Proportional Solenoid Valve Controller - How They Work
Figure 1: Burkert proportional valve controller 8611 (left) and 8605 (right)
Proportional solenoid valves are used to regulate fluid flow rate by varying the position of the valve plunger. The regulation of flowrate is most commonly used to control pressure, level, and temperature. Other process variables such as pH value, conductivity, and humidity can also be affected by flow rate. The plunger position is controlled by a magnetic field, which is generated by applying a power supply to the solenoid coil. Typically, proportional solenoid valves are normally closed (close with no power) but can also be normally open (open with no power). This article will discuss normally closed proportional solenoid valves.
When power is supplied to the coil, the plunger rises and pushes against the spring force to open the valve. Without a power supply, the plunger is pushed against the valve seat by a spring force to close the valve outlet. In contrast to a standard direct-operated solenoid the plunger position of a proportional solenoid valve can be controlled across a range of stroke positions by varying the power supplied to the solenoid coil. Valves suited for proportional control have a slightly different design to improve the position stability of the plunger at intermediate positions. The valve power supply is typically automatically controlled using a pulse-width modulation (PWM) controller such as the Burkert 8611 (Figure 1 left) or 8605 (Figure 1 right) controllers. They are referred to as proportional valve controllers
The Burkert 8611 and 8605 controllers are both designed to control Burkert proportional valves, but can also control other brand’s valves. The 8605 is a dedicated proportional valve controller, and the 8611 is a universal controller. The 8611 is capable of controlling proportional valves and other process valves and devices. Each controller is equipped with configurable control functions to ensure accurate and reliable process control. Additionally, multiple design options are available to meet the needs of a process control applications.
This article will discuss how proportional solenoid valve controllers’ function with pulse width modulation (PWM) and PI (proportional and integral) control theory and show an overview of the 8611 and 8605 Burkert proportional solenoid valve controllers. In addition, we have articles on the 8611 and 8605 discussing those controllers in more depth.
Table of contents
- Pulse width modulation (PWM)
- PI (proportional and integral) control theory
- Overview of 8605 vs 8611
- Additional Information
Pulse width modulation (PWM)
Burkert’s 8611 and 8605 types are both equipped with control technology to generate pulse width modulation (PWM) output signals to control proportional solenoid valves in a system.
The PWM signal is a pseudo-analog signal manufactured from a digital signal that is switched ON and OFF. PWM consists of two main components that influence the output, the switching frequency and duty cycle. The switching frequency determines how fast a cycle between ON and OFF is completed. For example, a switching frequency of 20 Hz would be 20 cycles per second. The duty cycle represents the percentage of the amount of time the signal is in the ON state over the full cycle. For example, at 75% duty cycle the signal output will be ON 75% of the cycle. The duty cycle and switching frequency will impact the effective current delivered to the solenoid during modulation.
Although the PWM voltage pulses to the coil are square, the current is delivered to the coil in a saw tooth curve due to coil inductance and duty cycle (Figure 2). The result is a steady oscillation of the plunger in a balanced state which is commonly called dither. Dither reduces static friction and decreases hysteresis (where actual plunger position lags behind command position). In Figure 2, the time off (toff) and time on (ton) are from the duty cycle and the nominal voltage (U) is what is supplied creating the resulting voltage (I) wave.
Figure 2: PWM signal
Valve sensitivity and dither movement are influenced by coil properties, switching frequency, and duty cycle. Valve reaction to a PWM signal is most sensitive when operated in a specific switching frequency range (f) at an optimal duty cycle. The point at which the valve is most responsive is known as the working point. The working point is shown in Figure 3 at the intersection of the upper frequency and 60% duty cycle. At its upper (fHI) and lower (fLO) frequency limits the valve will react more slowly to a PWM signal. An example of the ideal operating range of a proportional valve is shown in Figure 3.
Figure 3: Proportional valve operating parameters
The limit frequencies, optimal duty cycle, and coil properties are all unique and crucial for accurate control of any valve. Malfunction of the valve or higher audible noise may result if a valve is controlled with incorrect operating parameters.
The benefit of using the Burkert proportional valve series with their controller, is that the correct switching frequencies and operating points are known for all valves. Burkert determined these values empirically for each valve type. The 8611 memorizes the switching frequency limits of all Burkert valves. During configuration of the controller the valve type can be entered, and the operating parameters are automatically loaded into the control scheme without further configuration. The 8605 includes most frequencies in memory as default, however, some may need to be reviewed before operation. All Burkert proportional valve operating parameters are provided by Burkert. When utilizing a different brand’s proportional solenoid valve with a Burkert controller, the operating parameters need to be inputted correctly for optimal performance.
The control functions used to regulate the PWM signal for control of the process will vary between the 8611 and 8605. The most notable difference between the controllers is that the 8611 uses closed loop control for PWM. The 8605 uses open loop control for PWM. The differences and control functions available to each model will be discussed further in this article.
PI (proportional and integral) control theory
A PI control loop (Figure 4) is a closed loop system which adjusts a control variable (i.e. PWM switching frequency) in relation to measured feedback provided from the process variable being controlled (i.e. pressure). The PI control loop uses parameters known as proportional and integral terms to dynamically adjust the control variable to reduce error. The 8611 can operate on a closed loop system, while the 8605 is only open loop.
Figure 4: PI control loop block diagram
- SP: Desired setpoint
- PV: Measured process variable
- e(t): Error value
- P: Proportional parameter
- Kp: Proportional tuning constant
- I: Integral parameter
- KI: Integral tuning constant
- MV: Measured (or control) variable
The proportional and integral parameters are calculated using system error. The proportional parameter is the product of a tuning factor and the difference between the desired setpoint (i.e. desired pressure setpoint) and a measured process variable (i.e. measured actual pressure). The integral parameter accounts for error over a given time period. The product of a tuning factor and the difference between the desired setpoint and a measured process variable integrated over a set time period is used to define the integral parameter. The equation in Figure 5 shows the sum of the proportional and integral parameters is used to determine the control variable value (u(t)).
Figure 5: PI control variable formula
The purpose of a PI control algorithm is to stabilize and reduce system error over time by accurately regulating a control variable. This method is widely used in fluid control processes. In theory, the algorithm is quite simple and relatively easy to implement in many applications. However, the controller must be tuned properly with the correct proportional and integral parameters. Without proper tuning, the control variable may overreact to system changes or oscillate around the desired setpoint. Since every process is different, each system will utilize different tuning parameters to achieve desired performance. Although several tuning methods exist, it is most common to manually adjust each parameter until stability is achieved. Most control system manufacturers will offer guidance for starting points. Without accurate inputs and optimized tuning parameters, control stability will be sacrificed.
The advantages of a closed loop control system such as a PI loop include:
- Reduced hysteresis from set point
- Automated process error correction
- Increased process stability