Motor Encoders Don’t Just Revolve – They Continuously Evolve!

by Richard Anslow and Michael Jackson

The previous blog in this series looked at the history and present state of motor encoders. Even though motor tracking has been around for quite a while, that doesn’t mean these technologies are standing still. In this blog, we look at some emerging trends, applications, and innovations in the field of motor encoders. 

Lower Latency Communications 

Research carried out by Rockwell on servo drives, encoders, and encoder communication ports shows an annual growth of 20% in transceivers for feedback communication. Single-pair Ethernet (SPE) transceivers that support 100 Mbps communications over two wires (IEEE 802.3dg standard 100BASE-T1L) 1 are currently under investigation, with future encoder drive interfaces benefitting from low latency, with ≤ 1.5 µs targeted. This low latency will support faster feedback data acquisition and hence shorter control loop response time.  

Condition Based Monitoring  

Condition-based monitoring (CbM) of robotic and rotating machines, such as turbines, fans, pumps, and motors, records real-time data related to the health and performance of the machine to enable targeted predictive maintenance, as well as optimized control. Targeted predictive maintenance, early in the machine life cycle, reduces the risk of production downtime resulting in increased reliability, significant cost savings, and increased productivity on the factory floor. Using MEMS accelerometers, placed in the encoder provides vibration feedback for machines where quality control is critical. Adding a MEMS accelerometer to an encoder is convenient as the encoder already includes the required cabling, communications, and power to provide vibration feedback to the controller. In some applications like CNC machines, the MEMS vibration data sent from the encoder to the servo can be used to optimize the performance of the system in real-time. Extending the useful life of industrial assets using CbM can be complemented with robust longer-life position sensors. Magnetic sensors, which produce analog outputs to indicate the angular position of the surrounding magnetic field, can be used instead of optical encoders. Magnetic encoders can be used in areas that have higher humidity, dirt, and dust. These harsh environments impair the performance and lifetime of optical solutions.  

ADI Multiturn Memory Stores Motor Position Even When Power is Lost 

For many applications, like robots, the position of the mechanical system must always be known, even in the event of a power failure. One of the major costs and inefficiencies associated with standard robots, cobots, and other automated assembly equipment is the downtime associated with rehoming and initializing power-up following a sudden loss of power while operating. Although this issue can be solved with backup batteries, memory, and single-turn sensors, these solutions have their limitations. 

Battery packs have a limited life span, and maintenance contracts are required to manage the battery replacement. In certain environments, where there is a risk of explosion, the maximum energy that can be stored in the battery pack is limited. However, reduced energy storage capacity means more maintenance cycles because the batteries must be replaced more frequently. An alternative to battery backup is the use of Wiegand wire energy harvesting modules. These modules make use of a specially treated wire where the magnetic coercivity of the outer shell is much higher than the coercivity of the inner core. The different coercivities create voltage spikes at the device output when a magnetic field is rotated. The spikes can be used to power external circuitry and record the number of turns in a ferroelectric random-access memory (FRAM). 

In contrast, the magnetic multiturn memory (ADMT4000) that Analog Devices have developed requires no external power to record the number of rotations of an external magnetic field, reducing system size and cost (Figure 1).

Figure 1 Analog Devices’ ADMT4000 Multi-turn Position Sensor Block Diagram 

For robots and cobots, motor encoders and joint encoders typically require 16-bit to 18-bit ADC performance, with 22-bit ADCs required in some cases. High-performance ADCs, with up to 24-bit resolution, are also required for some optical absolute position encoders. Figure 2 captures graphically these emerging and interrelated trends.  

Figure 2 Emerging trends in motor encoder technology 

The next blog in this series will look at position encoder technology variations and associated tradeoffs.