Feedback connections

The Nix Servo Drive has a feedback connector and an absolute encoder connector dedicated to the following feedback options:

Additional feedback connections can be found on I/O connector:

Nix also provides a 5V, 200 mA outputs for feedbacks supply. This output is overload and short circuit protected.  

Digital Halls interface

The Hall sensors are Hall effect devices that are built into the motor to detect the position of the rotor magnetic field. Usually, motors include 3 hall sensors, spaced 120º apart. Using these 3 signals, the drive is capable to detect the position, direction and velocity of the rotor. Next figures show examples of digital halls signals. 

Digital halls signals example

Digital halls can be used for commutation, position and velocity control. Resolution using these sensors is much lower than using encoders. Nix can use single ended Hall sensors to drive the motor with trapezoidal commutation, but not with sinusoidal commutation. 

This interface accepts 0-5 V level input signals. Inputs are pulled up to 5 V, so industry standard open collector and logic output hall effect sensors can be connected. Next table summarizes digital halls inputs main features: 

Specification

Value

Type of inputs

Non-isolated
Single ended with pull-up and low pass filter
ESD protected

Number of inputs

3

ESD capability

IEC 61000-4-2 (ESD) ± 15 kV (air), ± 8 kV (contact)
IEC 61000-4-4 (EFT) 40 A (5/50 ns)

Voltage range

0 ~ 5 V

Maximum voltage range-0.5 ~ 5.5 V
Maximum recommended working frequency1 kHz

1st order filter cutting frequency (-3dB)

160 kHz

Sampling frequency10 ksps

Type of sensors

Open collector
Logic output
Push-pull output

Pull-up resistor value1 kΩ (The pull-up is activated only when the drive is configured to use digital hall sensors)

Digital and analog Halls

Digital halls input pins are shared with Analog Halls interface pins.

The 1 kΩ pull-up resistors are disconnected when Analog-halls input is selected to prevent analog data corruption.

Next figure shows the circuit model of the digital Halls inputs. 

NixDigitalHallsCircuit

Next figure illustrates how to connect the digital halls to the Nix Servo Drive. Refer to Feedback wiring recommendations  for more information about connections and wires. 

NixDigHalls


Velocity control with Halls

Due to inherent low resolution of motor mounted Hall sensors, they are not recommended for velocity feedback in low speed applications.

Analog Halls interface

The Nix Servo Drive can operate with analog Hall sensors (also known as linear halls) as feedback option. Signals provided by these sensors are typically 5 V peak-to-peak sinusoidal signals, with 2.5 V offset and a phase shift of 120 degrees. These sensors can be used for a fine positioning of the rotor. Nix analog halls inputs main features are shown in next table:

Specification

Value

Type of inputs

Non-isolated
Single ended analog filtered
ESD protected

Number of inputs

3

ESD capability

IEC 61000-4-2 (ESD) ± 15 kV (air), ± 8 kV (contact)
IEC 61000-4-4 (EFT) 40 A (5/50 ns)

Maximum recommended working frequency1 kHz

2nd order filter cutting frequency

11.9 kHz

Sampling frequency10 ksps

Voltage range

0 ~ 5 V (10 bits)

Maximum voltage range-0.3 ~ 5.3 V
Input impedance> 24 kΩ

Next figure illustrates the circuit model for one of the linear Halls inputs. An active Sallen-Key low pass filter provides immunity to motor and feedback noise. Note that analog halls pins are shared with Digital Halls interface, to avoid any signal distortion, when analog halls interface is selected, the 1 kΩ pull-up is disconnected automatically.  

NixAnalogHallsDiagram

Next figure shows how to connect the linear Halls to the Nix Servo Drive. Refer to Feedback wiring recommendations for more information about connections and wires.

NixAnHalls

Digital Incremental Encoder

Nix can use single ended or differential digital incremental encoder inputs (also known as quadrature incremental encoders) for velocity and/or position control, as well as commutation sensor. The encoder provides incremental position feedback that can be extrapolated into precise velocity or position information. Using high resolution encoders allows Nix Servo Drive to use sinusoidal commutation.

Channel A and channel B signals should have a phase shift of 90 degrees, indicating the rotation direction. Based on the pulses frequency, the drive can calculate the motor velocity and position.

Example of single ended digital encoder inputsExample of digital differential encoder signals

DigitalEncoderSingle

  NixDigitalEncoderDifferentialInputs

High precision applications

High resolution motor mounted encoders allows excellent velocity and position control at all speeds. Encoder feedback should be used for applications requiring precise and accurate velocity and position control. Digital encoders are especially useful in applications where low-speed smoothness is the objective.

The Nix Servo Drive has one differential digital encoder interface, with optional index signal input. Index signal (Z) is a single pulse per revolution signal that can be used to know absolute positions. Next table illustrates digital encoder inputs main features. 

Specification

Value

Type of inputs

Non-isolated
Differential or single ended
ESD protected

Number of inputs

3 (A, B and Index)

ESD capability

IEC 61000-4-2 (ESD) ± 15 kV (air), ± 8 kV (contact)
IEC 61000-4-4 (EFT) 40 A (5/50 ns)

Nominal voltage range

0 ~ 5 V

Maximum voltage range

-0.5 ~ 5.5 V
Maximum recommended working frequency10 MHz (differential)

1st order filter cutting frequency (-3 dB)

6.6 MHz

Maximum readable pulse frequency30 MHz
Termination resistor120 Ω (between ENC_x+ and ENC_x-)
Bias resistors

ENC_x+ (positive input) 1 kΩ to 5 V

ENC_x- (negative input) 1 kΩ to 2.5 V (equivalent)


For encoder signal reception, an analog differential line receiver with an hysteresis comparator is used. The high signals (ENC_A+, ENC_B+ and ENC_Z+) are pulled up to +5 V, and the low signals (ENC_A-, ENC_B- and ENC_Z-) are biased to 2.5 V. This arrangement let the user to connect either differential output encoders or single ended encoders (both open collector and totem pole). 

The encoder interface also accepts an RS-422 differential quadrature line driver signal in the range of 0 V to 5 V, up to 10 MHz. When single ended encoder is connected, only high signals (ENC_A+, ENC_B+ and ENC_Z+) must be used. 

Next figures illustrate how to connect a differential and a single ended encoder to the Nix Servo Drive. Refer to Feedback wiring recommendations  for more information about connections and wires. 

NixDigEncDiff

NixDigEncSing

Next figure shows the circuit model of the digital encoder inputs.

NixDigEncDiagram

Digital encoders with single ended 24 V outputs

Nix Servo Drive can also interface single ended digital encoders with output voltages higher than 5 V, for instance 24 V PLC level encoder. With the use of series connected limiting resistors, Nix is able to read encoder counts correctly while the inputs are correctly protected.

It is recommended to use a 4.7 kΩ 1/4 W resistor in series with the ENC_X- (inverting) input and leave the ENC_X+ floating. 

Digital encoders with differential 24 V outputs

To interface with 24 V push-pull style differential encoders, it is recommended to connect 4.7 kΩ 1/4 W resistors in series between the encoder signals and the corresponding drive inputs.

This ensures a correct differential signal reading as well as limiting currents to safe levels. Note that this additional resistance may limit the maximum encoder frequency to approximately 1 MHz by making a low pass filter with the 100 pF input capacitance.

Encoder broken wire detection

Nix Servo Drive includes a broken wire detection circuit. The circuit is based on 3 EX-OR gates that will generate an error if the encoder is disconnected or a wire is broken. This system only works for differential encoders.

NixEncoderBrokenWireDiagram

Encoder without Index (Z) line

To avoid a broken wire fault when the differential encoder has no index (Z) line, connect the negative pin (ENC_Z-) to GND (this ensures the XOR result = 1) or configure the encoder as single ended in MotionLab.


Analog encoder (Sin-Cos encoder) interface

The Nix Servo Drive can use analog encoder (also known as Sin-Cos encoder) as a position and velocity feedback element. This sensor provides a pair of quadrature sine and cosine signals as the motor moves, which frequency depends on the motor speed. The signals may be generated by optical or magnetic means. For noise immunity, the signals are typically transmitted differentially from the encoder to the sensor interface electronics.

PinSignal descriptionSignal example

SIN+

Sine wave with 2.5 V offset and 0.5 Vpp

SIN-

Same as SIN+, but with 180º phase shift

COS+

Cosine with 2.5 V offset and 0.5 Vpp

COS-

Same as COS+, but with 180º phase shift

REF+

One sine half wave per revolution as index pulse. Optional.

REF-

Same as REF+, but with 180º phase shift. Optional.

Sin-Cos calibration

Analog encoder signals are not always perfect sine and cosines. For this reason, Nix includes sin-cos calibration and adjustment parameters. For further information see the E-Core registers for Sin-Cos encoder configuration.

An automatic calibration based on Lissajous curves is included in MotionLab, which allows an easy feedback adjustment.


Next table summarizes analog encoder inputs main features. 

Specification

Value

Type of inputs

Differential analog input (switching to digital automatically at high speed)
ESD protected

Number of inputs

3 (SIN, COS, REF)

ESD capability

IEC 61000-4-2 (ESD) ± 15 kV (air), ± 8 kV (contact)
IEC 61000-4-4 (EFT) 40 A (5/50 ns)

Optimal voltage range

Each individual input: 2.25 ~ 2.75 V (0.5 Vp-p)

Differential inputs (SIN+ - SIN-, COS+ - COS-): 1 Vp-p

Maximum absolute voltage rangeAll inputs-0.5 ~ 5.5 V

Maximum recommended working frequency

1 kHz used as analog encoder

10 MHz used as digital encoder

1st order filter cutting frequency (-3 dB)

6.6 MHz

Sampling rate (analog)10 ksps
Maximum readable pulse frequency (digital)30 MHz

Input impedance

120 Ω resistive differential

100 pF capacitive

1 kΩ to GND

Resolution

10 bits

Next figure shows how to connect a Sin-Cos encoder to Nix Servo Drive. Refer to Feedback wiring recommendations  for more information about connections and wires. 

NixSinCos

For differential Sin-Cos encoders with peak to peak voltage exceeding 1 Vpp it is recommended to use a series resistor on ENC_X+ and ENC_X- pins. The value of that resistor should be chosen to attenuate the voltage at the input around 1 Vpp ± 20%. Use the following formula to determine the resistor that must be placed on each input. Rin = 50 ·  ( Vinpp - 1 V). Example: for 3 Vpp Sin-Cos, the resistance in series with each pin should be Rin = 50 · (3 - 1) = 100 Ω.

If no REF (Or Zero index) signal is available leave the ENC_Z- and ENC_Z+ pins floating. 

For single-ended analog Sin-Cos encoders. Connect the encoder signals to the ENC_x+ (positive) inputs.

  • If the average of the sine and cosine is 2.5 V±0.1 V.  ENC_x- inputs should be left floating.
  • If the average is different to 2.5 V±0.1 V. connect all the ENC_X- inputs to the average value. Some encoders provide this output and name (confusingly) Vref. The value of the average must be between 0.6 V and 4.4 V.

Circuit model for each differential channel (A, B, REF) is shown in the next figure.

NixSinCosDiagram

Absolute encoder interface

The Nix has an Absolute encoder connector that can be used as position and velocity feedback element. This sensor generates digital data that represent the encoder actual position. From the position information, speed and direction of motion is calculated. The position is not lost even if the encoder is powered down, this means it is not necessary to move to a reference position as with incremental type encoders.

Next table shows the absolute encoder inputs electrical specifications.

Specification

Value

Type of inputs

Non-isolated
Differential
ESD protected

ESD capability

IEC 61000-4-2 (ESD) ± 12 kV (air), ± 12 kV (contact) 
IEC 61000-4-4 (EFT) ± 4 kV

Number of inputs2 (CLK and DATA)
Nominal voltage range0 ~ 5 V
Maximum voltage range-13 ~ 16.5 V
Maximum readable frequency (SSI)1 kHz
Termination120 Ω on data line

Next Figure shows how to connect an Absolute encoder to Nix Servo Drive. Refer to Feedback wiring recommendations for more information about connections and wires. 

NixAbsEncoder

Circuit model for the absolute encoder receiver channels is shown in the next figure.

NixAbsEncDiagram

Digital input feedback - PWM encoder

Nix Servo Drive can also use a PWM encoder connected through the I/O connector as a feedback element. A PWM encoder provides a Pulse Width Modulated (PWM) signal with a duty cycle proportional to the angle (position) of the rotor. This feedback can be interfaced through the high-speed digital input 1 (HS_GPI1). Both differential and single-ended PWM encoders can be used. Further specifications about the PWM input can be found in I/O connection section.

Next figure illustrates PWM feedback input for different rotor positions:

Next figure illustrates how to connect a differential PWM encoder to the Nix Servo Drive: 
NixPWMEnc

Single ended operation

In order to use the high-speed digital input in single ended mode, connect the negative terminal (HS_GPIx-) to 2.5 V. This voltage can be achieved with a voltage divider from +5V_OUT.

Voltage_divider_NIX_Fb

For a 24 V input, the negative terminal (HS_GPIx-) can be connected to 5 V (+5V_OUT).


Next figure illustrates how to connect a single ended PWM encoder to the Nix Servo Drive:

NixPWMEncSing

Refer to Feedback wiring recommendations  for more information about connections and wires.

Analog input feedback

Nix Servo Drive can also use analog feedback systems connected through the I/O connector. From the voltage level of one analog input, the position or velocity of the rotor can be calculated. The Nix have 2 analog inputs that can be used for feedback input, each one with a different input range. The input used as feedback can be selected by software. Further specifications about the analog inputs input can be found in I/O connection section.

Refer to Feedback wiring recommendations for more information about connections and wires.

Potentiometer

A typical analog sensor used for position feedback is a potentiometer. This sensor provides a voltage proportional to the rotor position.

The following picture shows how to connect a potentiometer as a position sensor using analog input 1:

NixAnIn1Pot

Recommended potentiometer resistance

Potentiometers with high values of resistance (> 10 kΩ) can result in non linear behavior due to its the drive parallel input resistors. High resistance values also reduce the signal to noise ratio, making it easier to have disturbances and reducing the quality of the measure.

However, a very small value of resistance may also consume too much power and cause self heating (which causes additional variations on resistance).

Therefore, use the smallest value of resistance that:

  • Does not exceed 1/2 of the potentiometer power rating (to allow safety margin and prevent self heating).
  • Does not exceed the +5V_OUT current capacity.

Typically 1 kΩ to 10 kΩ will be preferred.

DC tachometer

The Nix Servo Drive can use a DC tachometer for velocity feedback through the I/O connector. a DC tachometer provides an analog signal whose voltage level is proportional to the rotor speed.

Next figure illustrates how to connect a DC tachometer with differential output to the Nix Servo Drive.

NixTachometer  

Feedback wiring recommendations

Signal distortion and electrical noise is a common problem in feedback signals. These problems can result in a bad position or velocity calculation for both digital feedbacks (gain or loss of counts) and analog feedbacks (wrong voltage levels).To minimize these problems some wiring recommendations are shown:

  • Use differential signals whenever is possible. That is, connect both positive and negative signals of differential feedback sensors. Use a twisted pair for each differential group of signals and another twisted pair for the +5 V supply and GND. Twisted-pairs help in elimination of noise because disturbances induced in twisted pairs 
  • Twisted-pairs help in elimination of noise due to electromagnetic fields by twisting the two signal leads at regular intervals. Any induced disturbance in the wire will have the same magnitude and result in error cancellation.
  • Connect the Nix and encoder GND signals even if the encoder supply is not provided by the drive. 
  • Connection between Nix PE and the motor metallic housing is essential to provide a low impedance path and minimize noise coupling to the feedback. For further information, see Protective Earth wiring.
  • For better noise immunity, use shielded cables, with the shield connected to PE only in the drive side. Never use the shield as a conductor carrying a signal, for example as a ground line.
  • It is essential to keep feedback wiring as far as possible from motor, AC power and all other power wiring.

Recommendations for applications witch close feedback and motor lines

In some applications, like in the subsea market, where additional connectors and cables are a problem, the feedback cannot be wired separately from the motor and power lines. This creates noise problems that could result in hall sensors wrong commutation errors or encoder loss of counts. For these applications we recommend:

  • Use a common mode choke on the motor phases. This single action can reduce common mode noise drastically and will solve most problems. See recommended wiring in Motor and shunt braking resistor wiring
  • Ensure the motor housing is well connected to protective earth and the system chassis (PE).
  • If possible, minimize power supply voltage. This will also minimize the electromagnetic noise generated by the motor switching.
  • Add additional RC low pass filters on the feedback inputs. The filter should attenuate at a frequency above the maximum speed signal to prevent loss of counts and signal distortion. Preferably use resistors with low values to prevent distortion to the servo drive input circuit at low frequency (< 500 Ω). Use ceramic capacitors with good quality dielectric, like C0G. 

For further information contact Ingenia engineers for support.