PD4-E-M EtherCAT Online Manual

Control modes

General

The control mode of systems without feedback is called open loop, the mode with feedback is called closed loop. In the closed loop control mode, it is initially irrelevant whether the fed back signals come from the motor itself or from the influenced process.

For controllers with feedback, the measured control variable (actual value) is constantly compared with a set point (set value). In the event of deviations between these values, the controller readjusts according to the specified control parameters.

Pure controllers, on the other hand, have no feedback for the value that is to be regulated. The set point (set value) is only specified.

In addition to the physical feedback systems (e.g., via encoders or Hall sensors), model-based feedback systems, collectively referred to as sensorless systems, are also used. Both feedback systems can also be used in combination to further improve the control quality.

Summarized in the following are all possible combinations of control modes and feedback systems with respect to the motor technology. Support of the respective control mode and feedback is controller-specific and is described in chapters Pin assignment and Operating modes.


Control mode Stepper motor BLDC motor
Open Loop yes no
Closed Loop yes yes


Feedback Stepper motor BLDC motor
Hall no yes
Encoder yes yes
Sensorless yes yes

Various operating modes can be used depending on the control mode. The following list contains all the types of operation that are possible in the various control modes.


Operating mode Control mode
  Open Loop Closed Loop
Profile Position yes yes
Velocity yes yes
Profile Velocity yes yes
Profile Torque no1) yes
Homing yes2) yes
Interpolated Position Mode yes3) yes
Cyclic Synchronous Position yes3) yes
Cyclic Synchronous Velocity yes3) yes
Cyclic Synchronous Torque no1) yes
Clock-direction yes yes

1) The Profile Torque and Cyclic Synchronous Torque torque operating modes are not possible in the open loop control mode due to a lack of feedback.

2) Exception: Homing on block is not possible due to a lack of feedback.

3) Because ramps and speeds in operating modes Cyclic Synchronous Position and Cyclic Synchronous Velocity follow from the specified points of the master, it is not normally possible to preselect these parameters and to ascertain whether a step loss can be excluded. It is therefore not advisable to use these operating modes in combination with open loop control mode.

Open Loop

Introduction

Open loop mode is only used with stepper motors and is, by definition, a control mode without feedback. The field rotation in the stator is specified by the controller. The rotor directly follows the magnetic field rotation without step losses as long as no limit parameters, such as the maximum possible torque, are exceeded. Compared to closed loop, no complex internal control processes are needed in the controller. As a result, the requirements on the controller hardware and the controller logic are very low. Open loop mode is used primarily with price-sensitive applications and simple movement tasks.

Because, unlike closed loop, there is no feedback for the current rotor position, no conclusion can be drawn on the counter torque being applied to the output side of the motor shaft. To compensate for any torque fluctuations that arise on the output shaft of the motor, in open loop mode, the controller always supplies the maximum possible (e.g., specified by parameters) set current to the stator windings over the entire speed range. The high magnetic field strength thereby produced forces the rotor to assume the new steady state in a very short time. This torque is, however, opposite that of rotor's inertia. Under certain operating conditions, this combination is prone to resonances, comparable to a spring-mass system.

Commissioning

To use open loop mode, the following settings are necessary:

  • In object 2030h (Pole Pair Count), enter the number of pole pairs (see motor data sheet: for a stepper motor with 2 phases, a step angle of 1.8° corresponds to 50 pole pairs and 0.9° corresponds to 100 pole pairs).
  • In object 2031h (Max Current), enter the maximum current in mA (see motor data sheet).
  • In object 3202h (Motor Drive Submode Select), set bit 0 (CL/OL) to the value "0".
  • If the clock-direction mode is to be used, then observe chapter Clock-direction mode.

If necessary, current reduction on motor standstill should be activated to reduce the power loss and heat build-up. To activate current reduction, the following settings are necessary:

  • In object 3202h (Motor Drive Submode Select), set bit 3 (CurRed) to "1".
  • In object 2036h (Open Loop Current Reduction Idle Time), the time in milliseconds is specified that the motor must be at a standstill before current reduction is activated.
  • In object 2037h (Open Loop Current Reduction Value/factor), the root mean square is specified to which the rated current is to be reduced if current reduction is activated in open loop and the motor is at a standstill.

Optimizations

Depending on the system, resonances may occur in open loop mode; susceptibility to resonances is particularly high at low loads. Practical experience has shown that, depending on the application, various measures are effective for largely reducing resonances:

  • Reduce or increase current, see object 2031h (Max Current). Excessive torque reserve promotes resonances.
  • Reduce or increase the operating voltage, taking into account the product-specific ranges (with sufficient torque reserve). The permissible operating voltage range can be found in the product data sheet.
  • Optimize the control parameters of the current controller via objects 3210h:09h (I_P) and 3210h:0Ah (I_I).
  • Adjustments to the acceleration, deceleration and/or target speed depending on the selected control mode:
    Profile Position operating mode
    Objects 6083h (Profile Acceleration), 6084h (Profile Deceleration) and 6081h (Profile Velocity).
    Velocity operating mode
    Objects 6048h (Velocity Acceleration), 6049h (Velocity Deceleration) and 6042h (Target Velocity).
    Profile Velocity operating mode
    Objects 6083h (Profile Acceleration), 6084h (Profile Deceleration) and 6081h (Profile Velocity).
    Homing operating mode
    Objects 609Ah (Homing Acceleration), 6099h:01h (Speed During Search For Switch) and 6099h:02h (Speed During Search For Zero).
    Interpolated Position Mode operating mode
    The acceleration and deceleration ramps can be influenced with the higher-level controller.
    Cycle Synchronous Position operating mode
    The acceleration and deceleration ramps can be influenced via the external "position specification / time unit" targets.
    Cycle Synchronous Velocity operating mode
    The acceleration and deceleration ramps can be influenced via the external "position specification / time unit" targets.
    Clock-Direction operating mode
    Change of the step resolution via objects 2057h (Clock Direction Multiplier) and 2058h (Clock Direction Divider). Optimize acceleration / deceleration ramps by adjusting the pulse frequency to pass through the resonance range as quickly as possible.

Closed Loop

Introduction

The closed loop theory is based on the idea of a control loop. A disturbance acting on a system should be compensated for quickly and without lasting deviation to adjust the control variable back to the set point.

Closed loop using a speed control as an example:

PII = Proportional-integral current control loop
PIV = Proportional-integral velocity control loop
Iactual = Actual current
Vactual = Actual speed

The closed loop method is also referred to as "sine commutation via an encoder with field-oriented control". At the heart of closed loop technology is the performance-adjusted current control as well as the feedback of the actual values of the process. Using the encoder signals, the rotor orientation is recorded and sinusoidal phase currents generated in the motor windings. Vector control of the magnetic field ensures that the magnetic field of the stator is always perpendicular to that of the rotor and that the field strength corresponds precisely to the desired torque. The current thereby controlled in the windings provides a uniform motor force and results in an especially smooth-running motor that can be precisely regulated.

The feedback of the control variables necessary for closed loop mode can be realized with various technologies. In addition to the physical feedback with encoders or Hall sensors, it is also possible to virtually record the motor parameters through software-based model calculation. Physical variables, such as speed or back-EMF, can be reconstructed with the help of a so-called "observer" from the data of the current controller. With this sensorless technology, one has a "virtual rotary encoder", which – above a certain minimum speed – supplies the position and speed information with the same precision as a real optical or magnetic encoder.

All controllers from Nanotec that support closed loop mode implement a field oriented control with sine commutated current control. Thus, the stepper motors and BLDC motor are controlled in the same way as a servo motor. With closed loop mode, step angle errors can be compensated for during travel and load angle errors corrected within one full step.

Commissioning

An auto setup must be performed before using closed loop mode. The auto setup operating mode automatically determines the necessary parameters (e.g., motor data, feedback systems) that are necessary for optimum operation of the field oriented control. All information necessary for performing the auto setup can be found in chapter Auto setup.

With the Plug & Drive motors, it is not necessary to perform the auto setup, as this was already performed at the factory

Bit 0 in 3202h must be set .

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