PALKI

Torque is proportional to current.

Torque*Speed = Power

KT =Torque constant (lb-in/A)

RMS torque is important for supply and thermal considerations.

The information within these three profiles is the foundation of system design.

From the velocity and torque profiles you can narrow your motor selection to models that are able to provide the torque and speed required

Based on the motor data (Kt - torque constant, Kv - voltage constant, Rm motor resistance) you can then determine the system current and voltage requirements

From the peak torque you can calculate the peak current (I). I = T / Kt

From the peak velocity you can calculate the peak voltage (V). V = Speed * Kv + I * Rm

When the current and voltage requirements are known you can then select a servo drive.

Starting with the velocity profile, the torque profile can be derived by taking the derivative of velocity. A positive slope in the velocity profile will be positive torque and negative slope will be negative torque. The steepness of the slop corresponds to the magnitude of the torque.

Next the power curve can be derived by multiplying the Velocity curve with the Torque curve (torque x speed = power).

The S-curve motion profile allows for a gradual change in acceleration. This helps to reduce or eliminate the problems caused from overshoot, and the result is a great deal less mechanical vibration seen by the system. The minimum acceleration points occur at the beginning and end of the acceleration period, while the maximum acceleration occurs between these two points. This gives a motion profile that is fast, accurate and smooth.

The trapezoidal motion profile slopes the velocity curve to create predictable acceleration and deceleration rates. A trapezoidal motion profile is shown in figure 3. The time to accelerate and decelerate is precise and repeatable. Ta and Td still exist, but they are now specified values instead of random values.

If ta = td = T/3 for a trapezoidal move profile, the overall power used is a minimum

Overshoot error still exists for a trapezoidal move, but this error is negligible for many systems.

Higher precision machines require a different motion profile.

This motion profile maintains a constant velocity between points (see figure 2a). This is the most basic motion profile because only a velocity command is used.

Constant velocity would be used in something like a conveyor or a fan.

Precision positioning machines do not use the constant velocity profile because a real world machine cannot change velocity instantly. There will be a time delay that will fluctuate with changes in the load and system. In figure 2B, the dotted line represents the actual velocity path the load will take. Ta and Td represent the time required to accelerate and decelerate. These times may vary with fluctuations in the load.

The purpose of all servo systems is to move some kind of load. The way in which the load is moved is known as the motion profile. A motion profile can be as simple as a movement from point A to point B on a single axis, or it may be a complex move in which multiple axes need to move precisely in coordination. An example profile is shown in Figure 1. The total distance traveled, D, is found by calculating the area under the curve. T is the total time required for the move. The slope of the velocity curve represents the acceleration or deceleration at that particular instant. There are several types of motion profiles used with servo control systems. The most often used are Constant Velocity, Trapezoidal, and S-Curve motion profiles.

Things to remember:

Velocity proportional to 1/T

Acceleration proportional to 1/T2

Power (peak) proportional to 1/T3

The implications of the last bullet point is profound. For example if you have an existing system and you want the moves to complete twice as fast, the system will require 8x the power!

A resolver is essentially a rotary transformer. This feedback is capable of resolutions above 16bit. Resolvers are the feedback of choice for high temperature and high vibration environment. These provide absolute feedback within one revolution.

This is a low resolution feedback that is often necessary for commutation control. This can also be used for velocity feedback at higher velocities. These provide 6 units of absolute feedback within each electrical cycle.

These use the same sinusoidal encoders as above in addition to a mechanical device or electrical circuit that can maintain absolute position information over many thousands of revolutions. These devices transfer the position information over a serial protocol such as: Hiperface®, EnDat® and BiSS.