There is common belief that two servos with the same power range from different manufacturers are roughly equivalent, and that the only other significant comparison point is price. This is not true!
Here are some of the important ratings and features you can’t afford to ignore:
Unfortunately, torque and speed ratings are not consistent throughout the motion control market. This makes it difficult to compare specifications. Let’s dive a little deeper to explain this further.
Servo speed: Servo motors typically have a rated speed and a max speed measured in RPM (root-mean-square, or roughly speaking, the average). Max RPM is dependent on a few factors – some of which are bearing ratings, tolerances within the motor and electrical construction.
The part that is not consistent across the industry is how long the servo motor can operate or the amount of torque available at max speed. However, each manufacturer has specifications and graphs to help you make the right selection.
Servo torque: As you might remember from science class: torque is required to produce work. But, did you know that a servo’s torque range is divided into two categories?
They are continuous and intermittent duty.
Generally, the continuous duty range should represent the torque the servo motor/drive system can deliver 24/7 without overheating or otherwise damaging the motor. The intermittent duty range refers to a much higher set of torque values the servo system can deliver (for only short bursts of time). These “bursts” are typically used for acceleration, deceleration, rapid movement with a long dwell or dealing with brief load disturbances.
Servo power: There is no industry standard that requires rated torque values to be measured at the same speeds for all servos, as opposed to NEMA induction motors. The equation for the power of a motor is expressed in terms of both torque and speed; neither have standard rated values for servos in the motion control industry.
Therefore, it is critical to select a motor that has the required torque at the machine’s operating speed, instead of fixating on the power rating. A servomotor’s internal stator windings can be wound to provide more speed and less torque, or more torque and less speed, at the same wattage.
The amount of time a servo can continue to deliver torque in the intermittent range (sometimes referred to as the “overload time”) varies widely among servo manufacturers; it is not always clearly specified. This feature alone can make a significant difference in the types of tasks a servo system can perform without premature failure.
When sizing a servo, remember that the torque requirements must be in the continuous duty range for the servo to operate without overheating. The duty cycle of a servomotor is limited by the heat it can dissipate.
Servo inertia: So far only torque, speed, and power have been discussed. Inertia is another important specification to consider when selecting a servo. The ratio between the servo motor’s rotor inertia and the inertia of the load (the load coupled to the motor’s shaft) is critical to achieve high performance.
By definition, a servo is a closed-loop system where its control algorithms are constantly changing the current in the motor. Good current control is vital and dependent on the impedance of the motor circuit.
The current sent to the motor is based on complex calculations that involve the differences between its feedback and its commanded values for position, speed, and torque. The digital drives available in today’s market boast the ability to have current regulation in micro seconds (uS) for faster calculations. The inertia ratio between the motor and the load will significantly affect the servo system’s ability to accurately control the motor.
If the ratio is too high, the motor will overshoot its target and cause oscillations. These oscillations can be minor, such as a slight wiggle when the motor stops, or major, such as violent and loud vibrations that can damage the machine. Faster execution of the calculations can help, but a proper inertia match is key.
High-performance servomotors available today have low or high inertia, permanent-magnet rotors and can provide a large amount of torque in a small package. It is important to select the proper mechanical transmission (such as a gearbox, ball-screw, or belt-and-pulley) to achieve a load-to-rotor inertia ratio within an acceptable range of:
10:1 (average performance), 5:1 (high performance) or 1:1 (highest performance)
Servo system resolution: Another important factor is the resolution of the feedback device.
Encoder resolution is constantly rising. It is not uncommon for encoders to have 20-bit or greater resolution. A 20-bit encoder has more than 1 million pulses per revolution! (2^20 = 1,048,576)
While this may seem like overkill, the purpose of a servo is to determine the difference between commanded and actual positions while also driving that error value as close to zero as possible. The higher the resolution, the finer the servo system can detect the movement; it will make the proper correction, resulting in more stiffness and tighter control of the speed/position of the load.
Servo system frequency response, bandwidth: The servo system’s ability to calculate and deliver current—and therefore torque—in real time can be another area where servos vary greatly.
The frequency response of a servo is a measure of its ability to follow changes in the command signal. You can have micro second calculations and high-resolution feedback, but if the system doesn’t have the band width to respond, you’re not in control.
A servo’s bandwidth measurement is defined when a sinusoidal signal is commanded into its speed loop. The frequency of the sine wave is raised until the servo cannot change the shaft speed to match the commanded signal.
When the actual speed falls to 70.7% (-3 dB) of the command signal, that frequency is measured as the bandwidth. Over the last 24 years, the speed loop bandwidths of high-performance servos have increased tenfold, from levels below 100 Hz to those now exceeding 1 kHz.
Servo system controls: As servo performance has increased, the size of the electronics in the amplifier and controller have decreased. Thermally efficient designs require less space between amplifiers. These advances help shrink the footprint of the electronics, which results in significant cost savings for the overall system. Smaller control cabinets and the resulting real estate savings can be used to create a more efficient use of the factory floor.
Along with advances in power electronics, there have also been many improvements in servo system controls. Most modern servo systems have network-based architectures, which lower the implementation costs and improve diagnostic capabilities. The reduction in wiring also increases the speed at which engineers can commission multi-axis systems, resulting in higher profits and greater throughput. Network connectivity from the machine’s servo system control to the factory SCADA and Manufacturing Execution System (MES) is an absolute must in this age of real time data. The added diagnostic capabilities will reduce downtime and allow for remote resources to quickly troubleshoot problems.
Of course, these features are of little benefit if the system suffers from inferior quality.
Cheap is not always better!
Supplying a machine that needs repair the day after warranty ends tends to label the OEM as a “junk supplier”. It is important to choose a manufacturer that has a track record of great quality and the data to support those claims.
Mean time between failures is a statistical measurement of the quality and reliability of a product. Asking for this information before your purchase can help you choose a motion control partner whose product offering provides a lower total cost of ownership.
There are many factors to consider before purchasing a servo system. When comparing servos, remember—the criteria for determining true value goes far beyond wattage and price!
Sales Manager & Engineering Manager
Axis NJ, is a high-tech distributor. We specialize in automation, drives, motion control, machine vision, machine safety and robots. Our staff of specialists are available to assist you with projects of any scale. Our headquarters is in Somerville, NJ and branch office in Long Island. Axis has been covering Metro NY and New Jersey for over 30 years.