Case Studies
Ultrasonic Medical Immersion System
The customer manufactures ultrasonic water immersion systems. A commercial ultrasonic transducer is positioned in front of a test object. The transducer, excited by driving electronics, emits a burst of ultrasonic energy. This same transducer then detects echoes reflected off the test object, which are synchronized with a trigger signal from a delay gate provided by the customer. Relevant ultrasonic echoes will be finished within 50 us.
As the customer is using ultrasonic transducers with center frequencies of up to 100 MHz, he must sample the reflected signal at 500 MS/s. In between successive ultrasonic record acquisitions, a robotic positioning arm laterally displaces the transducer by 1 mm. The customer would like the transducer to move at an average speed of 250 mm/s and will write his own DOS based applications in C.
GAGE'S SOLUTION
The solution to this problem is the CompuScope 8500, an 8-bit 500 MS/s analog to digital conversion card, with an on-board memory depth of 128 kilosamples.
Is 128 kilosamples of on-board memory enough?
With a maximum record interval of 50 us and a sampling rate of 500 MS/s, the maximum record length is 250 kilosamples, as shown below.
Max Record Length = 50 us x 500 MS/s = 25000 (us)(Msamples)/s = 25000 samples = 25 kilosamples (Metric 1u x 1M = 10 -6 x 10 6 = 1)
This 25 kilosample maximum possible record length is more than accommodated by the lowest memory depth (128 kilosamples) available for the CompuScope 8500.
The Importance of the Required Pulse Repeat Frequency (PRF)
Next we must consider the required Pulse Repeat Frequency (PRF). As the customer wants to move in 1 mm steps and maintain an average speed of 250 mm/s, the CompuScope 8500 must be able to maintain a PRF of at least 250 Hz, as shown below.
Minimum PRF = (250 mm/s) / (1 mm) = (250) (1/s) = 250 Hz
Calculating the CompuScope 8500's Pulse Repeat Interval
To determine the CompuScope 8500's maximum obtainable PRF, we must first calculate the minimum possible Pulse Repeat Interval (PRI). This PRI is equal to the sum of the capture time, the PCI bus transfer time, and the CompuScope 8500's re-arm time.
- Total capture time is 50 us, as shown below:
Capture time = maximum record length / capture rate = 25 KS / 500 MS/s = 50 us
- The PCI transfer time is 250 us, as shown below. The CompuScope 8500 is an 8-bit card, so one sample of data takes up one byte of memory. The maximum PCI transfer rate is thus 100 MB/s or 100 MS/s.
PCI transfer time = Maximum record length / PCI transfer rate = 25 KS / 100 MS/s = 250 us
- The CompuScope 8500 re-arm time is approximately 30 us.
We can now calculate the minimum PRI that the CompuScope 8500 can accommodate.
Minimum PRI = Capture time + PCI transfer time + re-arm time = 50 us + 250 us + 30 us = 330 us
Determining the PRF
The maximum PRF is the inverse of the minimum PRI.
Maximum PRF = 1 / 330 us = about 3 KHz
The CompuScope 8500's PRF of 3 KHz easily accommodates the customer's PRF requirement of 250 Hz and leaves over 90% of the time available to the application program for controlling the positioning motors.
Customer's Application Program
The customer will write his DOS based applications in C. This application will set up the scan of the test object, control the positioning motors, and then will simply call C driver subroutines to acquire and download the requested records between displacements of the ultrasonic transducer.
Since the customer is pushing the PCI speed limit, it is recommended that the CompuScope 8500 be housed in a reliable and tested GagePC/PCI 586 industrial grade computer.
PRODUCT RECOMMENDED
CS8012/DIM24/PCI
24 bit, 40 MS/s dual-channel Digital Input Module for PCI Bus
This application note is provided "as is" without any warranties of any kind, either expressed or implied, including but not limited to the implied warranties of merchantability or fitness for a particular purpose. Gage Applied Technologies further does not warrant the accuracy and completeness of the material contained herein. Gage Applied Technologies may make changes to this material, or to the products described in it, at any time without notice.
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