Capitalizing on Valve Mechanics With a Hit and Hold Circuit

Hit and hold circuits enable solenoid valves to be energized to full power and held for a short duration of time before voltage and current are reduced significantly to lower levels, while still allowing the valve to remain open and energized. Hit and Hold circuits enable OEM system designers to operate solenoid valves at a reduced input voltage rating while maintaining the intended valve function. When implemented properly, significant power savings and heat reduction benefits can be realized in the application. The following blog explains the mechanical states of the valve when a power source is applied and instructs the end-user on how to determine an appropriate hold voltage parameter.

Reducing power consumption and heat generation within the valve

Now that we understand basic solenoid valve mechanics, how do these operating principles tie into reducing power consumption and heat generation within the valve? The key here is understanding the difference between the maximum current, at maximum rated voltage, and the current required to maintain the actuated state. A plot of the inductive response time of the valve is helpful in distinguishing the difference between the two states. Figure-6 below illustrates a typical electrical circuit for conducting a response time test and the resulting voltage vs. time plot captured on an oscilloscope.

Figure-6: Response time test circuit and typical oscilloscope output

It should be noted that to provide a clean voltage signal to the oscilloscope, a low resistance resistor, resistor-2 or R2, must be added to the circuit. (R2 ˂ 1% of R1). Once the manual switch is closed, the voltage across R2 increases linearly until the armature begins to move as illustrated in Figure-7 and 8 below.

Figures-7 and 8: Oscilloscope outputs

Once the armature is in motion, the inductance of the circuit increases and the voltage signal across R2 begins to decrease until the armature is fully actuated (Figure-9). Once fully actuated, the input voltage across R2 returns to the same increasing trajectory until a steady-state, the maximum voltage/steady-state condition is achieved (Figure-10).

Figures-9 and10: Oscilloscope outputs

It is important to understand that the voltage required to actuate the valve will vary with changes in coil temperature and the coil resistance dependent on coil temperature. Therefore, as the coil temperature increases, the actuation voltage will increase proportionally to compensate for the increased coil resistance. The opposite is true when the coil temperature decreases. In either case, the actuation current will remain consistent with variations in coil and ambient temperatures.

For the remaining portion of this discussion, the maximum voltage parameter will be replaced by the corresponding maximum current parameter so that the difference between the maximum current and the drop-out current can be assessed. Figure-11 below illustrates the location of the corresponding maximum current and actuation current parameters.

Figure-11: Response time plot

As discussed earlier, the valve will return to the nominally closed state when the power source to the coil is disconnected. Rather than suddenly disconnecting the source, if the input current is gradually decreased, the current at which the valve state change occurs can be determined. This current value is referred to as the drop-out current and is typically detected by monitoring flow rate across a normally closed orifice. Figure-12 illustrates arbitrary test results from a drop-out current test.

Figure-12: Drop-out current/valve state versus time

Once the actuation and drop-out current parameters are known, the end-user must establish a minimum hold current (MHC) specification for the application. The MHC must include a safety buffer which accounts for drop-out current performance variations due to application parameters including, but not limited to, system vibration, ambient temperature variation, elastomer swell (due to media compatibility and/or elastomer temperature), valve function/porting, and any other application parameters which influence the drop-out current. Figure-13 illustrates an arbitrary MHC value based on the established performance margin.

Figure-13: Drop-out current/valve state versus time

The maximum current (I_Max) and drop-out current (I₄) parameters illustrated in figure 13 are not specified given that they vary with valve type and application parameters. Therefore, in order to understand the power savings potential in a hit and hold application, arbitrary values will be selected for (I₄) and (I₅) in the following example.

Let us begin by considering a 13.6 VDC / 136 Ohm valve configuration. For discussion purposes, the coil resistance will remain constant. Therefore, we are effectively ignoring any self-heating and ambient thermal effects on the coil resistance. Based on the input voltage and resistance of the valve, the (I_Max) value is calculated at 100mA. [I = V/R] The arbitrary values selected for (I₄) and (I₅) are 25mA, and 50mA respectively.

In this example, rated voltage is applied to the valve for a short duration, (typically less than 100 msec), to ensure actuation at 100mA, (I_Max). Once actuated, the input current is reduced 50mA, (I₅), and the valve remains in the actuated state. The input current/actuation state scenario described in this example is illustrated in Figure-14 below.

Figure-14: Hit and hold current state versus valve state 

Given the arbitrary parameters above, we can now calculate and compare the power consumed by the valve at the selected current states, {(I_Max) and (I₅)}, using the power equation P = I²R. During the initial “hit” current state, (I_Max = 100 mA), the effective power consumed by the coil is 1.36 Watts; this calculation is based on nominal 136 Ohm resistance. Once the current is reduced to the MHC or “hold current” value, (1₅ = 50 mA), the power consumed by the valve effectively drops to 0.34 Watts. Again, this calculation is based on a constant 136 Ohm coil; there is no change or increase in resistance due to the internal heating of the coil. Comparing the two power states in this example, the end-user has effectively reduced the continuous power consumption of the valve by 75% while maintaining the desired function.

It is important to understand that the pull-in response or response time of the valve will vary within the same valve family due to the differences in valve types, (2/3-way normally closed, 2/3-way normally open, distributor or directional, and universal type configurations), and variations in the magnetic gap. Therefore, in order to select a proper hit current duration, the end-user should reference the response time specification of the valve prior to designing the circuit.

Now that we understand a method to characterize the actuation and drop-out parameters of the valve and have re­viewed a theoretical/arbitrary example for power savings, we can now discuss optimum strategies for implementing “hit and hold” power delivery.

By Phil Dodge, senior engineer at Parker Precision Fluidics

Source: blog.parker.com