Closed-loop testing

Because of safety concerns, all combustible and/or toxic gases must be used in outdoor test loops or in a special indoor test building with the required safety monitoring equipment. The gas cost factor makes the problem even more difficult. The problem of known gas properties adds another complication. Despite all the negative aspects just mentioned, most performance tests are closed-loop tested. 

B. Tuning relations based on closed-loop testing and the Ziegler-Nichols ultimate-gain (cycle) method with given ultimate proportional gain Kcu and ultimate period Tu. 

Ziegler-Nichols Continuous Cycling (empirical tuning with closed loop test) Increase proportional gain of only a proportional controller until system sustains oscillation. Measure ultimate gain and ultimate period. Apply empirical design relations. 

The control loop was effectively opened for the open-loop test by injecting a voltage source directly into the FB/OCP pin. This voltage was adjusted to provide an output voltage of approximately 102 V. For the closed-loop test, this external voltage source was not used. 

Closed loop test apparatus for determining the heat transfer characteristics of high energy monoproplnts 3 C345... 

Closed Loop Test Apparatus for Determining the Heat Transfer Characteristics of High Energy Monopropellants has been described in PicArsnTechMemorandum 1119 (1962), by J. R. Grossman... 

Vibration test equipment using digital control techniques and feedback or closed loop test equipment and software therefore capable of vibrating a system at 10 g RMS or more between 20 Hz and 2000 Hz, imparting forces of 50 kN (11,250 lbs) or greater. 

Fatigue crack propagation tests were performed on 73-nmi x 73-mm (2.9 in. x 2.9 in.) compact-tension specimens that had been machined from the plaques and then precracked. An electrohydrau-lic closed-loop test machine was used to produce a constant-amplitude, 10-Hz sinusoidal load. Host of the tests were performed in duplicate at room temperature in laboratory air at an average ambient RH of A0%. In view of the slow rate of moisture equilibration in air at room temperature (15), the difference between the ambient RH and that of the nylon being tested was presumed to be unimportant within the time period of the test (3-5 hr). 

FFTF was designed for a cycle of 102 days of operations and 28 days for refueling. In a refueling outage, typically one-third of the reactor core would be replaced. This included 24 driver fuel assemblies, 6 control rods, 12 reflector assemblies, and 2 closed-loop test assemblies. 

The example in Figure 10.2 required 112 min (dead time plus 5 times the capacitance lag times) for the process variable to move 99% of the way to the new steady-state value. This time or more is needed for deterrniriing the steady-state process gain, Kp. By comparison, the time to reach the first peak and valley in a closed-loop test took 25 to 30 min in Chapter 9, Figure 9.1, for the same process. 

Guided-fault isolation (GFI) is practical with an emulative tester because the instrument maintains control over the entire system. Automated tests can isolate faults to the node level. AU board information and test procedures are resident within the emulative test instrument, including prompts to the operator on what to do next. Using the microprocessor test connection combined with movable probes or clips allows a closed loop test of virtually any part of a circuit. Input/output capabiUties of emulative testers range from single point probes to weU over a hundred I/O Unes. These lines can provide stimulus as weU as measurement capabilities. 

Personal computer programs are av2ulable to control the closed-loop test machine during cm LCF experiment and to anal3Tze load-displacement-time data. 

Validation of such flap actuation solutions have been performed in wind tunnel tests on a one-seventh downscaled Bell-412 Mach-scaled rotor hub [87]. It has been shown that trailing edge deflections of 4° to 5° can be achieved at up to 1800 rpm which allowed suppression of vibratory bending moments imder an open loop control condition. Even some preliminary closed-loop tests using a neural network controller were performed which however required simultaneous actuation of all four blades. In [88] an induced-shear piezoelectric actuator has been described to actuate trailing edge flaps... 

Table 9.16 only shows the steady-state characteristics of each control structure. However, good steady-state characteristics are not a sufficient condition for good dynamic control system performance. Thus, Aspen Dynamics will be used to evaluate the control system performance for the alternative control structures. Since the unmeasured feed composition changes are the more severe closed-loop test in comparison with the feed rate changes, these load changes will be made in the closed-loop dynamic simulations for comparison. 

Figure 9J27 Closed-loop test with +10% feed F3 water composition change.

Phase 2—Closed Loop Test. This checks the operation of each instrument loop. Proceeding as in the sequential test discussed above, an operator should drive all inputs to the values needed to test each mode of operation of a loop. Most control loops have one manual mode, one hold mode, one or more automatic modes, and one or more fault modes. For each of these, proper operation of all the following must be verified ... 

Model identification is the process of quantifying process dynamics. The techniques available fall into one of two approaches - open loop and closed loop testing. Open loop tests are performed with either no controller in place or, if existing, with the controller in manual mode. A disturbance is injected into the process by directly changing the MV. Closed loop tests may be used if a controller exists and already provides some level of stable control. Under these circumstances the MV is changed indirectly by making a change to the SP of the controller. 

This model identification technique can be applied to both open and closed loop tests. Multiple disturbances can be made in order to check the repeatability of the results and to check linearity. However it is important to avoid correlated steps. Consider the series of steps shown in Figure 2.11. There is clearly a strong correlation between the PVand the MV, with Kp of 1.0 and 9 of around 3.0 minutes. However, there is an equally accurate model with Kp of —1.0 and 9 of around 33.0 minutes. 

Ercolani et al. (1988) conducted extensive closed-loop tests on fine coal and water mixtures at shear rates of 20/sec, 50/sec, and 80/sec. They concluded that pumping at power rates in the range of 0.1 to 2.0 W/kg (0.045-0.90 W/lb) did not seem to affect the degradation of coal under mechanical stress. 

Fortunately there is a quick and easy method for obtaining enough information to suggest corrective measures in most instances. The method consists of one open-loop and one closed-loop test. In the latter case, the proportional mode of a controller serves as the test instrument. The procedure is as follows ... 

Since these tests are made only at one operating point, they will not disclose any nonlinear properties. Closed-loop response should be observed at other flow conditions to detect any change in damping. If the period changes with flow, a variable dynamic element is present. An extremely nonlinear measurement, such as pH, is identified by the distorted waveform it produces, as in Fig. 2.13. A less severe nonlinear measurement may not be detected without changing the set point. In short, if a thorough analysis is to be made, the closed-loop test should be repeated at other values of flow and set point. 

Run closed-loop testing. In this step, the steady-state model is used to simulate the candidate control scheme and to test its robustness to feed flow and feed composition changes. This step consists of a series of runs (sensitivity studies) aimed at locating a set of operating conditions that meet or exceed the product specifications for all expected disturbances in feed flow and compositioa... 

The primary element may be installed within a continuous section of pipe flowing full or at the inlet or exit of a pipe or a plentun chamber. Orifice and venturi tube are installed within the pipe in a closed-loop test. The flow nozzle may be installed within, at inlet, or at outlet of the pipe. 

It is normal practice to use a venturi tube installed within a continuous section of pipe in piunp-acceptance tests and a flow nozzle at the exit of the discharge pipe in compressor-acceptance tests. More closed-loop testing has recently been required in compressor testing. Industry normally uses the nozzle configuration shown in Fig. M-26 with a closed loop. The construction of the primaiy elements and examples of their installation are given in the following paragraphs. 

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