- DC Servo Motor Definition: A DC servo motor is a DC motor configured to respond to control signals from a servomechanism for precise motion control.
- Control Methods: DC servo motors can be controlled either by adjusting the field current (field control) or the armature current (armature control), each with distinct advantages.
- Field Controlled Motor: This type of motor adjusts torque through field current changes, suited for applications needing stable but less dynamic response.
- Armature Controlled Motor: In this setup, motor response is quicker due to direct armature current adjustments, ideal for applications requiring fast response times.
- Permanent Magnet Motor: Permanent magnet motors use a constant magnetic field, and control is achieved by varying armature current, simplifying the control mechanism but restricting field adjustability.
An electrical motor becomes a servo motor only when a servomechanism closes the loop around it. A DC motor used that way is a DC servo motor. Among types of DC motor you will see a shunt wound DC motor, a series DC motor, a separately excited machine, a permanent magnet DC motor and a Brushless DC motor. Servo work usually picks the separately excited machine, the permanent magnet DC motor or a brush less DC motor. A series machine is a poor servo plant because torque and speed are tightly coupled.
Separately Excited DC Servo Motor
DC Servo Motor Theory
A wound-field DC servo usually has separate supplies for the field and the armature winding. You can command torque by field current or by armature current. Field control is slower. Armature control is the quicker loop. Pick the scheme to match the plant.
Field control and armature control for DC servo motors are below.
Field Controlled DC Servo Motor Theory
The figure is a field-controlled DC servo. The field is driven by the amplified error. The armature is fed from a constant current source.
The field is held below the knee of the magnetizing curve, where flux rises almost linearly with field current. With armature current fixed, shaft torque then tracks field current.
From the torque equation of DC motor, T ∝ φIa. φ is field flux. Ia is armature current. Field control holds Ia constant, so T ∝ φ.
The amplified error drives the field, so it also sets torque and shaft motion. A larger fixed armature current makes a small field-current step produce a useful torque step.
Reverse the field polarity, or use a split-field winding, to reverse rotation. In a split-field machine the two halves are wound opposite. The error voltage decides which half’s magnetic field wins, and that sets direction. Field L/R is large, so If lags a step in error voltage. That slow field loop is why this scheme is mostly seen on small servo motor applications.
The amplifier only has to feed the field. Field-circuit power is far below armature power.
Armature Controlled DC Servo Motor Theory
The figure is an armature-controlled DC servo. The armature is driven by the amplified error. The field is fed from a constant current source.
The field sits well past the magnetizing knee. Flux is nearly flat there, so a large swing in field current barely moves φ. The loop is then almost deaf to field-current drift, which is the intent.
At saturation φ is large and almost fixed. The torque equation of DC motor is still T ∝ φIa. With φ large, a small Ia step makes a clear torque step. The shaft then tracks armature current.
The armature winding is mostly resistance, so its electrical time constant is short. Armature current therefore follows a voltage step quickly. Armature-controlled response is faster than field control. Reverse the error polarity to reverse torque.
Permanent Magnet DC Servo Motor
A permanent-magnet DC machine has no field winding, so field control is off the table. Its DC servo motor working principle is the armature-controlled case.
