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The control loop for digital axes/ spindles

Machine tools normally function on the principle of cascade control. Here the position control loop is prior to the speed and current control loops.

Benefits of cascade control:

„Transparent structure of the individual control loops.

„Disturbances can be compensated through the subsequent controllers. This relieves the prior

controller.

„The respective outer control loop protects the inner control loop by limiting the command

variable.

The position, speed and current controllers are located in the control. The power module is driven by the CC 42x through PWM signals.

PWM is the abbreviation for pulse-width modulation. The information content in this signal depends on the relation of pulse duration to pulse-off duration.

Principle of operation of iTNC530

Nominal and actual values for the controllers

The position controller receives its nominal value, e.g., from the NC program; the actual value is normally provided by a linear encoder (scale). The actual position value can also be provided by a motor encoder instead of a scale. The position of the machine table depending on the number of counting pulses or revolutions of the motor encoder is set in the machine parameters (e.g., one revolution of the encoder changes the table position by 10 mm).

The speed controller receives its nominal value from the position encoder. Thus the output quantity of the position controller is the input quantity of the speed controller. This is why this interface is also designated as "nominal speed value interface". With analog axes, the control leads the nominal speed value interface (± 10V) "outside" to the analog servo amplifier. With digital axes, this interface is "inside" the control!

The actual value for the speed controller is supplied by the motor encoder.

The current controller receives its nominal value from the speed controller. The actual value is provided by current sensors in the power module.

Cycle times The position controller cycle time is the time interval during which the interpolation points on the path are calculated. The speed controller cycle time is the time interval in which the actual speed value is compared to the calculated nominal speed value. The current controller cycle

time is the time interval in which the actual current value is compared to the calculated nominal

current value.

Detail

current controller

6, 10 or 12 digital current controllers for the axes and spindle(s) are integrated in the iTNC 530: The nominal values for magnetizing current Idnom and torque current Iqnom are divided into the PWM signals U1, U2 and U3 through a PI controller and vector rotator VD+, and are transferred to the power module through X51 to X60.

The actual current values I1act and I2act are determined by the power module and are transferred to vector rotator VD– through X51 to X60. The vector rotator determines the actual values of magnetizing current Idist and torque current Iqnom.

Circuit diagram: Position

Detail

speed controller

6, 10 or 12 digital speed controllers for the axes and spindle(s) are integrated in the iTNC 530: The actual speed values are measured directly at the motors with HEIDENHAIN rotary encoders. The position controller provides the nominal speed value. The speed controller is driven by the difference between nominal and actual speed values. It provides the nominal current value as output.

Position feedback control with servo lag

Following error (also known as servo lag) is a gap that remains between the nominal position commanded by the NC and the actual position.

Simplified representation:

The nominal position value snoml for a given axis is compared with the actual position value sactl and the resulting difference is the following error sa:

sa = snoml – sactl sa = following error

snoml = nominal position value sactl = actual position value

The following error is multiplied by the kv factor and passed on as nominal velocity value: vnoml = kv · sa

vnoml = nominal velocity value

kv factor during control with following error

The control loop gain, known as the kv factor, defines the amplification of the position control loop.

The kV factor is set by the machine tool builder.

For axes that are interpolated with each other, the kv factors must be equal to prevent contour deviations.

Interrelation of kv factor feed and following error

The following formula shows the interrelation of kv factor, feed rate, and following error: Or

kv = loop gain [(m/min)/mm] ve = rapid traverse [m/min] sa = following error [mm]

v_Noml S_Noml

S_Actl

DANGER

Control-loop parameter may only be changed by the machine manufacturer or after consultation with the machine manufacturer!

An increase of the kv factor could lead to damage or injury of property or persons!

k_v ve s_a --- = s_a ve k_v --- =

Position control with velocity feedforward control

The nominal velocity value consists of an open-loop and a closed-loop component.

With velocity feedforward control, the machine-adjusted nominal velocity value is the open-loop controlled component. The closed-loop velocity component is calculated through the following error. The following error is small.

On the basis of MP 1392 (for the operating modes Positioning with manual data input,

Program run / single block and Program run full sequence) and MP 1391 (for the operating

modes Manual operation and El. handwheel) you can find out whether the traverse is performed in the following error mode or feedforward mode. --> With velocity feedforward control, the bits are set to 1.

Block diagram:

Unlike operation with following error, the optimum kv factor for each axis when operating with interpolated axes is set by the machine manufacturer.

v S_a + – S_a + + v Δ t MP1510.x MP1080.x v_Noml Δ SNoml S_Noml S_Actl

Position control with velocity semifeedforward control

MP1396.x allows the operator to switch to velocity semifeedforward control.

Normally, work will be carried out using velocity feedforward. Velocity semifeedforward is activated, for example, by an OEM cycle before roughing, in order to permit a higher following error and thereby a higher velocity, combined with a lowered accuracy, in order to traverse corners.

Before finishing, another OEM cycle can be used to switch back to velocity feedforward, in order to finish with the highest accuracy possible.

In order to use velocity semifeedforward, a factor must be entered for every axis in MP1396.x, where values toward 0 control the following error more, and values toward 1 control the velocity feedforward more.

As soon as a factor between 0.001 and 0.999 has been entered in MP1396.x, the kV factor from MP1516.x becomes effective.

The values for position monitoring are interpolated according to the factor in MP1396.x between the values for servo lag (MP1710.x, MP1720.x) and the values for velocity feedforward control (MP1410.x, MP1420.x).

Note

For axes that are interpolated with each other, the kv factors must be equal. In this case the smaller kV factor determines the input value for these axes.

Feedback control with following error (servo

lag)

Feedback control with velocity semifeedforward

Feedback control with velocity feedforward MP1391 bit x = 0 MP1392 bit x = 0 MP1396.x = nonfunctional MP1391 bit x = 1 MP1392 bit x = 1 MP1396.x = 0.001 MP1396.x = 0.999 MP1391 bit x = 1 MP1392 bit x = 1 MP1396.x = 1