Anal. Chem. 2005, 77, 948-951
Directly Digital Flow Controller Denise M. McClenathan, Andrew Alexander, John Poehlman, and Gary M. Hieftje*
Department of Chemistry, Indiana University, Bloomington, Indiana 47405
An improved directly digital flow controller is evaluated for its ability to modulate gas flow rates. As in the older device, the “GasDAC” (named for its similarity to a weighted-resistor digital-to-analog converter) is capable of controlling gas flow in a linear and reproducible manner with the advantage of having an adjustable range of flow rates. The new design incorporates venting to prevent “puffing” when the individual flow channels are opened. The temporal characteristics of the GasDAC are also examined; modulation frequencies of 10 Hz with various types of waveforms are possible with the new device. Recently, our laboratory became interested in modulated ion sources for mass spectrometry, where the need to modulate gas flows on rapid time scales (∼10 Hz) arose.1,2 Unfortunately, although mass flow controllers (MFCs) are used in most ICPMS instruments and are deservedly popular for their accuracy and reproducibly, most have relatively long response times, commonly 1-3 s. Consequently, gas-flow modulation with such devices will be limited to frequencies below 0.5 Hz. A few commercial MFCs specify response times of 150-300 ms, so modulation at frequencies of ∼2-3 Hz should be possible; however, such devices are still below the desired frequency for our investigations. Although one MFC (Aera Mach One digital MFC, Advanced Energy, Fort Collins, CO) has the tens of milliseconds response time necessary for 10 Hz modulation, it is relatively expensive, so alternative approaches to modulating gas flow were explored. One such alternative for regulating gas flow is the directly digital flow controller reported by Hunter and Hieftje.3 Being the gas-flow equivalent of an eight-bit weighted-resistor digital-toanalog converter, it is colloquially referred to as a GasDAC. A conceptual diagram of the flow network is shown in Figure 1. Here, an input supply of gas at constant pressure enters the input channel and is split into eight parallel paths or “bits”. A needle valve is used to restrict the flow through each channel, and each successive channel is set to twice the value of the preceding one. For example, if the least significant bit (bit 0) is set to 0.01 L/min, then bit 1 would be 0.02 L/min, bit 2 is 0.04 L/min, etc., with the most significant bit (bit 7) set to 1.28 L/min. Solenoid valves are placed at the exit of each channel to open or close the channel and, hence, to turn each bit on or off. The flows from the open * To whom correspondence should be addressed. Phone: (812) 855-2189. Fax: (812) 855-0958. E-mail:
[email protected]. (1) McClenathan, D. M.; Wetzel, W. C.; Hieftje, G. M. Use of Rapid Modulation Techniques in Inductively Coupled Plasma Time-of-Flight Mass Spectrometry. The Pittsburgh Conference, Chicago, IL, 2004. (2) Wetzel, W. C.; McClenathan, D. M.; Hieftje, G. M. Use of Modulation Techniques to Overcome Limitations in Inductively Coupled Plasma Timeof-Flight Mass Spectrometry. The Pittsburgh Conference, Chicago, IL, 2004. (3) Hunter, T. W.; Hieftje, G. M. Anal. Chem. 1978, 50, 209-212.
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Figure 1. Conceptual diagram of the GasDAC. An “X” through the symbol for a particular solenoid valve signifies that it is closed. In this particular situation, the total flow through the GasDAC would be 1 + 8 + 32 + 64 ) 105 flow units, where the flow units can be adjusted in absolute units of L/min by changing the input gas pressure.
channels are recombined in an output conduit, where the total flow out of the GasDAC is the sum of the flows through all open channels. Because changes in flow rate are obtained by changing the state of the solenoid valve(s), the response time is dependent on the open and close times of the valve, typically on the order of 10 ms. Therefore, modulation should be possible at least at 10 Hz with this device. Moreover, since the selected channels represent a binary number proportional to the total flow, the system is directly compatible with the binary output of a digital computer. Since the total flow out of the GasDAC is dependent on the restrictions in the open channels and on the pressure applied to the flow network, this device also has a tunable dynamic range. Once the flow through each channel has been set, the flow range of the GasDAC can be adjusted without needing to recalibrate each bit, simply by changing the input pressure to the device. An additional advantage is that it is relatively simple and inexpensive to construct. However, one limitation of the device is that pressure can build up in the closed channels, resulting in a “puffing” effect when that bit is turned on. In the present study, improvements to the original GasDAC design were devised to minimize this problem and to substitute modern components for those originally used. Below, the temporal characteristics of the new device are examined. DESIGN CONSIDERATIONS The original design employed a delicate network of welded pipes and tubing that was susceptible to leaks and cumbersome to assemble. Consequently, the new GasDAC was constructed from a solid piece of aluminum to which the needle valves (Swagelok, Solon, OH) and solenoid valves (model HK5 B3 24 A NR, Humphrey Products, Kalamazoo, MI) were attached. The 10.1021/ac048726r CCC: $30.25
© 2005 American Chemical Society Published on Web 12/30/2004
Figure 2. Photograph of the assembled GasDAC. The inset of the picture shows the hole in the valve seal plate used to vent each channel.
Figure 3. End view of the flow network assembly. The solid arrow shows the flow path when the valve is actuated, and the dotted arrow depicts the direction of flow when the valve is closed. The portion of the electronic circuit used to control an individual solenoid valve is shown in the inset.
three-way solenoid valves used in this design were arranged in the flow network in a normally closed manner. The valve housing was open on both sides, and the top side of the valve was sealed with an O-ring and a flat plate. Another O-ring was used on the underside of the valve, and these pieces were fixed to the aluminum block with screws. A photograph of the device is shown in Figure 2, and Figure 3 depicts the GasDAC assembly from the viewpoint of a slice through an individual bit. Two lengthwise channels drilled in the aluminum block serve as the input and output chambers of the device. The bits of the
GasDAC consist of eight parallel channels that branch from the input and recombine at the output, and all bits are identical with the exception of the setting of the respective needle valve. From the input plenum, the flow is directed to the needle valve where the path is restricted to the desired flow rate. After exiting the needle valve, the gas is redirected with PTFE tubing back into the block, where the flow is guided to the solenoid valve. When the solenoid valve is actuated, the gas travels through the block to the output chamber where it is combined with the flow from the other open channels. However, when the solenoid valve is Analytical Chemistry, Vol. 77, No. 3, February 1, 2005
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unactuated (i.e., closed), the gas is allowed to vent to the atmosphere via a hole in the valve plate (see the inset of Figure 2). Use of such “vent plates” reduces pressure buildup in the channel while the solenoid is in the closed position and is a distinct feature of this new GasDAC design. The authors note, however, that slight changes to the design would be necessary to accommodate toxic or reactive gases. In such cases, an additional block could be used in place of the vent plates to properly dispose of the vented gas. Control of the solenoid valves can be performed via two approaches. Ideally, a digital output would be created to manipulate each bit directly. For steady-state flow values or even simple square waveforms, this scheme is not difficult. However, timing the eight digital outputs to produce an eight-bit waveform becomes more complicated for complex waveforms. As an alternative, a generic function generator can be used to produce an analog waveform, and an external analog-to-digital converter can then be used to generate the desired individual bit logic. An in-house LabVIEW program (National Instruments, Austin, TX) is used to produce the bit logic for the group of solenoid valves. The analog voltage that corresponds to the desired flow rate is output from the data acquisition card (PCI-MIO-16E-1, National Instruments, Austin, TX). This output is fed into a 10-bit analog-to-digital converter (AD571JD, Analog Devices, Norwood, MA) where the two least significant bits are left open. To prevent loading the circuit, the input voltage to the ADC is buffered by an operational amplifier (OP77, Analog Devices). A pulse generator (model 101, Systron Donner, Concord, CA) operating at 10 kHz was used to clock the ADC. The output from the analog-todigital converter is then sent to a latch (74HC273N, National Semiconductor, Santa Clara, CA) which holds the bit logic until the next clock cycle. A hex inverter (74HC123AN, National Semiconductor) is used to time the data-ready line from the ADC with the clock pulse of the latch. The logic from the latch is sent to the solenoid drive circuitry. To permit each solenoid valve to be operated or accessed individually, each valve has it own dedicated drive circuitry, which is shown in the inset of Figure 3. The solenoid requires 24 V at ∼65 mA to open. The bit logic is connected to the base of an NPN transistor (2N6045, ON Semiconductor, Phoenix, AZ). The emitter is connected to ground and the collector is connected to the negative terminal of the solenoid valve. The positive terminal of the solenoid valve is connected to +24 V. When the bit logic is low, the transistor is off and the collector is held at +24 V. Since there is no voltage drop across the solenoid valve, it remains closed. When the bit logic is held high, the transistor is pulled on and the collector is held at ground. The resulting current through the solenoid valve drives it open, and it remains actuated as long as the collector is at ground. A Zener diode (1N4753A, ON Semiconductor) is placed in the circuit to ensure fast switching of the solenoid valve, and the additional diode (1N4004, ON Semiconductor) is used to prevent current in the forward direction of the Zener diode. Additionally, an LED was added to the circuit to indicate the state of the solenoid valve. EXPERIMENTAL SECTION All flow rate measurements were made using a fast-response flow meter calibrated for nitrogen (model 41216, TSI Inc., Shoreview, MN). The sampling rate was 4 ms, and the measure950
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ments were made 10 cm from the GasDAC exit. For steady-state measurements, the flow-meter response was read using a digital multimeter (model 175, Fluke, Everett, WA). For transient flow rate measurements, both the LabVIEW output voltage and the flow-meter response were recorded with a digital oscilloscope (Tektronix TDS2024, Beaverton, OR). To investigate the effect of venting, the vent plates used to seal the solenoid valve were replaced with solid plates on all of the channels. RESULTS AND DISCUSSION Gas Flow Calibration and Range. The GasDAC was calibrated by opening each channel individually and adjusting the corresponding needle valve to yield the appropriate flow rate. With an input pressure of 10 psi, the most significant bit was set to 1 L/min. After calibration of bits 3-7 the input pressure of the device was adjusted until bit 3 was 1 L/min and then bits 0-2 could be tuned. After calibration of all eight channels, the output of the GasDAC was tested with flow rates from 0.1 to 2 L/min in 0.1 L/min increments. Between each pair of replicates the input pressure to the device was turned off and set back to ∼10 psi to yield 2 L/min with all bits “on” to signify day-to-day repeatability. Measurement precision (RSD) at full scale was 0.6% and ranged from 0.4% (0.5 L/min) to 1.2% (0.1 L/min). The linearity of the GasDAC output was compared to that of the response from an MFC. The linear response curve from the GasDAC was within the confidence limits of that from the MFC (FC-280 SAV, Tylan General, Carson, CA). The flow range of the device can be adjusted without recalibration simply by changing the input pressure applied to the GasDAC. When calibrated for 2 L/min at 10 psi, the flow range at 5 psi becomes 1 L/min. The maximum flow rate is 16 L/min and is governed by the pressure limit (100 psi) of the solenoid valves. However, if the desired flow rate were beyond this range, the individual channels could be reset and recalibrated to yield a new flow range; this versatility is unique to the GasDAC. Reduced Puffing. To evaluate the effect of puffing, the vent plates on the solenoid valves were replaced with solid plates to prevent venting. The output from the GasDAC with the solid and vent plates was compared for a 0-2 L/min 1 Hz square wave. As shown in Figure 4, the vent plates significantly reduce the puffing that is observed when the pressure is allowed to build up in the channels, even for the “worst case scenario” of having all bits turned off and on simultaneously. A significant advantage over the previous design, use of the vent plates permits modulation waveforms free of pressure effects. Temporal Characteristics. The GasDAC output for several 0-2 L/min square waves ranging from 1 to 20 Hz is shown in Figure 5. From this behavior, it is clear that the GasDAC is capable of modulating gas flows at frequencies up to 10 Hz while preserving the general shape of the incoming waveform. At higher frequencies, the rise and fall times of the modulation system begin to dominate and the shape of the waveform is degraded. The response time for the GasDAC output was determined from the 2 Hz waveform shown in Figure 5. The “on” time was calculated to be 8 ms (10-90%) and 20 ms (0-100%). The “off” time was measured as 20 ms (90-10%) and 36 ms (100-0%). These values are consistent with the timing of the valves, which typically open and close in 10 and 4 ms, respectively. It should be noted that
Figure 4. Output of the GasDAC while all bits are turned “on” (2 L/min) and “off” (0 L/min) at 1 Hz (a) without and (b) with venting during the off cycle of the waveform. The effect of “puffing” caused by pressure buildup during the off half-cycle is apparent in (a). Small fluctuations in each trace are the result of digitization errors in reading the fast-response flow meter.
Figure 5. GasDAC output for 0-2 L/min square waveforms at 2, 5, 10, and 20 Hz.
oscillations were observed on the rising and falling edges of the waveform that can interfere with the temporal response. These oscillations were found to agree with those expected on the basis of organ-pipe resonance calculations. Additionally, because the flow meter sampled only every 4 ms, it is difficult to determine more accurate response times for the present GasDAC. Sample Waveforms. Because each solenoid valve can be accessed individually and eight bits are used, the flow resolution should be 1 part in 256. Thus, many different waveforms can be used to modulate the output flow in addition to the simple on/off square wave. An example of such waveforms is shown in Figure 6, where sine, triangular, and sawtooth waveforms are shown at a frequency of 1 Hz. Although the waveform shapes produced by the GasDAC are accurate, spikes are observed at regular intervals along each waveform. These spikes are likely artifacts from the control and timing of the solenoid valves. As mentioned earlier,
Figure 6. GasDAC ouput for 0-2 L/min sine, sawtooth, and triangular waveforms at 1 Hz.
the solenoid valve on and off times are not symmetrical as the valves typically open in 10 ms and close in 4 ms. Because the array of solenoid valves is controlled by a single analog voltage, the logic for all bits changes simultaneously. Thus, as some valves are closing, others are actuating. As a result, there may be an instant where all bits are closed, and hence, there is no flow. This effect will be most visible when bits 0-6 turn off and bit 7 turns on. By producing the digital output for each bit directly rather than via an analog approach, this effect could be reduced by compensating for the timing differences between the opening and closing of valves in each channel. Despite such effects, the GasDAC affords a practical means for fast modulation of gas flows with various waveforms. CONCLUSIONS An improved GasDAC was designed for the purpose of gasflow modulation. The use of vent plates in the design eliminated the puffing observed when the bits were turned on, a significant improvement over the previous design. The temporal characteristics of the new GasDAC were evaluated. Modulation waveforms up to 10 Hz could be used without degradation of the waveform shape, unlike the
