The loop-controller in the Lexium LXM52 Drive, Lexium LXM62 Drive, or Lexium ILM62 Drive Module is a cascade controller including:
oA current control loop
oA velocity control loop
oA position control loop
At power stage frequencies of 8 kHz and 16 kHz, the current control loop works with a cycle time of 62.5 µs as well as at a power stage frequency of 4 kHz with a cycle time of 125 µs.
The current control loop is implemented as PI controller and can be parameterized using the parameters Curr_P_Gain and Curr_I_Gain. The current control loop is self-optimizing. Using data from the motor nameplate, it automatically adapts to suit the motor. Therefore, as a rule the default setting can be used. A voltage feed forward is applied to the output of the current controller which however cannot be parameterized.
The velocity loop works with a cycle time of 125 µs and can be parameterized using the parameters Vel_P_Gain, Vel_I_Gain, VelFilter, and CurrRefFilter. Additionally a notch filter with the parameters NotchFilterDamping, NotchFilterFrequency, and NotchFilterBandwidth is available for velocity control loop. The velocity is calculated by differentiation from the position. The velocity controller is realized as PI controller. Using the parameters Vel_P_Gain and Vel_I_Gain, the proportional (P) and integral (I) gain of the controller is parameterized. The moment of inertia has to be parameterized correctly using the parameters J_Load, GearIn, GearOut, and J_Gear so that the correct controller gain can be set internally. If the moment of inertia has been parameterized correctly, most mechanical equipment can be operated with default controller parameters. However, as a rule the parameters have to be optimized in order to minimize tracking deviation. Using the filters VelFilter and CurrRefFilter it is possible to dampen high-frequency vibrations (resonances) which can occur between motor and the mechanical equipment or within the mechanical equipment. These two filter parameters are parameterized with the same values. The higher the filters are parameterized, the stronger is the damping effect on high-frequency vibrations and the lower are the dynamics of the control loop. The notch filter is used to dampen a specific resonance frequency, typically the one with the highest resonance amplitude.
The position control loop works with a cycle time of 125 µs and can be parameterized using the parameter Pos_P_Gain. The most mechanical equipment can be operated using the default settings. However, as a rule the parameter has to be optimized in order to minimize tracking deviation.
As input quantities for the controller, the MechRefPosition and RefAcceleration are transferred via the Sercos III interface in the real-time channel with the defined Sercos cycle time (SercosCycle). Before these values are applied to the controller, they are interpolated. Further, the reference velocity is calculated from the two transferred quantities. The quantities are interpolated to 125 µs from the Sercos cycle time.
The value for the current feed forward (CurrFeedForward) is calculated from the acceleration. The moment of inertia with the parameters J_Load, GearIn, GearOut, and J_Gear taken into account in this calculation.
The quantities CurrFeedForward and the reference velocity are feed forward values which enable a direct, quick passage of the reference values to the controller. The command action of the controller is optimized and the tracking deviation reduced.
Another interpolator interpolates the reference current value from 125 µs to 62.5 µs if the power stage frequency is 8 kHz or 16 kHz. At 4 kHz, no interpolation is required since the current controller works with 125 µs as well.
The reference current can be dynamically limited by using UserCurrentLimit. UserCurrentLimit is parameterized as a percentage of UserDrivePeakCurrent and transferred via the real-time channel of the Sercos III interface in each cycle.
The speed of rotation is limited by UserMaxVel, with 110 % of UserMaxVel being allowed as the limitation value so that any control deviations can be compensated also at maximum speed.
The tracking deviation is the difference between MechRefPosition and MechPosition with a compensation of the delay between these two signals. The tracking deviation is compared to the parameter TrackingDeviationLimit. If this limit value is exceeded, a diagnostic message (DiagMsg) is triggered.
The position (MechPosition) and the current (Current) are transferred as output values to the controller via the real-time channel. At the moment, the FeedbackCurrent can only be read out via the Sercos parameter channel.
Servo amplifier controller structure:
This chapter contains the following topics:
