Shock - CS18
Shock calibration systems differ on one hand, with respect to the amplitude range of the shock exciter and on the other hand on the duration of shocks. Both parameters must match the technical parameters of the samples to be calibrated.
One goal of shock calibrations usually is to check the amplitude linearity of the specimen. Therefore, the shock exciter should cover most of the specified amplitude range as provided in the datasheet. The following maximum shock amplitudes can be achieved with various shock exciters:
- Shock pendulum 2 km/s2 (200 gn)
- Pneumatic Shock Exciter 100 km/s2 (10,000 gn) (experimentally up to 150 km/s2)
- Hopkinson-bar Exciter 2,000 km/s2 (200.000 gn) (experimentally up to 5,000 km/s2 or more)
In addition to the shock amplitude, the width or length of the shocks need to be examined and suited to each device to be tested. For crash tests in the automotive industry, for example, the signal of many sensors is limited to a few kHz. A cutoff frequency of 1 kHz corresponds approximately to a shock with a duration of 1 ms. Therefore, for calibration, a shock exciter should be used that allows shocks of a few milliseconds, otherwise sensor sensitivity would be too low in the damped area. Undamped sensors, however, can be damaged by shocks that are too short because they may cause resonances in the sensor element. Typical shock durations of various exciters are:
- Shock pendulum 1 ms ... 8 ms
- Pneumatic Shock Exciter < 0,1 ms ... 5 ms
- Hopkinson-bar Exciter < 40 µs
SPEKTRA manufactures a wide-ranging family of different shock exciters that are based on two working principles:
- Hopkinson-bar Shock Exciter
- Hammer-Anvil Shock Exciter
These include the manually operated shock pendulum SE-210 SP-LS (low shock) and the automated pneumatic shock exciter SE-201 PN-LMS (low medium shock). A hammer or projectile is accelerated by gravity or compressed air and hits an anvil on which the specimen and possibly the reference sensor is attached. A so-called mitigator - made of an elastic material - is positioned at the interface between the hammer and anvil and provides the pulse shaping.
Hammer-anvil exciters produce roughly a half-sine acceleration pulse of 50...100,000 m/s². The pulse width depends on the acceleration and mitigator and is 0.1...5 ms. The used reference is usually a Back to Back accelerometer.
Hopkinson-bar Shock Exciter
These include the medium Shock Exciter SE-221 HOP-MS with a piezoelectric exciter and the pneumatically operated high and very high Shock Systems SE 221. At one end of the Hopkinson bar (a titanium rod of 2 m length), an acceleration pulse is initiated by a projectile or piezo transducer. The bar shapes the pulse and provides, among other things, low transverse acceleration and no reflection within a given period. At the other end of the bar the device under test is attached. The Hopkinson bar produces a full sine pulse. References can be: Back to Back accelerometers, strain gauges, laser vibrometer.