The Basics of a Brushless Servo Motor
A brushless servo motor (also known as a BLDC motor) is an electric motor that doesn’t use brushes or a mechanical commutator. Instead, it uses electronic commutation and feedback devices.
They are used in many important applications because of their power-to-weight ratio, compact size and electrical efficiency. They are also quieter and eliminate sparks and fire hazards.
Electronic Commutation
In DC motors, brushes physically slide on a commutator to switch current from one coil to the next in order to develop the necessary back-and-forth torque to move the rotor and load. This process, called commutation, creates waste heat from the air gap between the commutator and the brushes, decreasing efficiency and increasing cost. The friction of brush-to-commutator contact also generates audible noise and increases electromagnetic interference (EMI), which can interfere with sensitive circuitry.
Brushless servo motors do not have mechanical commutators, so they need to use some other kind of method for commutation. This can vary depending on the design, but typically involves some combination of Hall sensors brushless servo motor and encoders. The Hall sensors provide information about the physical position of the rotor magnets, and the drive electronics uses this information to synchronize voltage passing through the stator windings with the rotational position of the rotor.
Some types of commutation, such as trapezoidal, can be implemented using simple combinational logic and do not require sophisticated control electronics. Other methods, such as sine commutation, do require more advanced drive electronics, but offer higher efficiency and better control. The encoders are used to monitor the rotor’s position and velocity, and they communicate this data to the servo drive that controls the electrical currents passed through the rotor. This allows the servo drive to precisely control the position and speed of the rotor.
Hall Sensors and Encoders
A servo motor needs a sensor to provide accurate position, speed and torque control. Designers have several choices for this task — optical or capacitive encoders, Hall sensors, and resolvers. Each technology has its strengths and trade-offs in terms of size, resolution, price, power, noise, and performance.
The choice of sensor depends on how the system will be used. For battery-powered handheld drills or other portable applications, smaller is better. This helps reduce user fatigue and battery cost. Smaller Hall position sensors also make it easier to include them in the motor case, which can be critical for compact motors that must pass light, wiring or plumbing through the shaft-hole diameter.
Encoders are available with a range of resolutions that can deliver arc second levels of rotary positioning precision. In addition, they run much cooler and draw less current than Hall sensors. However, they are more expensive and require more complex circuitry for initial rotor absolute position estimation. This can be accomplished by using the servo drive’s internal PID controller, but requires some knowledge of how the motor is configured and the mechanics of your specific motor. For simpler setups, SOLO supports the use of analog Hall sensors that generate a PWM signal whose duty cycle directly correlates with a motor shaft angle. This option eliminates the need for an encoder, and is supported by FAULHABER controllers that are designed specifically for this type of analog Hall output.
Shaft Design
When a servo motor is coupled to a load it introduces extra shaft inertia, which must be minimized. Inertial mismatch is best if it is 1:1 and can be further reduced by using a gear or coupling.
Servo motors are used in a variety of applications from robots to electric bicycles and scooters, electric car and truck hybrids, and drones. All of these systems must meet a high level of positioning accuracy with continuous motor torque available.
To achieve this a servo system uses a feedback device (either a hall sensor or encoder) to monitor the actual position of the motor shaft. This information is passed to the internal servo drive electronics which use brushless servo motor supplier a control method called proportional control. The servo drive reads the PWM signal from the feedback device and compares it to the desired position. If the error is large the servo drive increases the motor speed and direction to move the load to the desired position. If the error is small the servo drive reduces the motor speed and direction to slow the movement.
To minimize torsional resonance that can cause vibration in the drive and load, use a compression-type coupling clamped as close as possible to the servo motor shaft flange. Avoid key-type couplings, which can decouple the shaft during operation and damage the motor bearings and encoder disc. A torsional damping scheme in the servo drive also helps to minimize the effect of resonance.
Speed Control
The control system of a brushless servo motor monitors the position, speed and load in real time through sensors. These sensors can include an encoder or a Hall sensor. The sensor sends the feedback signal to the controller which continuously calculates and adjusts, enabling it to accurately perform the required motion tasks.
Because a brushless servo motor does not have mechanical commutators, the voltage of the coils is controlled by electronic current control devices like high-performance microprocessors. These processors constantly monitor the currents of the rotor and stator to ensure that they are aligned with the desired position, speed or torque that is sent through an EtherCAT network interface.
If the current is not aligned with the desired state, the controller will increase or decrease the switching frequency of a network of field effect transistors (FETs). The higher or lower current through these FETs results in the speed of the motor changing.
Servos are widely used in conveyor systems that move materials and products from one point to another, sorting and automating the process. They also feature in drones and electric vehicles where speed and torque control are critical. Whether you’re a hobbyist building robots, an engineer designing industrial systems or simply curious about electronics, the more you learn about motors and servos, the more doors open for your projects and your imagination.