Motion System Design

Rotary motion sensor combines long life with low cost

Until now, users had to choose between economical optical encoders and durable resolvers for sensing rotary motion. But, a new type of resolver reportedly offers the best of both worlds: durability and low cost.

When manufacturers need to sense angular shaft position, they traditionally must choose between low-cost, but delicate, optical encoders and durable, but expensive, resolvers. The durability and reliability of resolvers makes them a good choice for severe operating environments, but high cost has prohibited their widespread use. As a result, many companies choose optical encoders because of their low cost, even though they are less durable.

Now, users have an added option — a new resolver-based device with analogto- digital conversion capability that offers the durability of a conventional resolver, but in a smaller, simpler, less expensive package. In its basic form, the new device will cost about $400, which is comparable to the cost of an incremental optical encoder. By contrast, a conventional resolver with its analog-to-digital conversion electronics costs at least twice as much. The new device, called a DuraCoder, was developed by Advanced Micro Controls Inc. (AMCI) of Terryville, Conn.

Conventional technology

To understand the operational benefits of the new device, first consider how conventional optical encoders and resolvers work.

Optical encoder. This device consists of a light source and a photodetector separated by a rotating glass disk. Light passes through openings in the disk and contacts the photodetector, which generates digital electrical pulses.

Optical encoders (hereafter called simply encoders) come in two basic types, incremental and absolute. Incremental encoders produce a given number of pulses for each shaft revolution. This type is useful for applications that require a precise measurement of machine position, such as X-Y tables. But, incremental encoders lose track of position if the power fails.

Absolute encoders provide absolute position information for each shaft position. They keep track of position even when power is lost. This type is useful in applications involving potential equipment damage after power is restored or where position data can’t be continuously checked visually, such as flood gates, overhead cranes, and elevators.

Though both types of encoders are accurate, their disks are very delicate and susceptible to damage in many plant environments. Thus, maintenance crews may need to frequently replace these encoders.

Conventional resolver. This device is essentially a rotating transformer, which consists of a rotor connected to a rotating machine member, and a fixed stator. AC excitation of the stator windings produces analog outputs from the rotor windings that are proportional to the shaft angle. Typically, a resolver must transmit these outputs to an analog-todigital converter before they can be processed by a PLC or motion controller. Resolvers inherently provide absolute position information.

Similar to a brushless motor that works on an electromagnetic principle, this type of device has few moving parts, and it contains no delicate mechanical components. Both of these factors enhance service life.

How it works

The DuraCoder operates in much the same way as a conventional resolver. But, it offers advantages over both conventional encoders and resolvers by incorporating recent technology advances:

• Simple design. This device has fewer parts than a conventional resolver, making it smaller and more reliable. To illustrate, a conventional industrial resolver contains two shaft-bearing assemblies. The first one, a 1/8-in. diameter shaft with bearings, supports the rotor. To protect this small shaft-bearing assembly from high loads, the small shaft is linked, via a flexible coupling, to a 3/8-in. (or larger) diameter shaft-bearing assembly that accepts radial and axial loads.

The new device, Figure 1, contains a single 10-mm (0.394-in.) diameter shaft and bearing assembly. This arrangement eliminates one shaft, two bearings, and a flexible coupling, thereby cutting both the length and cost of the unit and increasing its reliability.

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• Onboard intelligence. The new device adds intelligence through the use of large scale integration (LSI) electronic components and surface-mount packaging. These technologies put most of the electronic components on a single circuit board, Figure 1.

With these electronics, the device produces either industry-standard 4 to 20- mA analog signals or digital pulses for use by a PLC or computer. Its electronics can be configured at the factory to work with networks such as the CAN-based DeviceNet from Allen-Bradley or SDS from Honeywell.

The unit handles temperatures up to 85 C, whereas most optical encoders are rated at 60 to 70 C.

• Programmability. By virtue of its electronic circuitry, the resolver-based device is programmable to fit different applications. The user programs the required number of counts per revolution by removing a cover plate and setting simple onboard switches, Figure 1. This feature eliminates the need to stock large numbers of incremental encoders, each of which has a fixed number of countsper- revolution.

Other benefits of this device include:
• Higher load capacity than a conventional encoder. This is achieved by using a 10-mm (O.394-in.) diameter shaft and bearing rather than the 3/8-in. (0.375-in.) size used in encoders. Though the size differential is small, it permits the use of larger balls in the bearing, which boosts the bearing load rating in a basic unit by about 40%.
• Because of its construction, the designers expect that the new device will better withstand severe vibration and shock commonly experienced with industrial machinery. AMCI hopes to verify this capability during initial service applications, which are planned within the next few months.

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