Brushless Servo Drives
Hudson brushless servo motors feature finished pigtail or on-body connectors to eliminate frustration with “flying leads”.
BLDC (or brushed) motors are permanent magnet, 3-phase, synchronous motors. They don’t use brushes, but instead have an electronic sensor that electronically switches transistors to commutate the windings.
High performance servo control requires a fast electrical time constant and fast mechanical response. Teknic’s servo drives deliver both.
The motor
A servo motor is an electric DC motor designed to turn a command signal into a precise position, velocity or torque output. It does this by comparing the feedback signal from an incremental encoder or resolver with the commanded status of the motor and changing the current and voltage sent to the motor in real-time.
In a brushless servo motor the brushes and commutator are brushless servo drive replaced with an electronic controller that uses timed power distribution to rotate the rotor. It also eliminates the mechanical noise and wear of the brushes and commutator and reduces electromagnetic interference (EMI).
The rotor has electromagnets on its exterior that interact with the motor’s permanent magnets inside the stator to generate rotational force. The rotor is supported by a pair of bearings fitted to both ends of the motor shaft. The electrical energy supplied to the rotor winds up in the rotor windings, which determine how much torque the motor can develop and at what speed.
A good servo motor is very responsive, both electromagnetically and mechanically. It must have a fast electrical time constant to minimize steady-state torque droop and respond quickly to the transients required by high performance servo control systems. It must also have a low inertia rotor to provide a high torque-to-inertia ratio to deliver the peak servo torque needed for dynamic positioning and speed regulation.
The drive
Brushless motors offer a number of advantages over their brushed counterparts. Because they don’t use brushes to mechanically commutate the rotor, they can run 5-15% more efficiently than brushed motors. This translates to lower energy costs and less maintenance. They also don’t produce the same kind of electrical and mechanical noise, sparks or short circuits that brushed motors do.
The drive is the device that pushes current and voltage through the servo motor to achieve position, velocity or torque control. It starts with the power stage, which uses transistors in an H bridge configuration to convert AC or DC electrical power to the required current for a given motor.
Once the current is flowing through the drive, it goes to the motor, where it attracts or detracts the rotor’s permanent magnets. This changes the motor’s speed, allowing it to meet or exceed its commanded velocity. It happens much faster than humans can perceive, with thousands of adjustments a second.
There are many different servo drives on the market, from panel mount devices with plastic or metal covers to PCB mount models designed to be plugged directly into another circuit board and used as a sub-component. They all work the same, but the differences are more about form factor and ease of installation. For example, a panel mount drive may have screw terminal lugs to allow for traditional wiring connections, while a PCB mount model will have pin connectors.
The feedback system
In order for a servo drive to operate correctly it must have feedback. Without feedback a servo would just run randomly.
The feedback system is how a servo drive learns to control the motor windings. It works by receiving a feedback signal from the motor and comparing it to its input. If the feedback signal is too large or too small the drive adjusts the motor winding currents to compensate and produce an output that matches the error signal.
This is known as a closed current loop. Some drives can also close velocity and position loops which are used in more advanced machine control.
To get the feedback signal to the servo drive the controller (which can be as simple as a dial) sends a pulse of voltage to the servo drive’s command input. This essentially tells the drive what speed, torque, and position to aim for.
Servo drives come in a variety of different form factors but performance-wise they all work the same. Panel mount drives have a plastic or metal case that encloses the PCB with holes for mounting to a flat surface using screws or bolts. Servo drives designed to be mounted directly to another circuit board use pin connectors and soldering instead of screws to offer more compact, reliable connectivity. They are most commonly found in industrial automation and robotics applications.
The control system
From your Nespresso machine to your father’s car and even animated Santa’s figures in malls – motors are everywhere. They convert electrical energy into mechanical energy converting mechanical inputs (pressure, speed, force) into linear or rotary outputs.
A servo drive takes the input signals and adjusts the motor current to achieve the desired output, whether that be velocity, position or torque. This is achieved by controlling the voltage and current flowing through each of the sets of coils that are fixed in the rotor and stator. The current is controlled by switching high power transistors on and off. This is known as electronic commutation and eliminates the need for a mechanical commutator.
The servo drive must be able to react quickly to fast servo control loops, this requires that the rotor and stator have both a fast electrical time constant and a low mechanical BLDC servo drive inertia. This can be accomplished with either a brushless DC or brushless AC motor.
Brushless servo drives can be used with a wide range of feedback devices including Hall effect sensors, encoders or resolvers to measure position, speed or position error. This data is sent to the servo drive where a microprocessor uses algorithms to predict system conditions and adapt the motor current accordingly. This means the servo drive can adapt to varying process operating circumstances to deliver accurate and repeatable motion.
