Vectors or Servos for positioning applications

Phil Strong of Baldor explores the basic characteristics of servo and vector technologies and illustrates that closed loop vectors can be used in many positioning applications that do not require the high acceleration of a permanent magnet motor.

Servos are commonly associated with high performance permanent magnet motors that are used in precise positioning applications. Vectors, on the other hand, are associated with industrial AC motors that offer tighter speed control than an standard AC motor and full torque at zero speed. If a feedback encoder is installed they also offer the possibility of position control.

Servo motors are permanent magnet motors with a low inertia rotor and some feedback device to sense the position of the motor shaft. More correctly, the word servo can be used to describe any drive comprising a motor, feedback sensor and an amplifier.

A position servo system is one that senses and controls the position of the motor. By this definition, closed loop vector systems can be used as position servos. They comprise a motor, a feedback encoder and an amplifier. Just like permanent magnet servo systems, closed loop vector systems can be used with a motion (position) controller in precise positioning applications. In contrast, encoderless vectors do not use a position feedback sensor – therefore they cannot be used in positioning applications unless a feedback encoder is employed separate from the drive. Here, we will use the terminology of servo and vector to mean permanent magnet servo motor and vector controlled AC motor, respectively.

Dynamic Performance

Perhaps the biggest difference between the servo and vector motors is their dynamic response. Permanent magnet servos have much lower rotor inertia/torque ratios than vectors, which means that for the same torque output, the servo will accelerate the same load much faster than a vector.

The typical torque and inertia for Baldor’s range of servos and vector motors, across a comparable torque range can be seen on the left. Above 10Nm, the vector motor has a rotor inertia of approximately ten times that of a servo motor which means that you can accelerate the same load at one tenth the rate you can with a servo. Below 10Nm, the difference is even greater.

Inertia Matching

When selecting the motor technology, consideration must be given to the required acceleration. The achieved acceleration is a function of the torque and also the total inertia of the system (inertia of load plus the inertia of the motor). For the best transfer of energy between the motor and the load, the inertia of the load should match the inertia of the motor. If the inertia of the rotor is much larger than that of the load, then more energy is used to accelerate the motor than the load. This means a larger more expensive drive is needed to provide the required performance.

If the inertia of the motor is much smaller than the load, it becomes difficult to control the position of the load during periods of high acceleration and deceleration. This is because most systems have some form of compliant coupling between the load and the motor. During acceleration this coupling is placed under torsion and `winds up’. The position of the load lags the position of the motor. During periods of deceleration, the coupling unwinds causing the load position to advance the position of the motor and ultimately oscillate to a final resting point.

With the low inertia and high acceleration of a servo it is important to match the inertia of the load and the motor to ensure good dynamic performance. In most practical applications it is not possible to achieve an exact match but as a rule of thumb a match of 10:1or less generally delivers good system performance. Mechanical reduction such as gearing or belts and pulleys can be used to achieve more exact inertia matching.

The higher inertia of a vector motor makes it easier to control the dynamic response of the load, albeit at lower rates of acceleration and deceleration.

Servo motors use permanent magnets to generate the magnetic flux. This flux is a constant magnitude and leads to an almost (constant) linear relationship between the speed of the motor and the torque output.

In contrast, a vector motor uses electronics to generate a rotating electric field which induces a magnetic flux into the rotor. The magnitude of the flux is a more complex function of rotor speed and consequently the relationship between speed and torque is less linear than a servo. The speed torque curves for both servo and vector motors is shown on the left.

Both vector and servo systems rely on some form of position feedback from the motor to the controlling drive. The drive will typically be controlling current (torque) and speed of the motor and some form of position controller (external or embedded ) will control the motor position.

The achievable positioning accuracy is largely a function of the characteristics of the position sensor: this is typically an incremental encoder for vector motors and a resolver or incremental encoder for servo motors. With comparable position sensors and comparable position controllers, it is possible to position a vector and a servo motor with the same accuracy. Positioning to within +/-1 encoder edge is quite achievable.

Physical Size

Servo motors offer much higher torque densities than vector motors. The use of permanent magnets allows much higher flux densities to be achieved in a given volume. Also with a permanent magnet rotor most of the heat losses are generated in the stator (the outer portion of the motor) and it is relatively easy to extract this heat through cooling fins and convected or forced air.

In contrast, a vector motor relies upon induced currents in the rotor to produce the magnetic field. These currents generate heat in the rotor which is difficult to extract because of the air gap between it and the stator.

Consequently the torque density achievable with a vector motor is limited by its construction and thermal characteristics. The torque densities for Baldor servo and vector motors is shown on the left.

Vector motors typically are available in much larger torque ratings than servos. Baldor offers servo motors up to 40Nm of torque and vector motors up to 2000Nm. This difference in offering is more a result of economic factors than technical ones.

In theory, a permanent magnet can deliver 2000Nm. However, the cost of such a unit would far out-weigh any of the benefits. Typically, at such high torques, the physical size of the motor becomes less significant when taken in the context of the overall size of the machine. Also, it is rare that such large machines require the high accelerations of a servo. The vector motor offers the most practical solution for these high torque positioning applications.