About the author:
Larry Sears is senior technical advisor for Hexagram, Inc., an ESCO Technologies Co. He can be reached by e-mail at [email protected].
Since the first public water systems were created in the 1800s, water meters have been installed in basements or pits to prevent freezing; unfortunately, their locations make them difficult to read.
Beginning in the 1960s, manufacturers began to develop equipment that permitted meters to be read without direct access. In the 1980s, interest grew in automatic meter reading (AMR), which allowed meters to be read even more efficiently.
All water meters that are to be electronically read must have some means of communicating the meter information to an electronic AMR device. The AMR device may be a remote product, such as a “touch pad” or electromechanical display that can be read or interrogated by an onsite meter reader. It may also be a short-range wireless device that is attached to the meter and communicates with an onsite meter reader. Finally, the AMR product may be a long-range wireless device that transfers the data over a fixed-network to the utility office.
In all of these cases, there is a requirement that the water meters have some type of electronic output. This article discusses the various technologies that are employed to provide this electronic interface.
Water meter basics
Measuring flow can be accomplished in a number of ways. For residential applications, the two most common approaches are turbine and positive displacement technologies. The turbine meters (known as single- or multi-jet meters) incorporate a turbine that rotates at a speed proportional to flow. By counting the rotations of the turbine shaft, flow is derived. These meters tend to be small and low in cost, but the turbine can provide very little output torque; even slight loading of the shaft will change the meter’s calibration.
Positive displacement meters employ a piston-cylinder arrangement, which continuously parcels out the water in a series of defined, known volumes. These meters are capable of providing reasonable torque.
Both turbine and positive displacement meters contain a small gearbox, or “register,” that is coupled to the output shaft. The gearbox divides the rotations by an initial scale factor and then drives a series of dials that represent decades of usage. These dials provide a basic visual readout for manual reading.
For AMR purposes, it is necessary to convert this mechanical readout to digital form. A device that converts mechanical location or displacement to an electronic signal is called an encoder. There are two types of encoders, incremental and absolute, and there are fundamental differences between them.
Incremental encoders
The incremental encoder is a relatively simple device that produces electrical pulses as a shaft rotates. In general, an incremental shaft encoder utilizes a sensing means that opens and closes a circuit or otherwise produces a voltage pulse with each shaft rotation.
The rotating wheel has a bump that closes a contact every time it completes one revolution. If there were two bumps, there would be two pulses per revolution. The number of shaft rotations can be recorded by simply counting the pulses. Incremental encoders may employ various types of sensors; non-contact magnetic sensors, such as reed switches, are very common.
When incremental encoders are used with water meters, the sensor is located in the register gear train so that it will produce one or more pulses for each gallon of flow. Note that the number of pulses per gallon is defined as the meter “resolution.” High resolution is valuable because it can enable leak detection and more precise billing.
Incremental encoders can be extremely effective. However, they can only register motion if some device is always available to count and store the output pulses. If the device misses pulses or loses track of the count, there is no way to go back and determine the number of rotations that have occurred; only new pulses can be recorded, and an error results. Thus we can never be absolutely sure about the amount of usage because only incremental usage is recorded.
Obviously, incremental encoders used with water meters must count reliably or else the value displayed by the AMR device will differ from the usage recorded by the mechanical register at the meter. Therefore, in addition to requiring an accurate sensor, the connection between the encoder and the AMR device must never be broken. For this reason, if the meter and counter are physically separate, there is usually a provision for sensing and reporting tampering.
Incremental encoders can be extremely reliable, may require zero torque, provide high resolution and can provide essentially infinite life. However, the need to record and maintain the count increases the complexity and cost of the electronics associated with an incremental encoder. In particular, a battery or other uninterruptible power source is required. This requirement is inherent in the use of incremental encoders, and it may be a disadvantage in some applications.
One special type of incremental encoder requires mention because of its important place in the history of AMR. The “generator remote” first appeared in the 1950s, and was the first practical solution to inaccessible meters. These devices were used exclusively with positive displacement meters. As the output shaft rotated, it would wind a spring, which stored mechanical energy. After exactly 50 or 100 revolutions, an escapement would trigger and release the stored energy into a mechanism that would spin a small electric generator. The generator would generate an electrical pulse, which would increment an electromagnetic counter that was remotely mounted in an accessible location and wired to the meter. The meter reader could then perform a visual read. This equipment was supplied by most meter vendors and was, for many years, the only solution to inaccessible meters.
Although conceptually ingenious, the generator remote suffered from poor engineering and general technical mismanagement. The generator “head,” although somewhat complicated, was relatively reliable. Unfortunately, the remote counter was a fairly precise device that had to be sensitive enough to be actuated by the limited energy provided by the meter generator. These counters could be inaccurate as they “lost” counts due to shaft binding caused by flexing of the mechanism, freezing and insect intrusion. Surprisingly, the counters would, occasionally, also register extra counts; this was usually due to the counter incrementing due to slamming doors or other mechanical shocks.
Currently few, if any, meter manufacturers continue to manufacture these devices. However, the history of the generator remote illustrates the crucial requirement that a meter-reading device using an incremental approach must be very carefully designed. It is also worthwhile to note that the unfortunate history of the generator remote has left a legacy of doubt regarding the inherent reliability of incremental encoders. Given the poor execution and support of the generator technology, this reputation may be unwarranted and unreasonable.
Absolute encoders
The absolute encoder is more complicated than the incremental encoder, but it offers the advantage that it can display the actual position of a shaft at any time, without depending on an external counter to derive it implicitly. An absolute encoder employs a group of switches or sensors to produce a unique digital “word” for each position of a wheel. In one common design called a “brush” encoder, the wheel contains four tracks of copper segments, with each track containing 10 segments. There are four brushes, or contacts, which slide over each of the tracks as the wheel rotates; each brush represents one “bit” of a four-bit digital word. The presence or absence of the copper trace makes or breaks a circuit and changes each bit from a digital “one” to a digital “zero.” The traces are arranged in a special pattern to produce a unique output for each of the 10 positions.
The segments are arranged in a special pattern so that 10 unique digital “words” are produced as the wheel rotates one revolution. The signals from the four brushes can be connected to digital logic, which converts the digital word to a format that can be displayed or transmitted.
The absolute encoder has the important advantage of providing the correct value at any point in time, without the need for a counter and memory to keep track of position. Therefore, an absolute encoder provides an absolute indication of shaft position. It is only necessary to momentarily energize the associated digital logic and read out the current position of the dials; indeed, it is not even necessary to be connected to the encoder except during a reading.
The inherent absolute nature of absolute encoders is very valuable, and the fact that power is required only for reading is important in many applications. The absolute encoder does, however, have several disadvantages compared to the incremental approach. First, whereas the resolution of the incremental encoder can easily be made very high, increasing the resolution of the absolute meter requires additional encoded dials, which increase cost. Also, the construction of the absolute encoder is complex; in particular, the common “brush” design contains numerous small parts and is subject to contact problems associated with insulating films and finite lifetime due to wear. Finally, all absolute sensors possess the problem of “transition ambiguity.” Because the absolute encoder steps through a series of finite steps, there always exists a region of uncertainty between transitions as the wheel rotates from one number to the next. Various electronic techniques can minimize the effect of transitions, and the quality and precision of the encoder will also have an effect. The common approach to overcoming this problem is to verify that the encoder output from each wheel is valid; if not, then an error code or special symbol is transmitted until the wheel fully rotates to complete the transition.
When the absolute meter is used with manual reading, it often is used in conjunction with a “touch pad” that is wired to the meter. When the meter is to be read, the meter reader places a probe near the pad. Both the probe and the touch pad contain a coil, which comprises one-half of a transformer. The probe transfers energy to the pad, which provides operating voltage and “wakes up” an integrated circuit in the meter. The encoder provides the dial position information, which the integrated circuit converts to a serial data stream. The data is transmitted to the touch pad, where it is received by the probe and sent to the handheld.
In drive-by and fixed-network applications, the AMR device provides the voltage to energize the meter circuitry when the AMR device requests a meter reading. The serial data produced by the meter is processed by the AMR device and transmitted to a nearby vehicle or back to the utility.
The absolute encoder may offer security advantages over incremental devices; if the wire between the meter and the pad or AMR device is broken, there will be no error as long as the wire is connected at the moment a reading is taken.
Water meters with absolute encoders contain the same basic mechanical elements as meters with incremental encoders. The meter body output shaft is connected to a gear reducer (register) that drives the display dials. Each of the six display wheels is equipped with a 10-position absolute encoder. Wires from each of the contacts (4 X 4 = 16 wires plus a common) connect to an integrated circuit within the meter that formats the data and prepares it for output.
The mechanical “brush” encoder has long been commonly employed in water meters, but long-term field experience has indicated the need for more reliable designs. Because the absolute encoder only needs to be energized when the meter is to be read, it is possible to employ “active” sensing instead of passive contacts and still obtain suitable battery life in AMR applications. A number of proprietary technologies have recently been incorporated into encoder meters. These include various optical, capacitive and inductive sensing means. In all cases, these approaches feature non-contact sensing, which is a tremendous improvement over the sliding brush contact. Non-contact sensing will provide essentially unlimited life, and its low-torque requirement improves meter accuracy and permits its use with turbine meters.
Absolute vs. incremental
Both incremental and absolute encoders are used in the water meter industry, and the issue of which is “best” has prompted some debate. Industry jargon refers to “pulse” meters and “encoder” meters, yet this terminology is inaccurate since both meters contain an “encoder,” which can be either incremental or absolute. What the water industry refers to as a “pulse” meter actually contains an incremental encoder, and an “encoder” meter contains an absolute encoder.
The question of which is “best,” absolute or incremental, is a somewhat provocative question in the U.S. water industry. Absolute encoders appear to be preferred by many in the U.S., yet European water utilities use incremental encoders almost exclusively. Also, U.S. gas utilities have millions of AMR devices in the field that use incremental encoders, and there is little concern about their performance. Also note that, in general industrial and commercial applications, the use of absolute encoders is actually relatively rare; incremental encoders are used in far greater quantity. As mentioned above, some of the opinions held by the U.S. water industry may be a result of past experience with poorly designed products. Nevertheless, there are very real technical issues that may favor one technology over another.
Although the differences may blur as technology advances, it is possible to make some general statements about encoder selection. Incremental or “pulse” meters offer the following advantages: greater resolution; less expensive; compact size; and touch pad circuitry is low in cost. Absolute or “encoder” meters, on the other hand, offer: less opportunity for tampering or data loss; lower battery drain; compact size; and no ambiguous states.
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