Alternative to digital rotary encoder

[lindajoewater]

@backflip

When you like to have precise control of a motor, it is normal to use a digital rotary encoder to provide a feed back signal to the microcontroller. The most common encoders are optical encoders with a transparent rotor with 20 up to 1000 lines blocking the light every revolution. Digital hall element encoders are in use as well, but they typically got about 11 to 21 steps each revolution. In most cases the encoder got two sensors, so you get directional information of the rotor position.

Two alternatives to the digital rotary encoders are small, brushed DC motors or small, brushless motors (synchronous motors). You want the brushed DC motor to have at least five rotor poles. Small DC motors often got only three rotor poles, but more poles provide less ripple voltage of the electromagnetic voltage force (EMF) from it.

Some applications need to have a large speed range of the motor drive. A DC motor for feed back is a suitable alternative in such cases. Furthermore, digital encoders with a high number of optical lines can be quite expensive, while a small DC motor costs below 0.5 $. But a DC motor does not provide the positional information, that a digital encoder does.

I have seen argued, that it is hard to provide a speed range of a motor drive above 100:1, but I think it might be because of the limitation you get from the digital encoder.

This video show a test set-up with a brushed DC motor and a brushed DC motor feed-back, that got a 1000:1 speed ratio.
https://youtu.be/oQxcpLYyCSM

In the video, an Arduino Nano controls the motor via an H-bridge. You will also need four quadrant control of the DC-motor to obtain the good low speed control, because the variable reluctance (cogging moment) of the motor will make you need to provide reverse current in parts of the rotation cycle. The feed back DC-voltage from the tiny tachogenerator use two operational amplifiers to provide a low-speed and a high-speed analog signal to the Arduino, because the 10 bit resolution of the ADC would not be sufficient otherwise.

Did you ever use other kinds of rotary encoders than the digital types?

When precision control of a motor is desired, digital rotary encoders are commonly employed to provide feedback to the microcontroller. Optical encoders with transparent rotors blocking light in increments ranging from 20 to 1000 lines per revolution are the most prevalent. Additionally, digital hall element encoders with approximately 11 to 21 steps per revolution are utilized, often featuring two sensors to ascertain rotor position direction.

However, alternatives to digital rotary encoders include small brushed DC motors or brushless motors (synchronous motors). For brushed DC motors, it's preferable to have at least five rotor poles to minimize ripple voltage of the electromagnetic voltage force (EMF). While small DC motors typically feature only three rotor poles, more poles can reduce EMF ripple.

In applications requiring a wide speed range for motor drive, a DC motor with feedback serves as a viable alternative. Despite being cost-effective compared to high-line count optical encoders, DC motors lack positional information provided by digital encoders.

Some argue that achieving a speed range above 100:1 is challenging due to limitations imposed by digital encoders. However, this assertion may overlook possibilities afforded by other types of encoders.

A video demonstrating a test setup featuring a brushed DC motor with feedback achieving a 1000:1 speed ratio controlled by an Arduino Nano via an H-bridge is referenced. To ensure smooth low-speed control, four-quadrant control of the DC motor is necessary, particularly to address variable reluctance (cogging moment) requiring reverse current during parts of the rotation cycle. The feedback DC voltage from the tachogenerator utilizes two operational amplifiers to generate low-speed and high-speed analog signals for the Arduino, compensating for the 10-bit resolution of the ADC.

While digital encoders are prevalent, this discussion highlights the use of alternative rotary encoders and their implementations, such as brushed DC motors with feedback, for specific applications requiring precise motor control.