MSC 01 through MSC 12 - Johnson Controls - Metasys - LIT-12011147 - Software Application - Controller Configuration Tool - 13.1

The MSC module provides sequencing operations for multi-stage systems (typically smaller tonnage rooftop multi-stage DX systems, heat pumps, and electric heat). This module uses relative capacities to determine which outputs to turn on or pulse (cycle a device on and off while respecting the Min On Time and Min Off Time) to meet a specific load requirement. The method is based on the split-range control concept and it allows multiple stages to be controlled by a single feedback controller, such as a PID. The module uses a patented PMAC.

Disabling this module is the same as setting the Instant Shutdown to True.

The Multistage Controller module establishes a means of activating 1 to 12 individual devices. The Multistage Controller algorithm performs the following main functions:

• Use to set up the number and capacities of configured devices. See Number of Devices and Device Capacity for details.

• Determines the appropriate device combination automatically. This module determines the switch points between stage combinations using the relative capacities, the capacity requirement, and the Staging Hysteresis. You do not need to provide make/break limits. See Determining Device Combinations for details.

• Provides a vernier output representing the difference between required capacity and actual capacity. The PMAC algorithm uses the vernier output to provide behavior that is similar to proportional operation between the current capacity and the next larger capacity. The module provides an output to indicate which device is being pulsed at the current capacity. See Vernier Output and Pulsed Device for details.

• Provides equipment protection by enforcing minimum on and off times. See Minimum On and Minimum Off for details.

  Note: You can bypass this protection by interacting directly with the Binary Output objects.

• Allows instant deactivation of all devices based on an Instant Shutdown input or Instant Shutdown command. See Instant Shutdown for details.

• Provides intelligent rotation of the start and stop order of the devices based on the runtime and the device status. The module also allows the user to force a rotation to occur immediately. See Device Runtime for details on lead rotation.

• Provides the relationship between devices (for example, runtime or start order) using Runtime. Devices with a lower runtime value start before devices with the same capacity but a higher runtime value, and devices with a higher runtime value are deactivated before devices with the same capacity but a lower runtime value.

• Indicates whether the device is enabled or disabled.

• Provides intelligent actions when a device is enabled or disabled. See Device Enable/Disable for details.

• Use to set up device groupings so that the first device always starts before the remaining devices in the group. You can set the action of the dependent devices to be either normal or inverse. See Device Grouping for details.

Number of Devices and Device Capacity

Select the MSC with the appropriate number of devices (for example, MSC 02, MSC 05, and so on). The relative capacity for each device is entered into the Device Capacity array.

Determining Device Combinations

This module creates and maintains a list of capacity combinations based on Device Capacity of all enabled devices. For example, four devices with the capacities 5, 5, 10, 10 can achieve the following capacities when they are all enabled: 0, 5, 10, 15, 20, 25, 30. The Input indicates a percentage of the total enabled capacity that is required. The module selects a new combination to use when either the required capacity exceeds the next larger capacity by the Deadzone divided by two or when the required capacity is less than the current operating capacity by the Deadzone divided by two.

The Multistage Controller uses the following criteria to narrow the possible combinations sequentially until a single combination is found:

• Selects the possible combinations of Device X Out that provides the capacity that is closest to but does not exceed the required capacity.

• Selects the combinations that have a pulseable device with the smallest capacity that is off.

• Finds the combinations that produce the least number of changes in Device X Out compared to the current status of Device X Out.

• Finds the combination with a Device X Out that has the lowest runtime of the devices that are on. See Device Runtime for an example of this selection process.

If multiple combinations still exist, find the combination with a Device X Out that has the second lowest runtime of the devices that are on. See Device Runtime for an example of this selection process.

If multiple combinations still exist, continue to test subsequent runtimes until a combination is found.

If all devices that are on have been compared with respect to runtime (the runtimes matched at every comparison), and one combination has fewer devices that are on than the other, choose the combination with fewest devices that are on.

If all devices that are on have been compared with respect to runtime and a single combination was not found, select the combination with the lowest number of Device X Out. See Device Runtime for an example of this selection process.

If Rotate Now transitions from False to True or a Rotate Now Command is issued, the reevaluation for the new combination occurs immediately with the selection criterion 4, 5, 6 (lowest runtimes) taking precedence over the selection criteria 3 (lowest number of changes).

For example, 4, 5, 6 takes precedence over 3 in case of Rotate Now. Otherwise, the normal sequence 1, 2, 3, 4, or 8 of selection criterion is applicable for finding the best combination out of the available choices.

Vernier Output and Pulsed Device

The vernier output is an indication of the proportion of the required capacity that is not met by the current output combination. This output takes into account the Deadzone. If there is only one device, the deadzone used to calculate the vernier is set to zero.

The following table shows the associated staging table that produces the vernier signal in the following figure. The different vernier output results when the combination of devices pulse a larger device to get to the next stage level. The first vernier is what would be produced while pulsing the 5-ton device. The second vernier would be produced when pulsing the 10-ton device.

The example shown in the previous table has three devices with the capacities 5 tons, 10 tons, and 15 tons. Only the first (5 ton) and second (10 ton) devices are pulseable. The first stage pulses the first device as the vernier signal goes from 0 to 100%. To go from the capacity of 5 tons to the capacity of 10 tons, the second device is pulsed while the first device is on continuously. When the capacity reaches 10 tons, the first device is pulsed while the second device now runs continuously.

If the current combination is the second-to-last combination from the table and the input reaches 100%, the last stage is not chosen if a pulsable device exists in the second-to-last-combination. But, the pulsable device (in the second-to-last combination) is controlled by PMAC. PMAC receives 100% from the vernier (in effect giving full 100% capacity) and the second-to-last combination is maintained. If the current combination is not the second to last combination and the input goes to 100%, the last stage combination (all 1s or On's) is chosen immediately, checking for existing timers first, by virtue of the Deadzone divided by two rule.

Minimum On and Minimum Off

The outputs are all subject to minimum on and minimum off timers. The Multistage Controller maintains individual timers for each device. These timers protect equipment from excessive wear due to short cycling. A timer is cleared and an appropriate timer is started when a Device X Out changes status.

If a minimum on or minimum off timer is active for any device that is changing state when a combination change is required, the change in combination delays until all timers associated with the changing devices have been met except for the following special conditions:

• Instant Shutdown is True

• Rotate Now changes from False to True

• Device Pulseable changes

• Device First Of changes

When a device is activated, it remains active for at least the Min On Time. This scenario occurs even if the input to the Multistage Controller indicates that it should stage. If a device that needs to be deactivated has an active minimum on timer, the Multistage Controller waits for that timer to expire before changing the state of any device. Commanding the module to Instant Shutdown or setting the Instant Shutdown input to True causes the Min On Time to be ignored and all devices are immediately deactivated. In addition, when a particular Device X Enable transitions to False, the corresponding Device X Out is set off ignoring the Min On Time.

When a device is deactivated, it remains inactive for the Min Off Time. This occurs even if the Multistage Controller indicates that it should stage. If a device that needs to be activated has an active minimum off timer, the Multistage Controller waits for that timer to expire before changing the state of any device. At startup or enable, the Multistage Controller does not consider minimum off timers (when disabled, all timers are canceled.) If the Instant Shutdown input is True, the minimum off timers are maintained for all outputs. If a device transitions to off due to the Instant Shutdown input, a minimum off timer is started for that device. If Instant Shutdown goes False and a device has its minimum off timer active, the Multistage Controller waits for that timer to expire before staging up and activating that device. If a device is disabled, a minimum off timer is started for that device if it turns off when it is disabled.

If the module is commanded to Rotate Now or the Rotate Now input transitions from False to True, the module evaluates and implements the appropriate device combination immediately, ignoring the Minimum On/Off timers.

If any Device Pulseable or Device First changes, the Multistage Controller immediately reevaluates the required outputs and changes the state of the outputs ignoring the minimum on and minimum off timers.

The Multistage Controller does not provide inter-stage timing. You can use a Sequencer module for inter-stage timing. As indicated in Determining Device Combinations above, the Staging Hysteresis provides protection from rapid cycling between adjacent stages.

In the past, PID controllers (set for proportional only) provided a signal to a stage sequencer to control the process variable. You manually set the sequencer’s make and break limits for each stage. Typically, large load disturbances or setpoint changes caused the system to overreact. To prevent using more energy than required and major overshoots of the process variable, inter-stage delays kept the system from staging too quickly. This method created sluggish response to large disturbances, although it worked for smaller disturbances.

The MSC with a self-tuning PID and the MSC Pre-Processor provide proper control for the current load. If a load change occurs, the MSC has an appropriate response in the outputs without excessive overshoot. Because of this operation, the MSC does not require Inter-Stage Timers.

Starts-per-hour is another method to make sure that the staged equipment does not cycle too frequently. The MSC provides the same functionality by selecting appropriate Minimum On Time and Minimum Off Time.

Instant Shutdown

When the Instant Shutdown input is True or the Instant Shutdown command is received, all of the devices are turned off ignoring the Min On Time. The vernier signal sent to the PMAC algorithm is set to zero and the PMAC algorithm is re-initialized. If the Instant Shutdown input is True, it must be set false for the Multistage Controller to control the devices. If an Instant Shutdown command is received, a Release Shutdown command must be received for the Multistage Controller to control the devices.

Device Runtime

Each Device X Out has an associated Device X Runtime. As described in the Determining Device Combinations section, this value is used to make a selection when more than one combination of devices can meet the required capacity.

The module always looks for the combination that has the device with the lowest runtime value.

For example, consider four devices with the relative capacities of 5, 5, 10, 10 and with runtimes of 1, 2, 3, 4. Assume that currently the Device X Out combination is 1, 1, 0, 0, which is a capacity of 10 and the input has changed to a value that requires a capacity of 15.

The combinations shown in row 1, 2, 3, 4 are evaluated. The search through the possible combinations follows a binary order, so combination 1 (0, 1, 0, 1) is checked and accepted with two changes from the current outputs. Then combination 2 (0, 1, 1, 0) is evaluated. Combination 2 also has two changes and it has the same runtime. The devices with the next Lowest Runtime are compared. The next lowest combination has the Second Lowest runtime and is selected over the first combination. Combination 3 (1, 0, 0, 1) also has the same number of changes but it has the Lowest Runtime device. Combination 3 is the best candidate at this point. Combination 4 (1, 0, 1, 0) is then evaluated. It has the same number of changes and the same Lowest Runtime device. The next Lowest Runtime devices is compared. It has the lowest Second Lowest Runtime and is selected as the current combination.

Another example is four devices with the relative capacities of 5, 5, 10, 10 and with runtimes of 0, 0, 0, 0. Assume that currently the Device X Out combination is 1, 1, 0, 0, which is a capacity of 10 and the input has changed to a value that requires a capacity of 15.

The combinations shown in row 1, 2, 3, 4 would be evaluated. The search through the possible combinations follows a binary order, so combination 1 (0, 1, 0, 1) would be checked and accepted with two changes from the current outputs. Then combination 2 (0, 1, 1, 0) would be evaluated. It also has two changes and the same matching runtime. It would be selected over the first combination because it has Device 3 on while the first row has Device 4 on. Combination 3 (1, 0, 0, 1) also has two changes, and it has the same runtime for all inputs that are on. It would be selected over the currently selected combination because it has Device 1 on while the other combination has Device 2 on. The final combination (1, 0, 1, 0) is then evaluated. It has the same number of changes and the same runtimes. They both have Device 1 on, but this combination has Device 3 on while the previously selected combination has Device 4 on. It would have the second lowest Device number and thus selected as the current combination.

Device Enable/Disable

When a Device X Enable changes state to False, the associated Device X Out is set to off, and the module determines a new combination appropriate to the required capacity as indicated by the Input. A minimum off timer is started for the disabled device. When a Device X Enable changes state to True, the module determines a new combination appropriate for the required capacity indicated by the Input. If a minimum off timer is active for the device, it must expire before the device is started.

Device Grouping

In some instances, devices are grouped together. This grouping is used when a device must be turned on before the module turns on any subsequent devices. The user sets up the group by indicating that this device is the first of x devices. The module then knows that this output must be on before it can turn on any of the subsequent devices.

An example is a compressor with two unloaders. Device 1 First Of is set to three. Any combinations that require Device 2 Out or Device 3 Out, must also have Device 1 Out on also. The available combinations for this compressor are: (0, 0, 0), (1, 0, 0), (1, 0, 1), (1, 1, 0) and (1, 1, 1). If the user prefers to always have Device 2 Out on before Device 3 Out, the user must make sure that Device 2 Runtime is equal to or less than Device 3 Runtime assuming that Device 2 and Device 3 are the same capacity.

When devices are grouped, they can also support inverting. This means that when the first device turns on, all of the devices turn on (the dependent devices do not begin tracking a minimum on timer). Then as the capacity requirement increases, the subsequent devices are turned off to increase the capacity. If the first device turns off, all of the devices are turned off.

PMAC Operation

The PMAC algorithm executes every PMAC Period (typically, the Time Constant input of the MSC Pre-Processor divided by 180 with a minimum value of 1 second). The PMAC algorithm determines the on time and off time of a device to keep the process variable within the control band while minimizing the number of times that the staged device turns on and off. The Multistage Controller operates similar to the Duration Adjust Output. Duration Adjust has a fixed cycle time and varies the on and off times based on the vernier signal. PMAC varies the on and off times as well as the cycle time. It generates a pulse train based on the vernier control signal. A pulse train is comprised of a sequence of on and off states. See the following figure for an example of a pulse. A cycle period is the time between two off to on transitions. PMAC varies both the on-time, off-time, and cycle period to constrain the variation in the controlled variable to be within the Control Band (as much as possible) while minimizing the number of cycles.

Re-initialization of the PMAC Algorithm

The PMAC algorithm is reinitialized and calculates a new pulse train when the following occur:

• A new Pulsed Device is selected.

• The PMAC Period changes.

The pulse train starts in an on state if the Vernier is greater than zero when re-initialization occurs. If the Vernier is zero, the pulse train starts in an off state.

Percent Request Output

The Percent Request output is connected to the fan determination module in the standard applications for terminal units. The Percent Request output has two modes of operation. Setting the Percent Request Mode input chooses the mode of operation.

Percent Request Mode = Demand

The Percent Request output is set to the value of the Input.

Percent Request Mode = With Output

When the Percent Request Mode is With Output, the Percent Request is set to zero if all of the device outputs are off (even if the input is non-zero). When one or more device outputs are on, the Percent Request output is set to the maximum of the Input or Min Percent Request. When connected with the fan determination module, this connection allows the fan to be turned on only when the pulsed heating or cooling devices are on (and turned off when the first device cycles off).

Merging the PMAC Pulse Train

The Multistage Controller merges the pulse train from the PMAC algorithm with the selected output states from the Multistage algorithm, and then these states are passed to the appropriate Device X Out. Changing the state of all Device X Out is subject to Minimum On and Minimum Off timers. See Minimum On and Minimum Off for details on Minimum On and Minimum Off timing.

You cannot view or modify the modules in this group’s logic.