Motion MMI-8S - Manual (Page 32)

PRODUCT DESCRIPTION. The MM & MS, Series 1 & 2, stepper motor driver, is a switching type, constant- current
regulator which drives current pulses through the windings of a stepper motor. All stepper motors are stepped or
rotated by changing the direction of the current flow through the windings in a unique sequence. Each change of
current direction results in a step.
The driver contains two sections: (1) the step generator; and the (2) power drivers. The step generator is a digital
logic system which receives input commands from a controller (typically a microprocessor) and generates a series of
step signals. The power drivers receive the step signals and switch the phase of current in the motor windings.
The driver requires a minimum of four input signals: (1) the step pulse - STP, (2) the direction level - DIR, (3) the
power level - PRK, and the enable signal - ABR. The step pulse (or step clock) to the input of the driver will cause a
corresponding change of the output current resulting in one step (one unit of motor rotation). The direction input is
a digital level signal which controls the direction of motor rotation. If the signal is true (HIgh), the motor rotates in
CW direction; if the signal is false (LOw), the motor rotates in CCW direction. In addition to the step and direction
inputs, the driver will accept an output power control input. This digital input, PARK, controls the amount of current
delivered to the motor windings either run power or park power. If the signal is HI or floating, the driver is at reduced
current; if LO the driver is at full current. The enable signal, ABoRt, sets the current to either off or on. If the signal is
HI or floating, the driver is FREE (no current); if LO, the driver is enabled.
In addition to the digital input signals, the MS driver also requires a power supply input of unregulated D.C. voltage.
The driver functions to control the current furnished by the D.C. supply. The combination of a D.C. supply and the
MS driver is referred to as a current-regulated power supply, or constant- current motor driver. The driver regulates
the current through the motor winding by rapidly switching on and off the D.C. voltage. This technique is referred to
as switch-mode or chopper stabilized regulation. The driver also requires +5 TTL logic supply for the digital
sections.
OPERATIONAL MODES. The driver can be operated in three modes: FULL-step or HALF- step, and QUAD-step only.
In each of these modes, the output power control, PRK, is controlled by an external microprocessor. PRK is used to
reduce driver and motor heating during non-step periods.
THEORY OF OPERATION. The unique element in the driver is the current regulator device, referred to as the "driver
chip". This driver has three main inputs: (1) the phase-control, F; (2) current-control, I0; (3) current-control, I1. The
outputs of a driver are the connections to a single motor winding. Internally an output section contains four power
transistors configured in an H-bridge with two pair sourcing current and two pair sinking current. The motor winding
is connected across the bridge. If one source transistor (at one end of the winding) and one sink transistor (at the
other end) are turned on, then current flows through the winding. Alternately, if the other pair is on, then the current
will flow through the windings in the opposite direction. The D.C. Supply is connected to the top (positive) and
bottom (negative) of th H-bridge transistor pairs. An external resistor (typically 1 ohm or less) is inserted in series
between the negative of the H-bridge and the negative of the power supply negative so that the total winding current
flows through the resistor. When full winding current flows, the small voltage (400 mv) across the resistor is fed
back to the comparator section and turns off the H-bridge transistors. After a fixed-time off to allow the transistors to
settle and the feed-back voltage to dissipate, the bridge again turns on and current builds up in the winding until the
voltage across the sense-resistor again trips the comparator.