User guide

22 6250 Servo Controller User Guide
When all gains are set to zero, the digital control algorithm is essentially disabled and the
system becomes an open loop system (see diagram below). During system setup or
troubleshooting, it is desirable to run the system in open loop so that you can independently
test the drive and motor operation (refer to the Tuning Setup Procedure section of this chapter
for instructions to run the 6250 in open loop).
Command
Digital
Control
Algorithm
Control
Signal
Offset
Drive Command =
Control Signal + Offset
Motor
Drive
Motor
Load
Encoder
Closed Loop System
Command
Offset
Drive Command = Offset
Motor
Drive
Motor
Load
Encoder
Open Loop System
Feedback Data
The 6250 has the capability of providing a ±10V analog voltage output for commanding the
motor's drive. After the digital control algorithm has calculated the digital control signal, this
digital value is sent out from the DSP (digital signal processor) to the Digital-to-Analog
converter (DAC). The DAC has an analog output range of -10V to +10V. It is often possible
that the digital control signal calculated by the control algorithm can exceed this ±10V limit.
When this happens, the analog output would just stay, or saturate, at the +10V or -10V limit
until the motion variables (position, velocity, acceleration) change such that the control
algorithm would calculate a control signal less than the limit. This phenomenon of reaching
the output limit is called controller output saturation. When saturation occurs,
increasing the gains does not help improve performance since the motor is already operating at
its maximum level.
Position Variable Terminology
In a servo system, there are two types of time-varying (value changes with time) position
information used by the controller for control purposes: commanded position and actual
position. You can use this information to determine if the system is positioning as you
expect.
Commanded
Position
The commanded position is calculated by the motion profile routine based on the
acceleration (A, AA), deceleration (AD, ADA), velocity (V) and distance (D) command values and
it is updated every servo sampling period. Therefore, the commanded position is the intended
position at any given point of time. To view the commanded position, use the TPC (Transfer
Commanded Position) command; the response represents the commanded position at the
instant the command is received.
When this user guide refers to the commanded position, it means this calculated time-varying
commanded position, not the distance (D) command. Conversely, when this user guide refers to
the position setpoint, it means the final intended distance specified with the distance (D)
command. The following plot is a typical profile of the commanded position in preset (MCØ)
mode.