Configuring Analog Connections

This section provides more general guidance for configuring analog controllers with various hardware (servo amps; encoders, etc.).

XMP-PCI, XMP-CPCI

Connections to Servo Motors

XMP-series controllers can control brush servo motors, brushless servo motors, or linear brush/brushless motors. Basic connections require an analog output signal (from the controller to the amplifier) and an encoder input (from the motor to the controller).

Most amplifiers support either Velocity mode (voltage control) Torque mode (current control) or both. The XMP controller can be used with either servo motor/amplifier package.

XMP-series controllers accept RS-422 compatible (0V to +5V, 40mA max) encoder input from either differential or single-ended encoders. Differential encoders are preferred due to their excellent noise immunity. The connections for a single-ended encoder are identical to a differential encoder except that references must be connected to channel A- and channel B-.

The controller reads the index pulse (either single-ended or differential ended). Typically, there is one index pulse per revolution of the encoder (rotary type), which can be used for homing. Encoder signals are read in quadrature. Every line on the encoder will produce a rising edge and a falling edge on channels A+ and B+ which is interpreted by the XMP controller as four encoder counts.

Brush Servo Motors

The minimum required connections to brush-type servo are: Analog signal ( ± 10V), +5V, Signal Ground, Encoder Channel A+, Encoder Channel B+. Typical connections for a brush servo motor with a differential encoder follow.

Typical brush servo motor connections.

Brushless Servo Motors

Typical connections for a brushless servo motor with a differential encoder are:

Typical brushless servo motor connections.

Typical brushless servo motor drives of this configuration are "intelligent." If single ended encoders are used, external references will need to be supplied.

Step-and-Direction Controlled Servo Motors

Some brushless servos are controlled by step-and-direction pulses. With this scheme, the position information is communicated by step pulses, and the PID loop is handled internally by the drive itself.

Connections to Step Motors

Open-loop Step Motors

XMP controllers can control step motors in open-loop (no feedback) configurations. The XMP can also be configured for STEP LOOPBACK. When this is enabled, the motor Position Error Limit and Axis Settling will depend on the output steps being read back into the actual position register. If STEP LOOPBACK is disabled, an external feedback device (e.g. encoder) will be needed for determining position error limits and axis settling. In either case, the output is generated regardless of actual position.

Typical open-loop step motor connections for step drives triggering on the rising edge.

For stepper drives that trigger on the falling edge, invert the step output transceiver. This can be done from Motion Console by enabling the XCVR A: Invert (or XCVR B: Invert, etc.) parameter in the Motor Summary / I/O page.

Connections for Dual-loop Control

XMP-series controllers can be configured for dual-loop control. In dual-loop control, the velocity information for the PID derivative term (Kd) is derived from a rotary encoder on the motor shaft, and the position information for the PID proportional and integral terms is derived from an encoder on the load itself.

The axis that will be used for the rotary encoder is configurable through software and can be any axis that is not controlling a motor. For example, if axis 0 is configured for velocity feedback and axis 1 is configured for positional feedback, your system would be connected as shown in the next figure.

Dual-loop encoder connections with differential encoders.

MPI Support for Steppers

Support for stepper motor control is included within the MPI (Motion Programming Interface).

Transceivers

All transceiver and user I/O are part of the "Motor" object. Each hardware axis has three (3) transceivers dedicated to it, labeled XCVRA, XCVRB, and XCVRC. The XCVRA, XCVRB, and XCVRC transceivers support:

Note that only sensible XCVRA/XCVRB configurations are permitted. For example, if XCVRA is configured for Step, then XCVRB should be configured for Dir. If XCVRA is configured for Dir, then XCVRB should be configured for Step. Refer to the next table.

Step/Dir & CW/CCW Specifications

The maximum separation between the Dir edge and the rising Step edge depends upon the sample rate and the commanded motion. A rule of thumb is that the Dir edge occurs at the start of the DSP's trajectory calculator (when the command velocity is non-zero), and the first Step occurs when the command position increments the first whole count. The time separation can be estimated from the commanded acceleration (Accel):

For example, if acceleration is 100,000 cts/sec2, then the separation between the Dir edge and the first Step edge is

which computes to Time = 4.5 milliseconds

The minimum separation between the Dir edge and the first Step edge is 125 nanoseconds. Note that the XMP increments its counter on each rising edge of the Step or CW signal.

Step/dir (and CW/CCW) and motor motion.

Step/dir and velocity.

Stepper Loopback

When the Loopback feature is enabled, the Step/Dir (or CW/CCW) logic is routed back into the encoder inputs. Note that the DSP doesn't use the feedback for control. There are two to three samples of latency between when the DSP's command position is updated and when the actual position (loopback) is updated. Also, loopback is not affected by "inverted" configurations.

The position error limit is still valid for loopback operations. If Loopback is not enabled, you can connect the encoder inputs to actual encoders.

HELPFUL TIP: Stepper loopback is very useful for motor simulation. When real servo motors are not available, the controller's stepper motor and loopback configurations make it possible to develop software.

Closed Loop Steppers

Currently, there is no explicit support for closed loop stepper configurations. But, it is possible to correct the final position based on the actual position via application software, since the encoder inputs are valid with Step/Dir and CW/CCW configurations.

Stepper Configuration using Motion Console

To configure a Stepper using Motion Console, you must set the following parameters:

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