Magnetohydrodynamic (MHD) Technology

ATA began its history by being involved in supporting government R&D organizations that were doing tests and demonstrations of laser weapon systems concepts. The capability of these systems to precisely point and stabilize a laser beam on long range targets was greatly influenced by linear and angular vibrations of the weapon’s host vehicle. As a result of the early test experiences it was observed that there was need for new technology in the area of accurate, high-bandwidth and rugged angular motion sensors. ATA won a competitive Air Force R&D contract to address this need. The development effort led to a new device for sensing inertial angular motions – an angular rate sensor based on the principles of magnetohydrodynamics (MHD). United States Patent No 4,718,276 was awarded in recognition of this novel means for sensing angular rates.

The MHD principle and its application to inertial angular motion sensing have been exploited in a number of different areas. Some of the areas in which MHD inertial sensors offer unique capabilities include: base motion environment characterization; automotive safety research instrumentation; projectile rate sensors; instrumentation of dynamic transients in weapon-release mechanisms; ultra-precise line-of-sight pointing and stabilization for optical systems such as laser weapons, surveillance imagers, and laser communications; spacecraft attitude control; high performance mirror and gimbal angle control; and true-north detection.

The MHD principle on which ATA’s inertial angular motion sensor operates is illustrated in Figure 1. At the heart of the sensing function is a rotational proof mass comprised of conducting fluid and a permanent magnet that is fixed to the sensor case. The case-fixed magnetic flux moves through the inertially-fixed conducting fluid with relative velocity as the case is rotated with angular velocity ω. This relative velocity between the magnetic flux and the fluid conductor generates a radially-oriented electric field, . This interaction or MHD effect produces a voltage difference between the electrode surfaces that may be amplified by a transformer or other active electronic amplifier configurations. The signal, Voltage Output, is proportional to the angular rate ω.

Angular Rate Sensor

Figure 1. The Principle of Magnetohydrodynamics Address the Behavior of a Conductive Fluid in the Presence of a Magnetic Field

The novel rate sensor idea illustrated above, via various physical implementations that exploit a variety of mechanical and electrical design configurations, offers unique products meeting the performance requirements for different application areas. The functional and performance characteristics of some of ATA’s available products are described in the subsections that follow below. Most of the products utilize the performance benefits of the MHD-based angular rate sensor. In other cases, the products are natural complements for systems employing ATA’s accurate, high-bandwidth, angular motion sensors and/or represents a subsystem in which the MHD sensor is a critical enabling technology.

The term Angular Rate Sensor (ARS) is used by ATA to designate all of the specific design implementations that have evolved capitalizing on the MHD principle for inertial angular motion sensing. Model numbers are added to the designation ARS to distinguish different designs. Some of the models are useful to a number of customers and consequently have become available as standard ARS products.

The specifications for the parameters defining the performance characteristics vary by ARS model. The three standard models currently being manufactured by ATA and that are available ‘off-the-shelf’ include the ARS-06, the ARS-14, and the ARS-15. Photographs and highlights of the key physical and performance characteristics for each of these models are described in subsections below. The descriptions also include a link to a detailed data sheet for the standard product in PDF format.

It is important that someone considering using an ATA ARS in an application have a good understanding of the physical operation, performance characteristics, and the distinction between an ARS and a gyroscope signal responses, both of which measure angular rate. This distinction is best observed by examining and comparing the generic frequency response functions of the typical ARS and the typical gyroscope which are displayed in Figure 2. Of special significance is the nominal measurement bandwidth. The gyroscope operates from DC (zero frequency) to some upper frequency (typically defined by the frequency in which the FRF magnitude is down by 3 dB). The ARS does not measure DC rate inputs, but operates over a bandwidth defined by a lower and upper -3dB frequencies. The lower corner frequency is a function of the parameters such as conductive fluid and the fluid proof mass geometry. The upper corner frequency is usually controlled by signal processing electronics employed in the ARS design implementation.

The measurement sensitivity of the ARS and the Gyro are defined with the bandwidth distinction being considered. The gyro sensitivity is the scale factor for a constant or DC rate (i.e., V/radian/sec or V/degrees/sec). The ARS sensitivity is also the scale factor for a rate, V/rad/s, but at a frequency in the measurement band, e.g. 10 Hz. The ARS does not have a response at DC frequency, i.e. constant rate inputs. The ARS is useful for accurately measuring environmental angular vibrations, transient motions, automobile crash test responses, and other motion cases in which constant rate sensing is not required.

Angular Rate Sensor

Figure 2. Characteristics of Typical ARS and Gyro Frequency Response Functions

The website subsections and linked references provide an overview of the sensitivity, the measurement dynamic range, and bandwidth of operation for the three standard ARS models. The least sensitivity is associated with an ARS-06 model; and the most sensitivity with the ARS-14 model. The ARS-15 sensitivity falls between. The errors in the measurement of angular rate, or measurement noise, are another important characteristic in the selection of the appropriate sensor model. The data sheet that is available for each sensor model provides guidelines for evaluating and determining which ARS model is the most appropriate for a user’s application. The data sheets for the sensor models also provide general information on mechanical and electrical interfaces. ATA also can provide custom ARS designs, manufacturing, and validation tests that meet special customer physical and performance specifications. ATA also offers ARS-based, multi-axis packages for measuring 2-D and 3-D angular motions suitable for ground, air and space vehicles. Data sheets and details of the specifications for these packages are provided in other subsections of the inertial sensor products listing.