Installation manual

77
Tuning Procedures
If you enter an inertia value lower than the actual inertia, but is between 10 and 90 percent of the actual, the drive will
tend to be more sluggish than optimum but will usually operate satisfactorily. If the value you enter is less than 10 percent
of the actual inertia, the drive will have a low frequency oscillation at speed.
There are three guidelines for defining the inertia ratio:
1. In most applications the default value of 0 (zero) will be used.
2. If the inertia of the machines varies or there is uncertainty in the estimate, use the lowest value for inertia.
3. The machine system bandwidth is reduced if the inertia estimate is low. Consequently a low inertia estimate
can sometimes add a level of robustness.
Friction
In the drive, this is a viscous friction parameter, characterized in terms of the rate of friction increase per 100 motor RPM.
The range is 0.00 to 100.00 in units of percent continuous torque of the specified motor/drive combination. The Friction
value can either be estimated or measured. For most servo drives viscous friction is 0.
If estimated, always use a conservative (less than or equal to actual) estimate. If the friction is completely unknown, a
value of zero should be used. A typical value used here would be less than one percent.
If the value entered is higher than the actual, system oscillation is likely. If the value entered is lower than the actual a
more sluggish response is likely but generally results in good operation.
Response
The Response adjusts the velocity loop bandwidth with a range of 1 to 500 Hz. In general, it affects how quickly the drive
will respond to commands, load disturbances and velocity corrections.
The drive’s position velocity loop is designed to be a second order system with a gain of one, a natural frequency
specified in the Response scroll box, and a damping factor of 0.8. If the drive’s bandwidth is defined to be the -
3dB point of the response, the idealized bandwidth of the system is approximately 2.2 times greater than the
natural frequency.
For example: When the Response is set to 50, the idealized bandwidth is 110 Hz.
High Performance Gains
Enabling High Performance Gains fundamentally affects the closed loop operation of the drive and greatly modifies the
effect of the Response parameter. High Performance Gains are most beneficial when the Inertia Ratio and Friction
parameters are accurate.
High Performance Gains, when enabled, make the system less forgiving in applications where the actual inertia varies
or the coupling between the motor and the load has excessive windup or backlash.
When using an external position controller, High Performance Gains should not be enabled.
Feedforwards
Feedforward gains are essentially open loop gains that generate torque commands based on the commanded velocity,
accel/decel and the known load parameters (Inertia Ratio and Friction). Using the feedforwards reduces velocity error
during steady state and reduces overshoot during ramping. This is because the Feedforwards do not wait for error to
build up to generate current commands.
Feedforwards should be disabled unless the absolute maximum performance is required from the system. Using them
reduces the forgiveness of the servo loop and can create instability if the actual inertia and/or friction of the machine
varies greatly during operation or if the Inertia Ratio or Friction parameters are not correct.
The internal feedforward velocity and acceleration gains are calculated by using the Inertia Ratio and Friction
parameters. The feedforward acceleration gain is calculated from the Inertia Ratio parameter and the feedforward
velocity gain is calculated from the Friction parameter.
When Feedforwards are enabled, the accuracy of the Inertia Ratio and Friction parameters is very important. If the
Inertia Ratio parameter is larger than the actual inertia, the result would be a significant velocity overshoot during
ramping. If the Inertia parameter is smaller than the actual inertia, velocity error during ramping will be reduced but not
eliminated. If the Friction parameter is greater than the actual friction, it may result in velocity error or instability. If the
Friction parameter is less than the actual friction, velocity error will be reduced by not eliminated.