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Page 24

MSRSAS - Postgraduate Engineering and Management Programme - PEMP

Electrical drives and control


2.3.1 Velocity profile improvement suggestions:

In the velocity verses time profile usually the motor takes trapezoidal moves were the

velocity ramps up linearly to a final velocity “V” over time “Ta” then the velocity ramps down

linearly from “V” to zero over time “Td” as shown in the Fig 2.8.

If the overall move time is “T” then the total distance travelled “D” by a motor in a cycle will be:

D = Da + Dv + Dd

Da = distance traveled during accelerating velocity.

Dv = distance traveled during constant velocity.

Dd = distance traveled during decelerating velocity.

Fig 2. 8 Velocity profile

As in the cup filling automation processes the cycle time is assumed to be 3 seconds as the

conveyor is indexed by every 3 seconds. As above the velocity profile has 3 components

accelerating velocity, constant velocity and decelerating velocity it is not possible for the stepper

motor to take the trapezoidal velocity profile as shown above in Fig2.9. Instead it will take the

velocity profile as shown below in Fig 2.10 for the maximum velocity Vmax, were there is no

constant velocity only the accelerating and the decelerating velocity will be available as the cycle

time is 3 second.

Page 25

MSRSAS - Postgraduate Engineering and Management Programme - PEMP

Electrical drives and control


Fig 2. 9 Velocity profile of the Stepper motor

The improvement in the velocity profile can be done by finding the RMS (root mean square) which

is calculated instead of normal average of velocity profile as the motor losses are mostly

proportional to I2-R losses. The most important conclusion from this formula is the square root of

the cycle time. Instead of triangular the profile can be modified as the equaling profile for the

triangular profile which is trapezoidal profile which is just above the “average” line.

Fig 2. 10 modified velocity profile by RMS

The most disadvantage of the trapezoidal profile is it will be resulting in the “jerk” as the velocity

changer linearly in the during acceleration and the deceleration to avoid these jerk the trapezoidal

profile can be modified into S- curve moves as shown in the Fig 2.11.

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