Oriental Motor of Japan is offering stepper motors with battery-free multi-turn absolute position encoding that can ride through power-loss.
The encoder, dubbed ‘mechanical absolute encoder’, combines mechanical gearing and magnetic sensing to provide unambiguous position feedback to the motor controller up to 1,800 revolutions, and so is suited to absolute position indication in machines with long leads-crews and high-ratio gear boxes.
The encoder uses four rotating dipole magnets, plus a fifth multi-pole magnet which joins one of the dipoles on the central shaft (diagram right).
Above the magnets is a PCB with a magnetic sensing ICs (top photo) picking up the rotational position of the magnets.
Gear-wise, the central shaft drives gears two and four, and gear two goes on to drive gear three. Tooth ratios are chosen so that the rotational position of gears one, two, three and four only assume any particular relationship once every 1,800 rotations – and so can be used for feedback on closed-loop servos up to that number of rotations.
Mechanical operation was chosen deliberately to allow the system to ride across any power failure, however sudden, without battery back-up, and the company selected a resin for the encoder gears to avoid the possibility of eroded metal dust contaminating the magnetic circuits.
It emphasises the amount of testing it did to prove their reliability: “The materials of these gears, resins which have a self-lubricating characteristic and are excellent for wear and abrasion resistance, and grease exclusively for resins, are used for high durability.”
The gear train was run in one direction for 7,000 hours at high speed and temperature (1,600rpm, 85°C), and continually reversed 200 million times with 5.46 x 104radian/s2 acceleration.
In both case, “no deterioration with time for backlash was detected”, according to the company, indicating no measurable wear.
Backlash was designed to be small enough not to introduce errors within the accuracy of the system. It is “even smaller than one pitch of a gear”, said Oriental.
The encoder comes built in to the company’s AZ series of dc and ac stepper motors (right), which have custom dc or ac driver boxes that connect with the encoders.
With the encoder and drivers “tt is not necessary to do return-to-home operations”, according to the company. “Even if a return-to-home operation is required, an external sensor is not needed since the motor has its own home position.”
It also points out that homing can be done at high speed as there is no external home position sensor that needs to be approached at low-speed to avoid crashing into it.
The driver box has a push button to store the nominal home position once during machine commissioning.
AZ series drivers are available with a Modbus RTU host computer interface, and are capable of reporting many parameters – serial number, power cycle count, motor position, motor speed and time-stamped alarm history are a small sample.
