimbal system involve low positioning accuracy, cogging effects, slow positioning, high current draw etc. A good start would be to design the gimbal with a slotless motor. This will eliminate the cogging torque as well as increase the number of poles and positioning resolution possibility. In combination with a high-resolution absolute encoder and pwm-step motor controller, a slotless motor will take the system accuracy to a high level. The slotless motor will have higher peak torque capabilities which will allow for fast position changes. And if possible, a winding configuration with a high torque constant is preferred to reduce the current needs.

Conclusion

Slotless BLDC motors are a great motor type for gimbal applications. Especially if they are designed for direct drive, and together with a high resolution absolute encoder and pwm-step motor controller.