Menu 3 − Speed Control and Position Feedback

Mode: RFC‑S

Final Speed Reference (03.001) shows the reference at the input to the speed controller, which is the sum of the Post Ramp Reference (02.001) if the ramp output is not disabled and the hard speed reference (if enabled).

The speed feedback can be selected with Motor Control Feedback Select (03.026) to be taken from either of the drive position feedback interfaces or from a position feedback interface in a position feedback category option module. It is also possible to selected sensorless speed feedback with RFC Feedback Mode (03.024). Speed Feedback (03.002) shows the level of the speed feedback selected for the speed controller.

The FI attribute is set for this parameter, so display filtering is active when this parameter is viewed with one of the drive keypads. The value held in the drive parameter (accessible via comms or an option module) does not include this filter, but is a value that is obtained over a sliding 16ms period to limit the ripple. The speed feedback includes quantisation ripple given by the following equation in rpm:

Ripple in Speed Feedback (03.002) = 60 / 16ms / Position resolution

The ripple for a linear system is given by the following equation in mm/s:

Ripple in Speed Feedback (03.002) = Pole pitch in mm / 16ms / Position resolution

The position resolution for each type of feedback device is defined in the table below.

For example the ripple in Speed Feedback (03.002) when a 4096 line AB type encoder is used is 0.23rpm. It should be noted that no filtering is applied to the speed feedback used by the speed controller or for the position feedback reference system unless the feedback filter for that particular interface is activated by putting a non-zero value in the appropriate set up parameter. The diagram below shows the filtering applied to the speed feedback when this is taken from the P1 position feedback option interface.

The speed feedback ripple seen by the speed controller and the position feedback reference is given by the following equations when the filter set up value = 0. 

Ripple for a rotary system in rpm = 60 / Speed controller sample time / Position resolution

Ripple for a linear system in mm/s = Pole pitch in mm / Speed controller sample time / Position resolution

The speed controller sample time is 250µs. If the filter set up value is non-zero the ripple is given by:

Ripple for a rotary system in rpm = 60 / Filter time / Position resolution

Ripple for a linear system in mm/s = Pole pitch in mm / Filter time / Position resolution

The description so far covers the P1 drive position feedback interface. Similar filtering is provided with the P2 drive position feedback interface and with position feedback interfaces in position feedback category option modules.

It is not advisable to use the speed feedback filter unless it is specifically required for high inertia applications with high controller gains, or if commutation signals alone are used for feedback, because the filter has a non-linear transfer function. It is preferable to use the current demand filter (Current Reference Filter 1 Time Constant (04.012)) as this is a linear first order filter that provides filtering on noise generated from both the speed reference and the speed feedback. It should be noted that any filtering included within the speed controller feedback loop, either on the speed feedback or the current demand, introduces a delay and limits the maximum bandwidth of the controller for stable operation.

The speed ripple seen by the speed controller can be quite high in some cases, for example with a 4096 line encoder the speed ripple is 14.6rpm with a sample time of 250µs. This causes high frequency torque ripple and acoustic motor noise. These effects increase with the level of speed feedback ripple and with the gains used in the speed controller. Therefore high speed feedback ripple usually limits the maximum possible gain settings for the speed controller, and so a position feedback device with high position resolution is usually required for a system with high dynamic performance or stiffness. It should be noted that the ripple caused by feedback quantisation and does not define speed feedback resolution. The speed controller accumulates all pulses from the position feedback, and so the speed controller resolution is not limited by the feedback, but by the resolution of the speed reference.

The speed error is the difference between the Final Speed Reference (03.001) and the Speed Feedback (03.002), and does not include the effect of the differential term in the speed controller feedback branch.

The output of the speed regulator is a torque demand given as a percentage of rated motor torque. It should be noted that this will be modified to take into account in the level of motor flux if field weakening is active before it is converted into the Final Current Reference (04.004).

If the Speed Feedback (03.002) is at or below the level defined by this parameter in either direction Zero Speed (10.003) = 1, otherwise Zero Speed (10.003) = 0.

At Speed (10.006) is set if the Speed Feedback (03.002) is on the boundaries or within the at-speed window. Above Set Speed (10.007) and Below Set Speed (10.005) are set if the feedback is above or below the window respectively.

If Absolute At Speed Select (03.009) = 0 reference window mode is used.
The "at speed" condition is true if Reference On (01.011) =1 and,

(|Pre-ramp Reference (01.003)| - At Speed Lower Limit (03.006)) ≤ |Speed Feedback (03.002)| ≤ (|Pre-ramp Reference (01.003)| + At Speed Upper Limit (03.007))

(If the lower limit is less than zero then zero is used as the lower limit.)

If Absolute At Speed Select (03.009) = 1 absolute window mode is used.
The "at speed" condition is true if Reference On (01.011) =1 and,

At Speed Lower Limit (03.006) ≤ |Speed Feedback (03.002)| ≤ At Speed Upper Limit (03.007)

See At Speed Lower Limit (03.006).

If Over Speed Threshold (03.008) is set to a non-zero value it defines the over speed threshold. If the Speed Feedback (03.002) exceeds this threshold in either direction an Over Speed trip is produced. If Over Speed Threshold (03.008) is set to 0.0 the threshold is based on the variable minimum/maximum for the references and is equal to 1.2 x VM_SPEED_FREQ_REF[MAX]. As the over-speed trip provides the ultimate protection against against the motor operating beyond the maximum allowable speed, the trip function is time deterministic and will disable the drive within 4ms of the over-speed condition being detected.

The motor speed and the motor voltage can be monitored to detect if the motor is accelerating in an uncontrolled way because position feedback is being used and the Position Feedback Phase Angle (03.025) has not been set up correctly. If the Over Speed Threshold (03.008) = 0 then position feedback phase angle error monitoring is enabled and a Phasing Error trip is initiated if a failure is detected. If the Over Speed Threshold (03.008) is not equal to 0 this feature is disabled, and the over-speed trip is the only protection against high motor speed due to an incorrect value of Position Feedback Phase Angle (03.025). It should be noted that phase angle error monitoring is not possible with high saliency motors, and so if Active Saliency Torque Mode (05.066) = 1 then phase angle error detection is suppressed and the over-speed trip must be used to provide protection.

If sensorless control is being used, a Phasing Error trip is also initiated if loss of control is detected. This trip is also suppressed if Over Speed Threshold (03.008) is set to a non-zero value.

See At Speed Lower Limit (03.006).

The diagram below shows a generalised representation of the speed controller. The controller includes a feed forward proportional gain (Kp), a feed forward integral gain (Ki), and a differential feedback gain (Kd). The description here refers to the first set of gains for motor map 1 (Speed Controller Proportional Gain Kp1 (03.010), Speed Controller Integral Gain Ki1 (03.011) and Speed Controller Differential Feedback Gain Kd1 (03.012)). See Speed Controller Gain Select (03.016) on how to select a different set of gains. 

Proportional gain (Kp) - Speed Controller Proportional Gain Kp1 (03.010) 
If Kp is non-zero and Ki is zero the controller will only have a proportional term, and there must be a speed error to produce a torque reference. Therefore, as the motor load increases there will be a difference between the reference and actual speeds. This effect, called regulation, depends on the level of the proportional gain, the higher the gain the smaller the speed error for a given load. If the proportional gain is too high either the acoustic noise produced due to speed feedback quantisation becomes unacceptable, or the closed-loop stability limit is reached.

Integral gain (Ki) - Speed Controller Integral Gain Ki1 (03.011) 
The integral gain is provided to prevent speed regulation. The error is accumulated over a period of time and used to produce the necessary torque reference without any speed error. Increasing the integral gain reduces the time taken for the speed to reach the correct level and increases the stiffness of the system, i.e. it reduces the positional displacement produced by applying a load torque to the motor. Unfortunately increasing the integral gain also reduces the system damping giving overshoot after a transient. For a given integral gain the damping can be improved by increasing the proportional gain. A compromise must be reached where the system response, stiffness and damping are all adequate for the application. The integral term is implemented in the form of ∑(Ki x error), and so the integral gain can be changed when the controller is active without causing large transients on the torque reference.

Differential gain (Kd) - Speed Controller Differential Feedback Gain Kd1 (03.012) 
The differential gain is provided in the feedback of the speed controller to give additional damping. The differential term is implemented in a way that does not introduce excessive noise normally associated with this type of function. Increasing the differential term reduces the overshoot produced by under-damping, however for most applications the proportional and integral gains alone are sufficient. It should be noted that the differential term is limited internally so that it is ineffective if speed in rpm x Kd x Ki is greater than 170.

To analyse the performance of the speed controller it may be represented as an s-domain model as shown below.

Kc' is the conversion between the speed controller output and the torque producing current reference. A value of unity at the output of the speed controller gives a torque producing current equal to Kc'. The drive automatically compensates the torque producing current reference for flux variations in field weakening, and so Kc' can be assumed to have a constant value even in field weakening. Kc' = Full Scale Current Kc (11.061) x 0.45.

Kt is the torque constant of the motor (i.e. torque in Nm per amp of torque producing current). This value is normally available from the manufacturer for a permanent magnet motor, however, for induction motors the value must be calculated from the motor parameters. In RFC-A mode this calculation is performed by the drive and the result is stored in Torque Per Amp (05.032).

L(s) is the transfer function of the load.

The speed controller calculations are provided for a rotary application. However, for a linear application it is possible to set Torque Per Amp (05.032) to the force per amp and the Motor And Load Inertia (03.018) to the mass, and all the rotary system equations still apply.

It should be noted that the gain levels are compatible with those in Unidrive SP. The internal resolution of the intergral gain parameter is twice that of Unidrive SP. In most applications this makes no difference to the performance, however, with Unidrive SP the internal value is zero (the integral term disabled) if the user parameter is less than 0.05. In Unidrive M the internal integral gain would be zero if Speed Controller Integral Gain Ki1 (03.011) is less than 0.03. However, if Speed Controller Integral Gain Ki1 (03.011) is non-zero and less than 0.03 (i.e. 0.01 or 0.02) the internal value is one, so that the integral term remains active, unless the user deliberately disables this term by setting Speed Controller Integral Gain Ki1 (03.011) to zero.

See Speed Controller Proportional Gain Kp1 (03.010).

See Speed Controller Proportional Gain Kp1 (03.010).

RFC Feedback Mode (03.024) selects the method used to provide rotor position feedback to control the motor. 

0: Position feedback
Motor Control Feedback Select (03.026) selects the position feedback used in controlling the motor. The maximum and minimum for the speed references are limited by the VM_POSITIVE_REF_CLAMP variable minimum/maximum which prevents the speed from exceeding the level where the position feedback can operate correctly.

1: Sensorless
A sensorless algorithm is used to provide position feedback used in controlling the motor. A filter with a 4ms time constant is automatically included in the speed feedback as this is required for this system to operate correctly. The possible speed controller bandwidth will be reduced by a factor of approximately 10 compared with the bandwidth possible when using a position feedback device. With this setting the maximum and minimum for the speed references are limited by the VM_POSITIVE_REF_CLAMP variable minimum/maximum as for the "Position feedback"  setting above.

NOTE: Sensorless mode can only be used with permenant magnet motors and not with synchronous reluctance motors.

2: Position feedback with no maximum speed limit
As for "Position Feedback" setting except that the maximum reference limit is disabled so that it is possible to change between operation with or without position feedback if the speed range needs to be extended beyond the limit of the position feedback device. Care should be taken not to exceed a speed that would damage the position feedback device.

3: Sensorless with no maximum speed limit
As for "Sensorless" setting except that the maximum reference limit is disabled

4: Feedback Correct
Motor Control Feedback Select (03.026) selects the position feedback used in controlling the motor. This mode is primarily intended to be used with incremental encoders that do not give the absolute position of the motor and where the number of lines per revolution is not known exactly or can vary in time or with motor speed. A once per mechanical revolution "marker" signal must be provided and this must be routed to the F1 freeze (see F1 Freeze Trigger Source (03.100)). The F1 Freeze must be set up so that freeze events occur on the leading edge of the marker pulse (falling for a low going pulse or rising for a high going pulse). The control alogrithm automatically resets the freeze flag after a freeze event, and so F1 Freeze Mode (03.101) can be set to the "1st" or "All" option for the selected edge. As the position feedback is not absolute a phasing test on enable is required and should be selected either to occur once or on every enable as required with Phasing Test On Enable (05.014). (A full stationary auto-tune can be carried out prior to running the motor as this will automatically select a suitable phasing test to use on enable.) The expected number of lines per revolution for the encoder should be set up for the selected position feedback. When the motor runs the drive uses the once per revolution marker pulse to adjust the ratio between the parameter that has been set up to define the number of lines per revolution and the actual lines per revolution which is written to Feedback Correction Ratio (03.077). The position of the first valid marker event in each direction is also stored internally within the drive and used to maintain synchronism between the feedback position and the once per revolution marker. Because the leading edge of the marker is always used the position of the marker event is different by the width of the marker pulse in each direction. Storing and using the marker position for each direction ensures that the feedback position is not shifted by the width of the marker pulse depending on the direction of rotation.

It is preferable to perform a phasing test on each enable, but for some applications this is not acceptable because of the acoustic noise produced by the test, and so the drive is set up just to perform a phasing test on power-up and position feedback initialisation (see Phasing Test On Enable (05.014)). When the motor begins to rotate the initial value of Feedback Correction Ratio (03.077) is used until at least two marker events have occurred so that the actual lines per revolution can be measured. This means that the position feedback used for motor control can drift during this initial period if Feedback Correction Ratio (03.077) is not accurate. To prevent any position error caused during this initial period from being retained, one additional phasing test is carried out on the next enable even if a phasing test only on the first enable is selected. Note that Feedback Correction Ratio (03.077) is a power-down save parameter, and so if the ratio changes with time the new value is automatically retained during power cycling.

It is expected that a reasonable approximation to the number of lines per revolution is entered in the appropriate user parameter. If an accurate value is not known and the motor can be rotated without enabling the drive, then the drive can be made to adjust Feedback Correction Ratio (03.077) for the inaccurate lines per revolution while it is still disabled. Once a suitable value has been obtained for the ratio the drive can be made to perform a phasing test on enable and will then run correctly.  

While RFC Feedback Mode (03.024) is set the "Feedback Correct" Feedback Correction Ratio (03.077) is constantly updated by the drive. If it is necessary to write to this parameter RFC Feedback Mode (03.024) should be set to a different value, Feedback Correction Ratio (03.077) should be modified and then RFC Feedback Mode (03.024) should be set to "Feedback Correct" again. (It is still possible for Feedback Correction Ratio (03.077) to be adjusted for 1s after RFC Feedback Mode (03.024) is set to "Feedback Correct". Because the default for RFC Feedback Mode (03.024) is "Feedback" this feature allows parameter files which first defaults the drive parameters before writing the difference from default values to include Feedback Correction Ratio (03.077) and write it successfully before it begins to be written by the drive.) 

The phase angle between the rotor flux and the feedback position must be set up correctly for the drive to control the motor correctly. If the phase angle is known it can be set in Position Feedback Phase Angle (03.025) by the user. Alternatively the drive can automatically measure the phase angle by performing a phasing test (see Auto-tune (05.012)). When the test is complete the new value is automatically written to Position Feedback Phase Angle (03.025). Position Feedback Phase Angle (03.025) can be modified at any time and becomes effective immediately. Position Feedback Phase Angle (03.025) has a factory default value of 0.0, but is not affected when defaults are loaded by the user.

The alignment required for zero position feedback phase angle (i.e. Position Feedback Phase Angle (03.025) = 0.0) is given below for different feedback devices. Although the drive output phase rotation and position feedback direction of rotation can be reversed, in the examples given forward rotation of the motor is produced when Vu leads Vv leads Vw, and the position feedback direction has not been reversed. Although it is not essential, forward rotation of a motor is normally defined as clockwise when looking at the motor shaft end. When the motor is rotating forwards the motor speed is shown as positive and the position increases.

AB Servo, FD Servo, FR Servo, SC Servo
The alignment required between the no-load motor voltages and the commutation signals for Position Feedback Phase Angle (03.025) = 0.0 is shown in the diagram below. It should be noted that if the encoder is ajusted with respect to the rotor so that the UVW signals are moved to the right with respect to the voltages the angle in Position Feedback Phase Angle (03.025) is increased from zero. If the encoder is retarded the angle changes to 359.9 and then reduces towards zero.

The encoder can be aligned statically by connecting the motor to a d.c. power supply as shown.

The motor will move to one of a number of positions defined by the number of motor pole pairs (i.e. 3 positions for a six pole motor, etc.). The encoder should be adjusted so that the U commutation signal is high, W is low and V is toggling in one of these positions.

Any other feedback device
The alignment required between the no-load motor voltages and the position feedback (i.e. P1 Position (03.029) for the drive P1 position feedback interface) with Position Feedback Phase Angle (03.025) = 0.0 is shown in the diagram below for a 2 or 4 pole motor. For higher numbers of poles the zero position should still be aligned as shown, but the one electrical cycle shown corresponds to 360° / (Number of poles / 2). It should be noted that if the position feedback device adjusted with respect to the rotor and the zero position is moved to the right with respect to the voltages Position Feedback Phase Angle (03.025) is increased from zero. If the position feedback is retarded Position Feedback Phase Angle (03.025) changes to 359.9 and then reduces towards zero.

The position feedback device can be aligned statically by connecting the motor to a d.c. power supply as already shown. The motor will move to one of a number of positions defined by the number of motor poles (i.e. 3 positions for a six pole motor, etc.). The position feedback device should be adjusted so that the position displayed by the drive is (n x 65536) / (Number of poles / 2), where n = 0, 1, etc..

The diagram below shows the relationship between the position feedback signals and P1 Position (03.029) for a 2-pole resolver. For higher numbers of poles the zero position is the same but the cycle repeats multiple times per revolution as the number of poles is increased. Where the waveforms are at their highest positive level they are maximum and in phase with the excitation, and where they are at their lowest negative level they are maximum and 180^o out of phase with the excitation.

Motor Control Feedback Select (03.026) should be used to select the position feedback interface for motor control. If the feedback interface does not exist then the drive will produce an Encoder 9 trip if it is enabled. Note that if RFC Feedback Mode (03.024) is set to 1 or 3 to select sensorless control then this trip is suppressed.

If Sensorless Mode Active (03.078) = 0 it indicates that the position feedback selected with Motor Control Feedback Select (03.026) is being used for motor control. If Sensorless Sensorless Mode Active (03.078) = 1 it indicates that the sensorless algorithm is being used instead. See RFC Feedback Mode (03.024) for more information.

When sensorless mode is active the estimated motor speed can include some unwanted noise. Some motors have concentrated windings which results is flux distortion when the motor is loaded, which in turn causes ripple in the motor currents and additional unwanted components in the calculated speed feedback. This usually has a more significant effect if the motor is running at high speed when the voltage controller may be active to limit the motor voltage. A filter is applied to the speed feedback with a time constant defined by Sensorless Mode Filter (03.079). The default value for this parameter gives the maximum amount of filtering, so that motors which introduce unwanted feedback noise will operate correctly. If the motor does not introduce this type of noise it is possible to reduce the filter time constant to give better dynamic performance when speed control is required.

When the drive is operating without position feedback Sensorless Position (03.080) gives the motor position where the least significant 16 bits represent a movement equivalent to one pole of the motor. The most significant 16 bits represent turns where one turn is the movement associate with one pole. For example in a rotary application with a 4 pole motor, the movement associated with one pole is a mechanical movement of 180°. In RFC-A mode Sensorless Position (03.080) is aligned with the motor flux and voltages, but this has no fixed relationship to the mechanical position of the rotor.