Servomotor and drive fundamentals

Perhaps the distinguishing feature of all servomotors is that they output motion under closed-loop control. They rely on feedback from encoders as well as control signals from acontroller and drive to operate the motor. Most servomotors are rotary motors and produce precise torque and speed, most often used in positioning applications.

Manufacturers classify motors for constant-speed tasks by horsepower or torque at base speed. In contrast, servomotors operate over varying speed ranges and aren’t rated in this way. Instead they have speed-torque curves that express continuous torque capabilities (that won’t threaten to overheat the motor) and intermittent or peak torque for acceleration.

Keep in mind that the term servomotor can mean different things depending on the context. Convention is that the term often (though not always) refers to what industry calls dc motors — both brushed and the costlier (but longer-lived) brushless servomotors.

There are also instances of classifying induction-motor-based designs running off vector controls as servomotor setups where the design incorporates feedback (usually from an encoder) to track and control speed and sometimes even position. These induction motors typically adhere to NEMA or metric standards, whereas other servomotor offerings are less uniform.

Many non-direct-drive servomotors have top speeds up to thousands of rpm. To better leverage their full capabilities, designers will often combine such motors with gearing to trade an increase in output torque with lower output speed. Much of the time, this gearing takes the form of planetary or harmonic gearheads, precision arrangements with high accuracy and efficiency. In many instances, gearing even lets machine builders use smaller motors on some axes. This equates to cost savings that may even offset the price of the additional gearing.

The 2G Motion System from Kollmorgen is based on the latest thinking about single-source, systems-based design. It harnesses the capabilities of Kollmorgen’s AKD2G servo drive and AKM2G servomotor. Motor and drive are precisely matched in every element, from drive switching frequency and commutation algorithms to motor magnetics. This avoids microincompatibilities that can occur when engineers select components from various manufacturers.

SELECTING A SERVOMOTOR

Here’s a quick guide to selecting the right servomotor for an application. First, start off by using the application inertia information to define how much load the motor will have to move. Next, determine the application speed or velocity, and how far and fast the load needs to travel. Calculating torque is next. Then, plot them on the prospective motors’ torque-speed curves — as the servomotors’ continuous and peak torque limits over the axis’ full speed range.

This bare-bones approach to picking and sizing a servomotor is a start, as the process may be a bit more complex. Still, plenty of manufacturers offer software programs to help simplify the selection process. What’s more, once a designer has the parameters for an axis and its motor, they can setup the drive to protect the rest of the system’s components by preventing excessive torques and other problematic conditions.

Servo systems are used when the application calls for precise control of position, velocity, or torque — or a combination of the three. Depending on the parameter being controlled, the servo system can be operated in torque mode, velocity mode, or position mode. Each mode requires control loops that allow the servo drive and controller to monitor the influencing parameters and provide the right commands to the motor to achieve the desired performance.

SERVO CONTROL – TORQUE MODE

In torque mode (also referred to as current mode), the current loop controls the motor’s behavior. Since torque is directly proportional to current, the servo controller obtains the actual motor current from the servo drive and uses this to determine actual motor torque. It then compares the actual torque value with the desired torque and adjusts the current delivered to the motor to achieve the desired torque. The current control loop is typically tuned with a PI (proportionalintegral) controller, and current loop parameters are often set by the manufacturer.

Applications that require torque mode control range from winding, where constant tension must be held on a web of material while it’s wound, to injection molding, where constant clamping force must be applied to the mold.

The amount of torque a motor produces depends on the amount of current it receives. And torque determines the motor’s acceleration, which affects velocity and position. Thus, servo systems always include a current control loop.

SERVO CONTROL – VELOCITY MODE

When an application requires that the motor maintains a set speed, even under varying loads, velocity mode is used. In velocity mode, the motor speed is controlled by the amount of voltage sent to the motor. But to change the motor’s velocity (to accelerate or decelerate) requires an increase or decrease in motor torque, so a current control loop is also required in velocity mode.

When more than one control loop is used, the loops are cascaded, with current control being the innermost loop and the velocity control loop added “around” the current loop. When a position control loop is used, it’s added around the velocity loop, forming the outermost loop. Tuning is done from the inner to the outer loop, so the current loop is tuned first, then the velocity control loop, then the position control loop.

Many advanced servo controllers can switch between control modes “on the fly” — transitioning from velocity mode to torque mode, for example, while the system is in operation without creating instabilities or interruptions.

The velocity control loop obtains speed information from an encoder or resolver to determine the error between actual and commanded velocity and uses this error to determine what current (torque) is required for the motor to correct the speed error. The velocity control loop is typically a PI controller, and servo systems operating in velocity mode sometimes include parameters that smooth acceleration and deceleration to minimize the effects of jerk.

Examples of applications that use velocity mode are conveyor tracking, dispensing, and machining processes such as grinding or polishing, where motor load varies but velocity needs to be maintained throughout the process.

WHEN THE APPLICATION CALLS FOR ALL THREE CONTROL LOOPS

Servo systems can also be operated in position mode, allowing the motor to move the load to a precise location, either relative to a starting location or based on an absolute position. To achieve position mode in servo control, all three control loops are typically required: torque, velocity, and position. This is because the motor’s speed must be monitored to determine its position, and torque must be monitored to determine how much current the motor needs in order to reach the commanded position, without undershoot or overshoot. The position control loop uses a PI or a PID (proportional-integral-derivative) controller.

WHAT ARE SERVO FEEDBACK GAINS, OVERSHOOT LIMITS, AND POSITION ERROR LIMITS?

Servo tuning can be accomplished by several methods, but the most common way is to use a PID algorithm. The PID algorithm uses three feedback gains—proportional gain, integral gain, and derivative gain—to compare the commanded position (or velocity) with the actual value and issue commands to correct errors between the two.

FEEDBACK GAINS

Proportional gain (Kp) determines the amount of restoring force (generated by the command voltage) that is applied to overcome the position error. The term “proportional gain” is used because its value is directly proportional to the positioning error. For example, if the proportional gain is 1.2 volts per encoder count, and the motor is 10 encoder counts from the commanded position, the command voltage will be 12.0 volts.

Proportional gain is the most important component of the PID algorithm, but a Kp value that is too high can cause the system to oscillate, to become under-damped, or to become unstable.

Often, as the servo controller works to decelerate the motor by reducing the command output, system friction overcomes the command voltage and causes the motor to fall short of the target.

Integral gain (Ki) overcomes this by producing a command that “pushes” the system to zero positioning error at the end of the move. The term “integral gain” is used because its command increases over time at the end of the move.

If sufficient positioning accuracy is achieved with the proportional gain, then integral gain may not be necessary. However, it is useful when steady-state (static) positioning is difficult to hold due to system disturbances, or when constant velocity motion is required.

Newer hybrid motor designs, like the TSM14POE StepSERVO integrated motor from Applied Motion Products, offer closed-loop servo control using high torque step motors. It uses a power over Ethernet (PoE) single-cable to satisfy motion-control applications requiring minimal wiring and a small footprint. Combining power and Ethernet communications in a single connector simplifies wiring to the integrated motor so it can mount almost anywhere.

Derivative gain (Kd) determines the restoring force that is proportional to the rate of change (derivative) of the positioning error. It works in conjunction with proportional gain to dampen the system response and reduce overshoot and oscillations.

Servo instability can occur if the derivative gain value is calculated too frequently, as it will begin to work against, rather than with, the proportional gain. To avoid this, the derivative sampling period can be increased.

OVERSHOOT LIMITS

Derivative gain is used to set the overshoot limit, or the acceptable amount by which the servo can exceed the target position. While perfectly accurate positioning is desirable in theory, in real world applications, an overshoot limit that is too small will likely cause the system to be over-damped. Conversely, a very high overshoot limit gives good system response, but can lead to oscillations.

POSITION ERROR LIMITS

The principle behind servo control is the comparison of the system’s actual value versus the commanded value, and the difference between the commanded and actual position is the position error (also referred to as the following error). The purpose of feedback gains is to reduce the position error. The position error limit, however, is used to indicate a problem with the servo or with the tuning algorithm. The position error limit should be set to a value that would not be seen during normal operation, and the controller response should be programmed to stop the axis when this limit is reached or exceeded.

Why is the bandwidth of a servo control loop important?

A servo drive can include any combination of three types of control loops—aposition loop, a velocity loop, and a current loop. While each loop’s purpose is tocontrol a different aspect of the motor’s performance, they are all characterizedby a common parameter: bandwidth. The bandwidth, or response time, of thesystem is a measure of how fast it responds to the changing input command. Inother words, the bandwidth of the control loop determines how quickly the servosystem responds to changes in the parameter being controlled—position, velocity,or torque.

In servo drives, the bandwidth of a control loop is defined as the frequency at which the closed-loop amplitude response reaches -3 dB. At this point, the output gain (ratio of output to input) equals approximately 70.7% of its maximum, and the output power (power delivered to the load) equals 50% of the input power.

While higher bandwidth generally provides stiffer motor performance, decreases error, and improves transient response time, there are also drawbacks to high bandwidth in servo systems. Specifically, the higher the bandwidth, the higher the frequency at which the motor responds to disturbances, which typically requires higher accelerations and forces.

Power dissipation has a squared relationship to force, so any increase in bandwidth significantly increases power dissipation (i.e. heat), and therefore, the temperature rise of the motor. And because temperature is a limiting factor in motor operation, the motor characteristics may actually limit the allowable bandwidth of the servo drive.

Note: Other components in the system — including the resolution of the feedback device, the update rate of the drive, the motor-load inertia ratio, and the rigidity of the motor-load coupling — also affect the maximum achievable bandwidth of the drive.