Signal path of motor drive

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In Granite Devices drives the torque, velocity and position limits and setpoints are defined as integer numbers. The vales are represented "hardware" scale which are described below.

Setpoint signal path

Setpoint signal path converts user setpoint to internal setpoint.

Setpoint signal path

Main parts are:

  • Input multiplier. Purpose of this is to increase resolution of input setpoint to allow more fine grained velocity & acceleration control in trajectory planner. By default Setpoint multiplierMUL value is 50.
  • Setpoint smoothing filter. If enabled, applies low pass filter to signal reducing jitter and roughness of signal but also introduces about some delay. By default the filter has 100% attenuation at 250Hz.
  • Trajectory planner. This limits rate of change of setpoint signal based on Velocity limitCVL and Acceleration limitCAL parameters. Output rate maximum rate of change:
    • Velocity changes max Acceleration limitCAL nubmer of units per control cycle (control cycle is 400µs in most GD drives)
    • Velocity maximum value is limited to Velocity limitCVL
  • Input divider. This divides setpoint signal by Setpoint dividerDIV to give desired output scale for internal setpoint. Combination of multiplier and divider can be used change total scaling of setpoint signal.


Driveblockdiagram setpoint v1.png

Setpoint source scales

Different setpoint sources have different range and scale:

Setpoint source Type Range Scale
Pulse & directon Incremental Infinite
  • Position and torque mode: one pulse changes setpoint by one
  • Velocity mode: pulse frequency input, setpoint = number of pulses per control cycle
Quadrature Incremental Infinite
  • Position and torque mode: one edge of either channel changes setpoint by one
  • Velocity mode: frequency input, setpoint = number of edges of either channel per control cycle
PWM Absolute Full input scale equals setpoint range of +/-16384. In loss of PWM signal, setpoint is 0.
  • Direct 1:1 absolute value in position mode
  • Velocity & torque mode: +/-16834 represents full torque or speed scale
Analog Full input scale equals setpoint range of +/-16384
Serial (SimpleMotion V2) Absolute & incremental Infinite

It should be noted that trajectory planner operates after multiplier meaning that Velocity limitCVL velocity limit value is not in equal scale with velocity setpoint value.

Internal setpoint

Internal setpoint is a predefined setpoint scale inside the drive. The scale of internal setpoint signals are:

  • Position mode: position sensor counter raw value
  • Velocity mode: internal goes through Velocity normalized that changes scales depending on setpoint source:
    • In PWM & Analog source: Internal setpoint of +/-16384 represents whole speed range covered by Velocity limitCVL parameter. I.e. 10V input to analog input runs motor at 100% speed and -5V at -50% etc.
    • In all other sources: number of feedback device counts per one control cycle. Obtained by calculating the difference of position feedback values at every control cycle.
  • Torque mode: Torque normalizer scales internal setpoint so that value of +/-16384 represents full torque scale (i.e. internal setpoint value 16384 outputs configured peak current Peak current limitMMC and 8192 outputs Peak current limitMMC/2)

Controller

The default controller type of GD drives is cascaded type where each controlled variable has it's own PI or P controller. In position mode such structure is called as PIV controller.

The block diagram below represents simplified structure of GD drives.


Driveblockdiagram controller.png



Calculation formulas and examples

These examples focus on calculating values on a rotary motor and linear axis.

Constants used later in calculations

Assuming control cycle to be 400µs / 2500 Hz (default in GD drives):

$ f=2500 $

Calculation of how many counts the feedback device produces per one physical unit:

$ X_{FeedbackDeviceCountsPerUnit}=4\frac{FBR}{AXS} $

AXS is a number that tells how many physical lenght units (such as millimeters a linear axis) translates per one rotary motor revolution.

Example 1 - Calculating setpoint in position mode

Here we convert physical units (such as millimeters) to setpoint value in position control mode:

$ setpoint=D_{DesiredPosition}\frac{MUL}{DIV}X_{FeedbackDeviceCountsPerUnit} $

Example 2 - Calculating value for CVL parameter

Here we convert speed (such as mm/sec, or whatever lenght units AXS represents) to Velocity limitCVL value:

$ P_{CVL}=\frac{V_{DesiredSpeedLimit}*X_{FeedbackDeviceCountsPerUnit}*P_{DIV}}{f} $

Example 3 - Calculating value for CAL parameter

Here we convert speed (such as mm/sec2, or whatever lenght units AXS represents) to Acceleration limitCAL value:

$ P_{CAL}=\frac{A_{DesiredAccelerationLimit}*X_{FeedbackDeviceCountsPerUnit}*P_{DIV}}{f^2} $

In alternative method we don't need acceleration value, but just time $ t $ in seconds to define how long motor should take to accelerate from zero speed to maximum speed defined by CAL:

$ P_{CAL}=\frac{P_{CVL}}{tf} $

Example 4 - alternative calculation methods

Here we use do practical calculation example for CVL and CAL values on a spinning motor. AXS is 1 and unit is revolutions.

  • Motor has 2500 P/R encoder (10000 CPR) and user wants to rotate it at 1000 rpm in velocity mode. Scaling is set to 1:1 (MUL/DIV ratio is 1)
  • Rotation speed is 1000/60 = 16.667 revs/s (RPS) Needed encoder count frequency is 16.667*10000 = 16667 counts/s.
  • Internal setpoint is the amount of counts per control cycle so in this case it's 16667/2500 = 66.667

Because set point is integer value user must round setpoint to 66 or 67. To achieve exact speed, he could change input scaling (MUL/DIV) to allow unrounded value here.

Calculating acceleration and velocity limit parameters

If DIV is 50, then user must have Velocity limitCVL value at least 3333 (66.6667*50).

If user wants motor to accelerate to 1000 rpm in 0.1 seconds, then Acceleration limitCAL is value should be 3333/2500/0.1 = 13.33. Value must be again rounded to integer and rounding error could be reduced by adjusting scaling (adjust DIV).

Granity calculates real world units thus it can be used to calculate and experiment with the scales. As scales are linear, interpolation of values is viable choice.

See also


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