TECHNICAL BULLETIN. Direct Impulse Control for Einstein RX

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1 P/N: Date of Release: 7/23/99 TECHNICAL BULLETIN Direct Impulse Control for Einstein RX Scope To explain setup and operation of the Direct Impulse feature of the Einstein RX series of CPC controllers. Overview Direct Impulse control is a form of refrigerated case control that uses a pulsemodulated valve to maintain a programmed superheat across an entire circuit. The pulse valve is opened and closed at a varying rate in an attempt to keep the difference between the saturation suction temperature and the circuit suction temperature at a user-defined temperature. This is meant to prevent liquid from re-entering the compressor rack while maintaining efficient refrigeration within the circuit. Valve Control The Direct Impulse application uses PID control to change the rate of aperture for the pulse valve. PID control compares the superheat measurement to the superheat set point, yielding a valve opening percentage. The control algorithm is designed to keep the pulse valve operating at about a 50% aperture when the superheat is equal to the superheat set point. The valve will open to a higher percentage when the superheat is above the set point, and it will open to a lower percentage when the superheat is below the set point. The size of the valve s reaction to a change in superheat is determined by the throttling range. Throttling range is the range of superheat values across which the valve will travel from 0% to 100%. The superheat set point in Direct Impulse control is always in the middle of the throttling range, which represents a valve opening of 50% when the superheat is at the set point. Small throttling ranges result in greater reactions to changes in superheat, and large throttling ranges result in smaller reactions.

2 For example, in a Direct Impulse application with a superheat set point of 25 and a throttling range of 10, the throttling range extends from 20 (at which the valve will be near 0% open) to 30 (at which point the valve will be nearly 100% open). As the superheat varies within the range of 20-30, the valve will vary its aperture to attempt to keep superheat as near the set point as possible. How Direct Impulse Interacts with Other Einstein Applications Direct Impulse relies on other applications to provide both necessary input values and commands to activate or deactivate refrigeration in the circuit. Direct Impulse calculates superheat by taking the saturation suction temperature of the circuit s suction group and subtracting the circuit suction temperature. Saturation suction temperature is a value that is calculated by the Suction Group application that controls the compressor rack (based on the current suction pressure and the thermal properties of the refrigerant being used). Therefore, Direct Impulse applications rely directly on Suction Group applications to provide it a real-time saturation suction temperature value. A Direct Impulse application also relies directly on a Standard Circuit application to tell it when it is time to activate refrigeration and when it is time to deactivate refrigeration. A Standard Circuit application will determine when refrigeration is necessary, and send a command to the Direct Impulse cell to begin or end refrigeration. Direct Impulse applications will fix the valve to 0% when refrigeration is not being called for. Recovery Time When Direct Impulse is inactive (i.e. when a Direct Impulse application is being told by a Standard Circuit to keep refrigeration OFF), no refrigerant is flowing through the circuit s evaporator coils (i.e. the pulse valve is at 0%). When refrigeration re-activates, the circuit will need some extra time to get the system going again. This time period is called recovery time. The user determines the length of a recovery time period, but it defaults at one minute. During recovery time, the valve remains fixed to a percentage for the entire recovery time, after which normal Direct Impulse control will begin. The percentage to which the valve will be opened during recovery time will be the last recorded valve percentage of the previous refrigeration cycle. For example, if a valve is at 75% when Direct Impulse receives a command to shut down refrigeration, this valve position will be recorded. When refrigeration begins again for the next cycle, recovery time will begin with the valve open to 75%.

3 Pulse Width Modulating for Valves A pulse valve can only be opened to two physical positions: open, and closed. When a Direct Impulse application calls for 50% OPEN, pulse valves are physically incapable of opening half-way. Instead, pulse valves achieve multiple valve positions by using pulse width modulation. Pulse width modulation pulses the valve ON for a percentage of a fixed amount of time (called the period). Therefore, if a valve is commanded to be 20% open and the period is 1 minute, the valve will be open for 20% of one minute (i.e. 12 seconds) and closed for the remaining 80% (48 seconds). Over a course of several periods, the overall result will be a flow of refrigerant equivalent to 20% of the maximum flow rate. The pulses that command the valve are supplied by an 8DO Digital Output board, which is attached to the Einstein RX via the RS485 I/O Network. The 8DO pulses a 12V signal to open the valve. Hardware / Wiring Setup Direct Impulse will require a minimum of the following: A pulse valve for the circuit, wired to a point on an 8DO Digital Output board. A suction temperature probe, mounted near where the pipes from each case meet and return to the rack. The probe must be connected to a 16AI or similar CPC-compatible input board. A suction pressure transducer. This transducer will actually be used for control purposes in the Einstein s Suction Group application, but it will also be used to calculate saturation suction temperature. Software Setup Standard Circuit and Suction Group Setup Before a Direct Impulse application can be programmed, you will need to set up applications to control the Suction Group and Standard Circuit. Refer to P/N , Einstein RX Installation and Operation Manual for instructions on how to do this. Create an Impulse Application If you haven t done so already, create a new Impulse application in the Einstein. From the Main Status Screen: 1. Press to bring up the Actions Menu. 2. Press Control Appl Setup. 3. Choose option Add Control Application.

4 4. Press the key with the cursor in the Type field, and select Impulse from the Look-Up Table. Press to select this option. 5. Move the cursor to the How Many field, and enter the number of applications you wish to create. 6. Press to create the applications Edit an Impulse Application From the Main Status Screen: 1. Press to list all types of existing applications. 2. Choose Impulse from the menu. 3. If a list appears showing multiple Impulse applications, move the cursor to the name of the one you wish to edit, and press. 4. Press followed by to begin editing the application. Screen 1: Setup 1. Enter a name for the Impulse application in the Name field. Options Filter Time OPN and Filter Time CLS The Filter Time fields are designed to slow down the rate at which a change is made to the valve percentage. When Direct Impulse determines the valve percentage must increase or decrease, the Filter Time will set the time duration across which the change will be made. When a Filter Time is set to 0 seconds, changes in the valve position happen instantaneously. Larger filter times will result in slow changes to the valve position, which likewise results in a slower overall system reaction to changes in the superheat. The Filter Time OPN time period applies when the valve percentage is increasing. The Filter Time CLS time period applies when the valve percentage is decreasing. It is recommended the Filter Time OPS and CLS fields remain at their default values (zero seconds). They should be used only when fine-tuning the system. Recovery Time The Recovery Time is the amount of time after beginning refrigeration that the valve will be opened to a fixed percentage before normal refrigeration control will begin. The default value, one minute, should be sufficient for most cases. PWM Period This value sets the period of time for the pulse width modulation of the valve. Across this period of time, the valve will be open for a percentage of

5 the time and closed for the remainder. The default value, 1 minute, is based on the Direct Impulse application s Update Rate, which is 4 seconds. NOTE: If you choose a PWM Period at or near 30 seconds, you will need to change the Direct Impulse application s Update Rate to 2 seconds. To do this, press followed by to turn on Einstein s Full Options feature, and change the Update Rate field that appears from 0:00:04 to 0:00:02. Screen 2: Setpoints 1. Enter the desired superheat set point in the SUPERHEAT SETPT field. 2. Enter the desired throttling range in the Throttle Range field. A throttling range of 10 should be sufficient for most installations. Screen 3: Inputs 1. In the SAT SUCT TEMP input definition, specify the location of your suction group s calculated saturation suction temperature output. With the cursor in the Application field, press and choose the name of your suction group. With the cursor in the Output field, press and choose SATUR SUC TEMP from the Look-Up Table. 2. In the CIRC SUCT TEMP input definition, specify the board and point location of the probe that will supply the circuit s suction temperature. 3. In the REFR ACTIVE input definition, specify the location of the refrigeration solenoid output from the Standard Circuit application: With the cursor in the Application field, press and choose the name of your suction group. With the cursor in the Output field, press and choose REFRIG SOLENOID. Options SHUT DOWN If you wish, you may connect a digital closure or signal to this input that will shut down the Impulse cell and close the pulse valve to 0% when ON. Screen 4: Outputs 1. Enter the board and point address of the 8DO point that will drive the pulse valve in the PWM OUT field.