LM628, LM629

SNVS781C – JUNE 1999 – REVISED MARCH 2013

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Figure 11. Typical Velocity Profiles

VELOCITY PROFILE (TRAJECTORY) GENERATION

The trapezoidal velocity profile generator computes the desired position of the motor versus time. In the position

mode of operation, the host processor specifies acceleration, maximum velocity, and final position. The LM628

uses this information to affect the move by accelerating as specified until the maximum velocity is reached or

until deceleration must begin to stop at the specified final position. The deceleration rate is equal to the

acceleration rate. At any time during the move the maximum velocity and/or the target position may be changed,

and the motor will accelerate or decelerate accordingly. Figure 11 illustrates two typical trapezoidal velocity

profiles. Figure 11(a) shows a simple trapezoid, while Figure 11(b) is an example of what the trajectory looks like

when velocity and position are changed at different times during the move.

When operating in the velocity mode, the motor accelerates to the specified velocity at the specified acceleration

rate and maintains the specified velocity until commanded to stop. The velocity is maintained by advancing the

desired position at a constant rate. If there are disturbances to the motion during velocity mode operation, the

long-time average velocity remains constant. If the motor is unable to maintain the specified velocity (which could

be caused by a locked rotor, for example), the desired position will continue to be increased, resulting in a very

large position error. If this condition goes undetected, and the impeding force on the motor is subsequently

released, the motor could reach a very high velocity in order to catch up to the desired position (which is still

advancing as specified). This condition is easily detected; see commands LPEI and LPES.

All trajectory parameters are 32-bit values. Position is a signed quantity. Acceleration and velocity are specified

as 16-bit, positive-only integers having 16-bit fractions. The integer portion of velocity specifies how many counts

per sampling interval the motor will traverse. The fractional portion designates an additional fractional count per

sampling interval. Although the position resolution of the LM628 is limited to integer counts, the fractional counts

provide increased average velocity resolution. Acceleration is treated in the same manner. Each sampling

interval the commanded acceleration value is added to the current desired velocity to generate a new desired

velocity (unless the command velocity has been reached).

One determines the trajectory parameters for a desired move as follows. If, for example, one has a 500-line shaft

encoder, desires that the motor accelerate at one revolution per second per second until it is moving at 600 rpm,

and then decelerate to a stop at a position exactly 100 revolutions from the start, one would calculate the

trajectory parameters as follows:

let

P = target position (units = encoder counts)

let

R = encoder lines * 4 (system resolution)

then R = 500 * 4 = 2000

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