Load Velocity in RPMLoads velocity for an axis or vector space in RPM.
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Velocity (500) is the double precision floating point velocity value in RPM. The Velocity range depends upon the motor counts or steps per revolution and the trajectory update rate. | ||||||
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Board ID is a unique number assigned by Measurement & Automation Explorer (MAX) used to send and receive commands and data to or from a specific NI motion controller. | ||||||
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Axis or Vector Space is the axis or vector space to control. | ||||||
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Inp Vect indicates the source of the data for this VI. Available input vectors include immediate (0xFF), variable (0x01 through 0x78), or indirect variable (0x81 through 0xF8). Refer to Input and Return Vectors for more detailed information. | ||||||
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error in (no error) describes error conditions that occur before this VI runs. The default input of this cluster is no error. If an error already occurred, this VI returns the value of error in in error out. The VI runs normally only if no incoming error exists. Otherwise, the VI passes the error in value to error out. The error in cluster contains the following parameters:
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Bd ID Out is provided for flow control. You can string together NI-Motion VIs by wiring the Bd ID Out terminal of one VI to the Board ID terminal of the next VI. | ||||||
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Resource Output is the Axis, Vector Space, ADC, or Encoder you wired into the VI. Use Resource Output to pass the resource to another VI and/or to display information about the device. | ||||||
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error out contains error information. If error in indicates an error, error out contains the same error information. Otherwise, it describes the error status that this VI produces.
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Using This VI
The Load Velocity in RPM VI specifies the maximum trajectory velocity for individual axes or vector spaces. When executed on a vector space, the value controls the vector velocity along the vector move path. For velocity control applications, the sign of the loaded velocity specifies the move direction. This VI requires previously loaded values of either counts per revolution (for servo axes) or steps per revolution (for stepper axes) to operate correctly.
RPM is double-buffered, so you can load it on the fly without affecting the move in process, and it takes effect on the next Start or Blend Motion call. After it is loaded, this parameter remains in effect for all subsequent motion profiles until re-loaded by this VI. You do not need to load velocity before each move unless you want to change the velocity.
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Note The velocity loaded with this VI is the maximum move velocity. Actual velocity attainable is determined by many factors including PID tuning, length of move, acceleration and deceleration values, and physical constraints of the amplifier/motor/mechanical system. You also can load velocity in counts/s or steps/s by calling the Load Velocity VI. |
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Tip Use Load Move Constraints instead of this VI for more move options. |
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Note Refer to VI Execution Times for benchmark timing information about your controller. |
Remarks
This section includes information about how the behavior of this VI differs among the controllers that support it.
NI 73xx Controller Considerations
The following list includes considerations you must make when you are using this VI with a 73xx motion controller:
- For 73xx motion controllers, RPM values stored in onboard variables are in double-precision IEEE format (f64). For information about the number of variables required to hold an RPM value, refer to Using Inputs and Return Vectors with Onboard Variables.
Note NI-Motion firmware uses a 16.16 fixed-point format internally to represent velocity in counts/sample period or steps/sample period and acceleration/deceleration in counts/sample period2 or steps/sample period2. If you change counts or steps per revolution or change the update rate using Enable Axes, you must reload the velocity and acceleration/deceleration. - Velocity values in RPM are converted to an internal 16.16 fixed-point format in units of counts (steps) per sample period (update period) before being used by the trajectory generator. NI-Motion can control velocity to 1/65,536 of a count or step per sample. You can calculate this minimum velocity increment in RPM with the following formula:
minimum RPM = Vmin × (
) × 60 × (
)where:
Vmin is 1/65,536 count/sample or step/sample,
Ts is the sample period in seconds per sample,
60 is the number of seconds in a minute, and
R is the counts/steps per revolution.
For a typical servo axis with 2,000 counts per revolution operating at the 250 µs update rate, the minimum RPM increment is:
× 4,000 × 
= 0.00183105 RPM
You can calculate the maximum velocity in RPM with the following equation:
maximum RPM = Vmax × 60 ×
where:
Vmax = 20 MHz for servos,
8 MHz for steppers on a 7350 controller,
4 MHz for steppers on a 7330/40/90 controller, and
R is the counts/steps per revolution,
and is constrained by acceleration/deceleration according to the following equation:velocity
(65536 × deceleration) – accelerationwhere velocity is in counts/sample and acceleration and deceleration are in counts/sample2.
From the example, the maximum RPM is:
(20 × 106) ×
= 600,000 RPM
- Use Load Velocity and Load Acceleration/Deceleration for faster performance.
NI SoftMotion Controller Considerations
The following list includes considerations you must make when you are using this VI with the NI SoftMotion Controller:
- Use Load Move Constraints to load the velocity in user units/second.
- The NI SoftMotion Controller does not convert velocity values in RPM to a 16.16 fixed point number. All calculations performed by the NI SoftMotion Controller maintain f64 precision.
- For the Inp Vect parameter, the NI SoftMotion Controller supports only the immediate return vector (0XFF).