Menu 3 − Speed Control and Position Feedback

Mode: RFC‑A

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].

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), which can be changed even when the motor is running, selects the feedback method used to provide position feedback to control the motor. The actual feedback method being used at any time is given by Sensorless Mode Active (03.078). When sensorless mode is used, 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 with a position feedback device. 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 cannot be interpreted correctly. This limit is disabled if RFC Feedback Mode (03.024) is 2 or 3, 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.

0: Position feedback
RFC mode is active using the position feedback selected Motor Control Feedback Select (03.026) to control the motor.

1: Sensorless
RFC mode using a sensorless algorithm to provide position feedback, is used to control the motor.

2: Position feedback with no maximum speed limit
RFC mode with position feedback selected with Motor Control Feedback Select (03.026)) is used to control the motor. The maximum reference limit is disabled.

3: Sensorless with no maximum speed limit
RFC mode using a sensorless algorithm to provide position feedback, is used to control the motor. The maximum reference limit is disabled.

It should be noted that when sensorless mode is active the maximum torque that can be produced at low speeds (i.e. < 2% or rated speed) is reduced.

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 measured speed can include some ripple, which increases as the drive passes into field weakening. A filter is applied to the estimated speed and Sensorless Mode Filter (03.079) defines the time constant. The default time constant is 4ms, but this can be extended to improve the filtering. This is particularly useful when using standard ramp or spinning start with a low friction high inertia load, and can prevent over voltage trips when the drive has no braking resistor.

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.