Servo System What Motion Control Feedback Devices Should Be Able to Do

Related Vendors

KOLLMORGEN Europe GmbH

FAULHABER Drive Systems

Wuxi InfiMotion Technology Co., Ltd.

The feedback device often receives too little attention when it comes to the efficient design of new machines. After all, it is usually installed invisibly in the motor housing. However, the importance of this small component for the precision, performance, reliability and integration capability of the overall system is enormous.

There is no room for deviation in industrial production. Machines have to work with extreme precision, which places special demands on the drive technology. Servo systems today therefore require precise and consistent feedback in order to maintain position, control speed and execute complex motion profiles. However, there are major differences between the feedback devices available on the market. In many applications, conventional encoders or resolvers still fulfill their purpose. However, modern applications increasingly require devices that can do more. Faster integration and shorter test times are also in demand.

With a modern feedback device, machine builders can simplify cabling and setup without having to compromise on resolution, accuracy and fluidity of movement. Machine builders should therefore ask themselves in every project: "What does my feedback device actually need to be able to do—and what is possible today?".

Understanding Your Own Requirements Profile

Very few engineers start a project by looking for the right feedback device. As a rule, the focus is on performance, reliability and selecting a system that works within the specified parameters at all times. To achieve this, you have to start with the right question. It's not about which drive technology is obviously the best. Rather, the question should be: "What performance must my application enable?".

In principle, a feedback device must provide the necessary resolution and accuracy to enable an efficient control loop. If the performance is too low, this can lead to movement disturbances, jerking in the holding positions, vibrations in the machine or positioning errors. Too much, and the required performance is far exceeded, often resulting in additional costs.

However, mechanical connections between the motor shaft and the element of the machine responsible for the movement only achieve limited accuracy. Other criteria must also be considered, such as how smooth the movement is, how noisy the signal is and how much bandwidth it offers in terms of requirements. However, these questions are usually easy to answer and most engineers know how to weigh them up.

Reduce the Complexity of the Overall System

However, modern feedback devices offer a further category of functions that go beyond simple technical performance data. They reduce the complexity of the overall system and therefore offer the greatest user benefit today.

• Simple cabling: Many servo systems have special feedback cables, sometimes with up to 13 or more wires for the feedback signal alone. However, this leads to larger cabinets, more connections, tight bends in the cable duct and a much higher installation effort. Modern single-cable solutions make all this easier. Apart from the cable for the power supply to the motor, the cable only contains two wires for the feedback data. This significantly reduces the time and effort required for installation. Especially with compact and space-optimized machines, single-cable technology makes a big difference and can even shorten the construction time of the machine.
• Integrated motor identification: Many modern feedback devices have an internal memory for the motor identification as well as the technical data and even the fine-tuning parameters. When connected to a compatible drive, the system automatically recognizes which device it is working with. This makes set-up faster, more consistent and less prone to human error. In systems where motors are easily confused—for example, a high-speed motor and a low-speed motor in the same housing—automatic detection avoids costly mismatches. Motor identification also simplifies global deployment, especially when machines are delivered to regions with different power supplies.
• Thermal monitoring: Temperature sensors are essential for motor protection, but until now they usually required their own cabling back to the drive. With the help of smart feedback systems, the temperature data can be sent via the same digital connection that is used for the position data. This combination simplifies the cabling and enables real-time monitoring of the winding temperature. As a result, the system is better protected against overheating, achieves more consistent performance and is less prone to errors.
• Functional safety: When people work in the vicinity of moving machines, safety is a top priority. Modern feedback devices support functional safety directly via the encoder. Using certified encoders, safety systems can reliably monitor the motor position and intervene in the event of movement outside the expected limits. Integrating safety at the feedback level also eliminates the need for additional sensors or relays and simplifies the system design. This allows compliance requirements to be met without overcomplicating the machine.

It's Time for A New Standard

One function that many machine builders can no longer do without is absolute multiturn feedback. The advantage: the system not only knows the shaft angle within one revolution, but also knows the total number of revolutions—even after switching off. In comparison, systems without multiturn tracking can no longer assign the position of the respective axis after a restart. In this case, the usual procedure is a reference run. The machine slowly moves to a reference point and initializes itself. Although this works, it is inefficient, time-consuming and prone to mechanical errors. With absolute multiturn feedback, on the other hand, the machine does not need a reference run at all; switch on and you're done. This means shorter start-up times, fewer moving parts (as no switches are required for the reference run) and fewer sources of error.

Multiturn functionality is realized in various ways. In some devices, small gears are integrated that count the revolutions. However, this increases the cost of the encoder, makes it more complex and lengthens its design. In addition, gears only offer a limited multiturn range of usually 12 bits. Other feedback devices rely on backup batteries. While this solution is compact and simple, it can potentially lead to maintenance issues in the future as well as reliability issues in the event of an unforeseen battery failure. Feedback devices with power harvesting technology are a forward-looking alternative. Here, just enough energy is recovered from the shaft rotation so that the number of revolutions can be stored in a non-volatile memory. This solution is compact, battery- and maintenance-free and requires the same space as a simple rotary encoder.

Choose the Right Feedback Device

This raises the question of which criteria machine builders should use to select the feedback system. For most applications, modern, digital feedback devices with absolute multiturn functionality using energy harvesting, single-cable technology, thermal monitoring and motor characteristics are the ideal and future-proof choice. One example of such a device is the SFD-M encoder from Kollmorgen, which has been specially developed for modern requirements. It is universally suitable for a wide range of motion systems and is characterized by high performance, low costs and reduced system complexity.

If systems are to be built in large quantities, it is also worth choosing coordinated components. Experienced drive manufacturers such as Kollmorgen offer a wide range of servo controllers, motors and feedback devices that are designed for optimum interaction. This simplifies integration and speeds up development. The following applies: Regardless of whether the topic of feedback is addressed at component or system level, a solution should always be selected that requires as few compromises as possible.

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Keeping An Eye on the Entire Drive System

In the past, the feedback device was not an issue. Mechanical engineers decided on a motor and received the corresponding feedback device. They wired it up and checked whether it met the resolution and performance requirements. Nowadays, the choice is a little more complicated, but modern solutions offer decisive advantages: These include single-cable technology, power-harvesting multiturn feedback, plug-and-play configuration and integrated safety and diagnostic functions. These are quickly becoming standard in mature motion control platforms.

Machine builders should therefore be aware that they are not just choosing a single drive technology, but an entire system that must function reliably and efficiently on a daily basis. It is therefore advisable not only to study the data sheet of the components, but also to consider how the feedback device supports the machine as a whole. Modern servo solutions such as the systems from Kollmorgen show how far this technology has already progressed. These complete systems simplify integration, are more reliable in the long term and give machine builders peace of mind.