ARMS Acoustic Radiometer Measurement System
- ARMS System Elements:
- The ARMS is a servo-force balance instrument for the measurement of ultrasound power. An absorbing target absorbs the ultrasound waves being measured, and thus receives a small downward force. The target is held in a nearly constant position by a servo system. By measuring changes in the current required to hold the target in place, we can measure the radiation force caused by absorption of the ultrasound. The ultrasound being measured is alternately turned on and off, and the differences in servo current are recorded and converted to measurements of ultrasound power. The forces involved are small - the radiation force resulting from the absorption of a 1 mW sound wave is equivalent to the weight-force of about 70 micrograms.
- Many servo force balance radiometer systems have been built and used in the past. In many cases, the force sensing mechanism (a modified laboratory balance) was not designed to be used under water. The NTR ARMS was specifically designed to be submersible. This eliminates the need for having part of the mechanism (a wire or fiber) pierce the air-water interface. It also allows the sound beam to be radiating downward, eliminating the need for sending the sound beam upward through a membrane.
- In many radiometer systems, the servo control system is implemented using analog circuitry. In the case of the ARMS, the servo control system is implemented in software. The position of the target is sensed and digitized, calculations are done, and a digital to analog converter controls the current to an electromagnet coil. This process of maintaining the servo loop is repeated many times a second by an interrupt service routine, which runs in the background, invisible to the user. This configuration, where the servo control system is in software, gives us far greater control over the servo loop than is possible with conventional analog circuitry. This gives us the ability to "tune" the servo loop characteristics for use with different types of targets. New target upgrades may be installed in the future without modifying the ARMS hardware.
- The ARMS consists of several main subsystems:
- Sensor Mechanism
- Control Electronics and Interface
- Tank / Frame / Vibration Isolation System
- System Controller (a PC compatible computer)
- ARMS Software
- ARMS Target Assembly ARMS Sensor Mechanism
- The ARMS target assembly is attached to the sensor mechanism by a threaded stem. Inside the sensor mechanism's stainless steel covers is a mechanism which allows the target assembly to freely move vertically over short distances. The target is made from an acoustically absorptive material, cast into a special shape to minimize reflections from the absorbing surface. The target fits into the top of the target holder. The target holder is hollow, containing air to make the target assembly and the arm portion of the sensing mechanism nearly neutrally buoyant. The volume of this hollow space (and therefore the buoyancy of the target assembly) may be adjusted by rotating the lower portion of the target holder in relation to the upper part.
- The standard target supplied with the ARMS is 76 mm (3.00") in diameter. Other target configurations can be used with the ARMS, such as a 1 cm wide target or a 45 degree reflecting target.
- The ARMS sensor mechanism uses a Hall effect sensor to sense vertical position of the target assembly. The position signal is digitized, the computer sends a calculated voltage to the A/D converter, which then controls the current through a coil in the coil assembly. The calculated voltage is chosen to keep the position signal as near to zero as possible. Above the coil assembly is a permanent magnet, which experiences a force proportional to the current through the coil. This force pushes (or pulls on) the target assembly.
- ARMS Control Electronics
- The electronics that control the ARMS are housed in a chassis as shown above. Because the servo loop is controlled by the computer, there are no front panel controls that pertain to the servo loop. The controls that are present on the front panel are for controlling the sound on / sound off status. The left hand portion of the panel is concerned with selecting a source for the signal that controls the sound. The choices are: Manual; Computer; External (Enable High); and External (Enable Low). While measuring, either the computer can be in control, or the computer can sense the state of the manual switch or one of the external inputs, to know hwne the sound is on or off. On the right-hand side of the front panel are various outputs for controlling the ultrasound being measured. These include: TTL outputs (one active high, one active low); an RF relay for switching low-level RF signals (up to a few volts); and a standard relay for switching DC less than 1 Ampere. In the middle of the front panel is a sound-on tor light.
- ARMS Tank / Frame / Vibration Isolation System
- In use, the sensor assembly is placed in an acrylic water tank. The tank is surrounded by an aluminum frame, which is suspended by four spring assemblies to isolate the tank and sensor from vibration. The spring assemblies are covered by rubber tubing to help damp out vibrations. Tubing connections with valves and standard hose thread connections are provided for filling/emptying, and for water circulation and conditioning.
