The PVDC logic block provides a positional percent as an output to an actuator based on a desired flow setpoint, a measured flow, and a current position controlling the flow. This block uses a fixed gain, proportional-only feedback design for systems with constant-rate actuators. The PVDC has the integrating effect that incremental motor actuators use to provide stable operation given the nonlinear flow through a damper or a valve.
Set the PVDC configuration parameters to reflect the characteristics of the environment. The output is sent to the object that controls the actuator, typically a Position Adjust Output (PAO) object. Configure the PAO object consistently with the PVDC block.
See the Damper Control (VAV Supply) v51 section for examples of how this block is used.
- Reliable: Flow input is Reliable and no error in PVDC algorithm.
- Unreliable: Flow input is Unreliable at the start of PVDC algorithm.
- Input Out of Range: One or more of the Inputs failed its validation checks.
- Math Overflow: Math overflow during PVDC algorithm.
- Math Underflow: Math underflow during PVDC algorithm.
- Divide by Zero: An attempt to divide by zero in PVDC algorithm.
- Math Error: Other Math error.
If the Flow is unreliable at the time the PVDC attempts to evaluate its logic, that execution of the PVDC is skipped and the outputs held at their last reliable values. The outputs retain their previous value on internal error. The Reliability of each output of the primitive is set to the Reliability of the primitive.
The following table describes the inputs and outputs of the PVDC logic block.
|
Name |
Data Type |
Exposed for Connection by Default |
Description |
Default Value |
|---|---|---|---|---|
|
Inputs |
||||
|
Effective Stroke Time |
Float |
No |
Indicates the effective time the actuator requires to drive the damper from 0 to 100%. For proper operation, this input must be configured based on the actual time it takes the actuator to drive the damper. Depending on the range of motion for a given VAV box, this value may be a fraction of the full actuator stroke time (for example, 1/2 of the 90-degree stroke time for a 45-degree damper). |
60 Seconds |
|
Flow |
Float |
Yes |
Indicates the measured volumetric flow, or the difference between two volumetric flow measurements (for example, the exhaust flow and the supply flow). |
0.0 |
|
Flow Setpoint |
Float |
No |
Indicates the desired amount of flow or the desired flow difference requested of the controller. |
0.0 |
|
Maximum Flow |
Float |
No |
Indicates the maximum flow that can be delivered by the box. This value defaults to the maximum flow setpoint. However, this value can be set manually in cases where the VAV box serving the space is oversized. You may need to break the default maximum cfm flow setpoint connection and set this value manually to the actual manufacturer-rated volume flow rate at a nominal static pressure (for example, 1 in.w.g. or 250 Pa). |
0.0 |
|
Min Pulse Width |
Float |
No |
Indicates the minimum pulse allowed for the motor actuator. |
0.25 Seconds |
|
Smooth Factor |
Float |
No |
Indicates the internal filter weight used for estimating the standard deviation of the flow error. |
0.015 Range: Greater than 0 and less than 1 |
|
Startup Value |
Float |
No |
Indicates the startup value of PVDC output. This value is used when startup or restart is the value of the PVDC Present Value. |
0.0% |
|
Outputs |
||||
|
Absolute Effort EWMA |
Float |
No |
Provides an EWMA value for the absolute value of the controller effort. |
0.0% |
|
Absolute Error EWMA |
Float |
No |
Provides an EWMA value for the absolute value of the flow error (flow setpoint - flow measurement). |
0.0 |
|
Error EWMA |
Float |
No |
Indicates the EWMA value of the flow error (flow setpoint - flow measurement). |
0.0 |
|
Present Value 1 |
Float |
Yes |
Indicates the position, in percentage of maximum output, being requested by the PVDC Controller of the motor actuator. The Present Value is limited from 0 to 100%. When the calculation results in a value beyond a limit, the Present Value should be set to that limit. |
0.0% |
|
Present Value EWMA |
Float |
No |
Indicates the EWMA value of the Present Value. |
0.0% |
|
Saturation Status |
Enum |
Yes |
Indicates the current operational status of the PVDC block. This value is based on the output and the amount of time that the output is at the high or low limits (100% or 0%, respectively). The Saturation Status is set to Normal the first execution after the PVDC is Enabled (or Re-Enabled). The Saturation Status is Normal for at least one execution after returning from a non-Normal state. For example, if the Saturation Status is Timing High and the Present Value becomes equal to the Low Limit, the Saturation Status is Normal for one execution before transitioning to Timing Low at the next execution. Note: The Enumeration Set for Saturation Status contains the Overridden state, but the PVDC does not set this attribute to that value. The Overridden state is used by other applications.
|
Normal |
|
Properties |
||||
|
Actual Period 2 |
Float |
N/A |
Indicates the actual instantaneous period of execution, measured as the actual time between successive periodic executions of the PVDC algorithm. This value is not computed after the first periodic execution (startup, restart, or enable after disable) or when the block executes for any other reason (for example, when an input changes value). |
1 Second |
|
Dynamic Deadband2 |
Float |
N/A |
Indicates the smoothed Deadzone value. |
0.0 |
|
Interval |
Number |
N/A |
Indicates the execution period of PVDC block. |
1 Second |
|
Noise Estimate2 |
Float |
N/A |
Indicates the smoothed standard deviation of the measured flow. |
0.0 |