Figure 1- Isolation mechanisms such as “trampoline” suspension (a) and strain relief mounting loops (b) help reduce vibration-induced frequency shift in crystal oscillators.
Certain designs have temperature shifts that affect vibration response, creating a need for combined tests.
In the development of components for military and aerospace applications, engineers must test their designs to stringent temperature and vibration specifications. Often these tests are performed separately, but for designs where temperature shifts can significantly affect vibration response, these tests must be combined.
A modified temperature chamber mounted above a vibration table provides an in-house alternative to specialized outside services for combined temperature and vibration testing of small components.
Background
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Creation of this combination test system arose from a need to test precision crystal oscillators. Crystal oscillators operate on the piezoelectric effect, in which a potential difference creates a deformation of the crystal AND the deformation of the crystal creates a potential difference across the crystal.
As part of an oscillator circuit, application of a periodic change in potential difference causes a periodic deformation at the crystal’s resonant frequency.
Also, any periodic mechanical deformation will create a corresponding periodic potential difference, resulting in the generation of unwanted sidebands on the oscillator’s output.
Crystal oscillators often find application in military and aerospace applications where they must maintain their precision under adverse environmental conditions. Typical mil-aero environmental specifications include an operating temperature range of -55° to +90°C with random and sinusoidal vibration as high as 10 g out to 2000 Hz.
Because the oscillator frequency depends on the crystal’s mechanical dimensions, thermal expansion and contraction over this temperature range can cause frequency shifts. Precision oscillators have internal heaters to stabilize the crystal’s temperature and thus frequency during environmental changes. Vibration, however, is another story.
Environmental vibration creates acceleration strain in the crystal element, which will shift its resonant frequency slightly. There are several approaches available to mitigate the effect. An active approach, useful for vibration from DC to 500 Hz, uses an accelerometer to monitor the environment and generate a compensation signal for a feedback control loop.
A passive approach, useful from 100 Hz up, isolates the crystal element from external vibration. Several such isolation techniques have been developed, as shown in Figure 1. One uses a “trampoline” type membrane (a) to suspend the entire resonator package (patent 5,896,000). Another, used in Vectron’s patented QRM (quad relief mount) precision resonators, employs mounting clips with strain relief loops (b) to suspend just the crystal element within the package (patent 6,984,925).
Combined Testing Requirement
While such techniques are highly effective in mitigating vibration-induced drift, they ultimately depend on mechanical properties and thus their operation exhibits some temperature dependence. The effects are small, but Vectron faced a design requirement that did not allow them to be ignored. A typical QRM resonator exhibits a g sensitivity of 0.1 to 0.2 parts per billion per unit of acceleration (ppB/g). Vectron had a design requirement an order of magnitude more stringent: < 0.02 ppB/g.
Mil-aero customers require that component specifications be verified by test over their full range of environmental conditions. For most components, vibration and temperature testing can be performed independently because the there is little coupling between ambient temperature and the component’s vibration response.
At this level of precision, however, the effect of temperature on the oscillator’s vibration mitigation mechanisms can be significant, making simultaneous vibration and temperature testing essential in order to demonstrate specification compliance.
This need for combined testing created a significant challenge for the design team. There are third-party facilities available that can provide such combined testing, but they are designed to test whole systems such as satellite assemblies. They utilize a vibration table set inside a large environmental chamber and are expensive to operate.
The need for design teams and their specialized equipment to be on-scene to perform iterative design modifications and re-test adds to the cost of this approach. Further, scheduling the use of outside facilities adds to the development cycle, increasing the design effort’s time to market.
Blending Test Equipment
The approach Vectron chose was to create a combination test system in-house by enclosing only the moving portion of the vibration table – the armature – within a temperature chamber (see Figure 2). This scale of this approach is more in line with the test of small components such as crystal oscillators than the system-test facilities. Further, it could be implemented by modifying standard equipment already in-house, helping to meet development cost and schedule constraints.
The design team needed to address several key concerns in creating the combination test system. One was a restriction on the weight the vibration table could handle. The table’s armature has a 400 force-pound limit. If the equipment mounted onto the armature weighed 20 pounds, for instance, the table could only generate 20 g of acceleration.
A second concern was the heat-sink effect that the armature would have inside the temperature chamber. A standard chamber does not have enough thermal capability to overcome the heat that the steel armature would transfer to the rest of the vibration table and its surroundings. Thus, some kind of thermal barrier needed to be in place between the device under test (DUT) inside the chamber and the vibration table armature outside the chamber.
A related concern was the effect of having an opening in the temperature chamber through which an armature extension could pass. A simple hole would allow outside air to enter the chamber, complicating the environmental control. It would also allow cold air to escape the chamber and trigger condensation on the armature that would drip into and damage the vibration table.
Entering The Chamber
The first attempt at marrying the two test systems involved replacing the chamber door with a panel that allowed the armature extension to pass through. This had two major drawbacks, however. One was that it required the chamber to mount face down to the vibration table, blocking access to the chamber’s control panel. The approach also made access to the DUT difficult and a dangerous. To give access to the DUT, the chamber had to be hoisted up off of the table, where it hung threateningly over the engineer’s head.
To address these issues, a hole was cut in the bottom of the test chamber to create a passage for the armature extension. As the chamber’s interior is stainless steel and the hole needed better than 0.5-inch dimensional accuracy, making the cut required the services of an outside machine shop.
Cutting into the chamber also required welding a collar inside the hole to retain the chamber’s inter-wall insulation. Coming through the bottom had the advantage, however, of allowing the chamber to be permanently mounted to the vibration table while allowing normal access to the chamber controls and, through the door, to the DUT.
Mounting Challenges
Normally on a vibration table, the DUT’s test fixture mounts directly to the armature. However, the armature’s heat sink effect along with the need to reach through the chamber floor into the interior called for use of an extension block that could double as a thermal barrier.
The block needed to be lightweight with low thermal conductivity. It also had to be non-hygroscopic so that it was unaffected by condensation, and machinable to permit creation of mounting holes.
The first material choice, wood, proved to have a limited operating life. Repeated heat/cool cycles quickly dried the wood, causing it to split. A materials search for a replacement turned up a type of phenolic that met all criteria. The use of helicoils in the mounting holes provided sufficient retention strength to withstand worst-case vibration.
To prevent air leakage around the armature extension, a gasket made of 1/16th-inch silicone rubber sheet material was used. Clamping the outer edge of the gasket to the collar made a good seal to the chamber. A large hose clamp was used to secure the gasket to the phenolic block. The material’s elasticity allowed it to move with the armature, minimizing wear.
The final configuration included a second block mounted to the armature extension. This second block, made of magnesium to minimize weight, served as the mounting point for the DUT test fixture. This configuration avoided needing to modify the armature extension for every new test fixture.
It also provided enough vertical surface area to mount the DUT on the side, simplifying 3-axis vibration testing. The total weight of armature extension, mounting block, and DUT test fixture came to about 10 pounds, allowing the combined test system to operate at up to 40g, well beyond current requirements.
The final working system (see Figure 3) met all the test requirements. The hole in the temperature chamber had no adverse effect on the chamber’s ability to maintain the required environment, nor did the gasket interfere with the vibration table’s operation. Vibration dampeners incorporated into the mounting frame prevented any potentially damaging resonances from appearing in the temperature chamber.
The combination test system thus developed to evaluate ultra-high precision crystal oscillator designs over the full range of mil-aero environmental requirements is able to handle a wide range of small components.
Vibration table weight limits and temperature chamber dimensions are the only significant restrictions on what the test system can handle. As a result, this marriage of vibration and temperature testing is now a permanent part of product development at Vectron International.