In a growing market of robotic applications, where many direct drive systems are used, there are considerations to be made unique to the robotic market with respect to power supply.
Precision motion relies on repeatable and stiff mechanics, high positional accuracy encoders and fast current / velocity and position PID control loops (in the motion controller). Gearheads with their inerrant lost motion are not welcome in said applications. This lost motion consists of two components; gear backlash and torsion. The latter is less well known (or even specified by manufacturers) yet it still contributes to positional errors. For these reasons direct drive from the motor is preferred.
When loads are driven directly from motors the reflected inertia of the load with respect to the motor is fully realised. When a gearhead is used the reflected inertia is represented by the equation below:
It is easy to see how a gear reduces the inertia that the motor “sees” and without it how high that it would be. Inertia is important as the magnitude of the inertia of a body gives us an idea of how difficult it is to move (accelerate/ decelerate or stop). As servo systems by their nature are required to start/stop or change direction quickly to complete the cycle in the minimum amount of time, thus increasing productivity.
All modern DC servo motor controllers are of 4 quadrant design. 4 quadrant controllers have the electrical capability to overcome the generated voltage of a motor that is being driven by the load or is already in motion. Flemings right hand rule dictates that if a conductor moves through a magnetic field a current will be induced in that conductor. Thus a free-wheeling motor or one that it being pushed by and external force (e.g. gravity) will produce a voltage at its terminals. For the DC servo drive to dynamically control the motion profile in this operating mode it has to be of 4 quadrant design else the dynamic and controlled decent or deceleration of the motor cannot be achieved.
The 4 quadrants of motor operation are shown below, with indication of the applied voltage, current flow, generated voltage and relevant torque and speed vectors.
Due to the construction of the bi-directional H bridge, the regenerated voltage from the motor is in the same polarity as the supply, so it “adds” to the voltage on the supply rail to the controller. The voltage increases rapidly at the supply side of the driver which results in the controller over voltage fault, as the power supply feeding the controller with DC voltage will protect its self from the rising voltage, “pushing” it back to the controller. This will result in the DC bus capacitor or / and the MOSFETS blowing in the servo controller. This is a costly and inconvenient failure that would not usually be granted warranty for from the manufacturer as it is often viewed as misuse.