Absolute encoder offers new options for controlling DC motors

The latest example of an innovative design philosophy applied to the control of DC motors with the emphasis on developing compact, flexible, energy-efficient solutions, has been introduced by Rotalink. The company's new output-shaft absolute encoders provide accurate positional information to a resolution of ± 0.35 degrees with a performance which is unaffected by power loss and is self-correcting in the event of electrical interference, says the company. The absolute encoders operate via contactless Hall-effect technology which provides either a linear analogue output (0-5V) or reliable 10bit positional data sent via a synchronous serial interface (SSI). Notably, when the digital option is chosen the benefits are longer life span, higher reliability and improved noise resistance. The encoder is particularly effective when used in conjunction with Rotalink’s 210 gearbox and 24mm brushed DC motor. The result is a compact and low-cost servo package measuring just 50mm x 50mm x 34mm, weighing 100g and offering important advantages over a hybrid stepper motor of equivalent size. For example, at low speeds (<10rpm) it can achieve up to twice the torque whilst offering the same levels of accuracy (+/- 1 degree). Whilst a stepper motor option will offer greater accuracy if micro-stepping is used, the downside is that it reduces torque to a fraction of its full step value. The high starting torque of the brushed DC motor makes the absolute encoder option particularly well suited to high-inertia loads. Also, closed loop positional feedback solves the problem of overshoot, or missed steps. Furthermore, if used with a high gear ratio long enough to provide less than 10rpm, the detent torque of a brushed DC motor is high enough to hold position. This can be achieved without the need for continuous power and therefore brings the added benefits of energy savings. The absolute encoders are available as single turn and multi-turn options. The single turn option operates through the full 360º of rotation to accurately monitor its position indefinitely and without power. The multi-turn unit operates to a maximum of 32000 revolutions whilst sill providing the full resolution of 10 bits per 360 degrees and is fitted with an integral rechargeable battery to provide full position monitoring for up to 100 hours when mains power is removed. The use of an extended SSI interface with the multi-turn unit allows the end-user the option of programming of zero position and maximum rotation limits as well as resetting any error codes, says the company. Typical applications include motorised valves, pan / tilt mechanisms and vending carousels.