TECHNOLOgy REViEW


determined whether incremental or absolute encoding is required, the key selection criteria are rotational speed, measured in revolutions per minute (rpm), and angular resolution, either quoted as the number of positions per revolution or the equivalent number of bits. Of course, if the intended application is going to subject the encoder to extended use at high speeds then some consideration of life expectancy is appropriate i.e. choosing an encoder with suitable high-performance ball bearings. Beyond this, normal considerations of quality, reliability and price will apply. Rather than considering these factors


from an abstract position, it is easier to look at a number of representative products from various manufacturers to understand their advantages in the type of applications they are intended for. Bourns Inc. manufactures a wide range of


encoders, featuring both magnetic and optical types. It places considerable emphasis on quality and reliability, especially for heavy-duty applications in tough environments such as industrial, medical and military. The AMS22S product is a single-turn, magnetic rotary encoder that is highly resistant to vibration


or shock and the ingress of fluid or dust. The robust design, shown in an exploded view in figure 2, features a sturdy mechanical construction to resist side loads and also axial forces acting on the shaft. The encoder uses a programmable quad Hall Effect IC to measure rotational angle, which can output either as a linear analogue voltage or a PWM waveform, in both cases with 12-bit resolution. These are factory pre-set options, as is the output voltage range, which can be programmed between 1-99% of VDD and the effective electrical angle, which can be defined between 10 and 360 degrees in 10-degree increments. From an environmental and mechanical perspective, the AMS22S is specified for an operating temperature from -40o to +125oC with a rotational life of 50 million revolutions at speeds up to 120 rpm. Target applications are controls (valves, pedals, steering), geared DC servo motor systems and camera pan/tilt positioning. For absolute encoding with a digital output,


Bourns offers the EMS22A, which is also a non-contacting magnetic encoder. This has 1024-position angular resolution with the data output serially as a 10-bit code. It is also suited


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to harsh environments and, because it uses a ball bearing, it can operate up to 10,000 rpm. Factory ordering options allow for 3.3 or 5V supply, clockwise or anti-clockwise coding, different bushing types and various shaft style/ length and terminal configurations. Its target applications are industrial automation, robotics and automotive. Bourns also manufactures optical


encoders for incremental applications where a simple 2-channel quadrature output, for interfacing directly to microcontrollers without A/D conversion, is sufficient to measure rotational position changes or speed. The EN series includes a ball-bearing bush option, supporting shaft speeds to 3,000 rpm and a rotational life of 200 million revolutions or a regular spring-loaded bush (for optimum feel) that is good for 300 rpm and 10 million revolutions. Different ordering codes allow a choice of resolution from 25 to 256 positions (i.e. 8-bit) as well as various mechanical and electrical termination options. Bourns’ EM14 optical encoder, with push switch, is clearly


continues p 14 > focus magazine - issue 16


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