Units

Acyclic Units (Except position)

All parameters, except for position values that are accessed via PNU read/write record requests, are presented in units according to the associated AXIS#.UNIT.* keywords and match associated keyword values provided in WorkBench. Position values are scaled using DS402 type units to provide the required precision in 32-bits that floats cannot provide.

Cyclic Velocity Units

All cyclic velocities are normalized per PROFIdrivespec N2 (NSOLL_A/NIST_A) or N4 (NSOLL_B/NIST_B/MDI_VEL) data normalization. These values are scaled as a percent of a reference value and are either 2 bytes (N2) or 4 bytes (N4). For N2 values, 100% of the reference velocity corresponds to 2^14 = 0x4000 and for N4 values, 100% corresponds to 2^30 = 0x40000000. For velocities, the reference velocity is stored at p60000 and is 3k rpm by default but is writable to allow for different resolutions.

Range and resolution using 3k rpm default:

Type Resolution Range
N2

5.46 RPM per count

((1/3k)*2^14)

 +/- 6k RPM
N4

0.01 RPM per count

((1/3k)*2^30)

 +/- 6k RPM

Example

p60000 = 3k rpm (default)

Desired RPM is 1k rpm

Cyclic Acceleration Units

Cyclic acceleration signals MDI_ACC and MDI_DEC use unsigned ProfiDrive N2 normalization similar to velocity values. The reference acceleration is P5208 and is defaulted to 65,536 RPM/s.

Range and resolution using 65,536 RPM/s default reference accel:

Type Resolution Range
N2

0.25 RPM/s per count

(1/65536)*2^14

 0.25 RPM/s to 262,144 rpm/s

Example

P5208 = 65,536 rpm/s (default)

If desired accel is 10k rpm/s

Cyclic Current Units

Cyclic current signals use signed N2 units similar to acceleration and velocity values. The reference current is P5223 and matches AXIS#.IPEAK in Amps by default.


  • IPEAK changes to match the drive model in use (i.e., a 3 Amp AKD2G will have IPEAK of 9 Amps)

Example

Using a 3 Amp AKD2G:

P5223 = 9.000 amps (default AXIS#.IPEAK for 3A AKD2G)

If desired current is 500 milliamps:

Position Units

The signals G1_XIST1 and G1_XIST2 are signed 32-bit position values. The resolution for these signals is fixed at 2^20 counts per revolution.

All other position values, including MDI_TARPOS, are signed 32-bit position values that are scaled according to DS402 units. These are described in examples below.

General Formulas

Given Convert to
Motor/Drive required value (physical units) Command value in master (fieldbus units)
Feedback value in master (fieldbus units) Motor/Drive actual value (physical units)

 

Setpoint: Calculate the position command in the master given a required motor position setpoint

 

Actual Value: Calculate the motor feedback position given a feedback position value in the master

Feed Ratio

The Feed Ratio sets the position resolution in the fieldbus. The feed specifies the number of counts and the driving shaft revolutions specifies the number of revolutions. The units are counts/revolution. The Feed Ratio can be any arbitrary value. It does not need to be associated with any physical units or scaling. The master controller can be programmed in 20 bits per revolution = 1048576 counts/revolution or 10000 counts/revolution, etc. It is purely arbitrary.

 

Encoder Resolution

Position encoder resolution used in these calculations is always 2^32 counts/revolution for the internal AKD2G position values regardless of the actual feedback resolution.

Gear Ratio

The Gear Ratio is a position ratio that can be used to scale the fieldbus position resolution. It can be used to account for a physical gear ratio but it is not necessarily related to physical mechanics. It can be arbitrary. It can be set to any value regardless of the system mechanics.

Complete Formulas

Convert to fieldbus units

fieldbus position

 

Convert to physical units
physical Position

Real-World Units

Fieldbus position is always in units of counts. All position values in the formulas must be in units of counts converting between fieldbus position (counts) and physical position (counts).

A physical position scale of 2^32 physical counts/revolution is used in the conversion between counts and real-world units.

Physical position can be converted to/from any physical units including:

  • inches
  • mm
  • degrees
Units Conversion from real-world units to counts for physical position “X”
inches
mm
degrees

Example 1

  • Desired Target Position = 180 degrees
  • Fieldbus Position units in counts based on 20 bits per rev = 1048576 counts/rev

Values assumed:

PNU Drive Parameter Value
P5204 AXIS#.PROFINET.FEEDRATIO.FEED 20 bits per rev = 1048576 counts/revolution
P5205 AXIS#.PROFINET.FEEDRATIO.SHAFTREV 1
P5206 AXIS#.PROFINET.GEARRATIO.MOTORREV 1
P5207 AXIS#.PROFINET.GEARRATIO.SHAFTREV 1

 

Formula for physical position in degrees; feed constant = 20 bits per rev = 1048576 counts/rev

Derivation:

Convert position from desired real-world units to counts based on 2^32 physical counts per revolution.

Convert from physical units to fieldbus units.

Substituting physical position (counts):

This means a position command of 32768 sent over the fieldbus will command a target position of 180 degrees.

Example 2

  • Fieldbus Position units in counts equivalent to hundredths of a degree (36,000 increments per revolution)
  • Desired Target Position = 180 degrees

Values assumed:

Object Drive Parameter Value
P5204 AXIS#.PROFINET.FEEDRATIO.FEED 36000
P5205 AXIS#.PROFINET.FEEDRATIO.SHAFTREV 1
P5206 AXIS#.PROFINET.GEARRATIO.MOTORREV 1
P5207 AXIS#.PROFINET.GEARRATIO.SHAFTREV 1

 

Formula for physical position in degrees; feed constant = 36000 counts/rev

Derivation:

Convert position from desired real-world units to counts based on 2^32 physical counts per revolution.

Convert from physical units to fieldbus units.

Example 3

  • Ballscrew lead = 2 mm/revolution
  • Gearbox ratio = 5:1
  • Desired Target Position = 25000 microns
  • Fieldbus Position units in counts equivalent to microns

Values assumed:

Object Drive Parameter Value
P5204 AXIS#.PROFINET.FEEDRATIO.FEED 2000
P5205 AXIS#.PROFINET.FEEDRATIO.SHAFTREV 1
P5206 AXIS#.PROFINET.GEARRATIO.MOTORREV 5
P5207 AXIS#.PROFINET.GEARRATIO.SHAFTREV 1

 

Formula for physical position in microns; feed constant = 2000 counts/rev; 5:1 gear ratio

Derivation:

Convert position from desired real-world units to counts based on 2^32 physical counts per revolution.

Convert from physical units to fieldbus units.