Differences between Force Servo and direct drives


Force

In Force Servo, there is a reduction gear between the motor and the force sensor that increases the torque. The reduction gear helps establish a balance between force and speed across a broad range of values.
A direct drive always works in an inefficient mode. The chart shows a comparison of torque values.

Speed

Force Servo is designed to work with a reduction gear that ensures the optimal balance of speed and power reserves. The reduction rate can be between 1 (fast rotation) to 1000 (high torque). As a rule, the optimal reduction rate for a small platform with a camera is around 100.
A direct drive does not use a reduction gear. Since the motor rotates a heavy platform with a camera, a direct brushless drive lacks the power to quickly turn or decelerate the payload. Most of the drive's energy is wasted on heating up the coils instead of rotation.

Accuracy

Force Servo features a force sensor, and the force it creates is always accurate, regardless of the position of the rocker. Mechanical noise in Force Servo is absorbed by an elastic element in the coupling.
In a direct drive, the force is determined by the current intensity, but cyclic torque variations and friction must also be taken into account. Because of the impulse control, the drive sends high-frequency vibrations to the payload. To minimize accoustic noise of the direct drive a high frequency PWM may be used. But higher PWM frequency means greater power loss due to switching. Additionaly, very short PWM pulses have no effect on the output due to transition delay. The wires that connect direct-drive motors emit heavy electromagnetic interference. The inductive reactance of the wires increases sharply during short PWM impulses. Because of their length, the short impulses fall into the range of radio waves, and the wires radiate them like antennas.

Control

In order to control a brushless direct drive, you have to consider the position of the rotor in relation to the stator. In other words, additional position-based feedback is a must. Force Servo is easier to control: direct and differential feedback is used to send motion commands for reducing the error margin relative to the set value. The set value can be defined by various sensors and their combinations: operator, gyroscope, accelerometer, optical marker, object shift in the footage.

Result

If we are talking about a stabilized platform for a video camera, it is correct to use measurable characteristics. Let's installe a laser pointer on the stabilized platform and measure the fluctuations from 10 meters. A laser beam helps distinguish the difference between the platform stabilization effect and that of optical or digital stabilization.





Comparison of well known samples



Limits of short PWM pulse



The inductive reactance of the wires (a direct drive with a long wiring)



Comparison of torque



Aleksey Zaitsevsky, 2014, http://f-servo.com/