STEPPER MOTOR DRIVER A4988 WITH HEAT SINK

DESCRIPTION

PROJECT NAME:  A4988 DRV8825 3D Printer Stepper Motor Driver Control Extension Panel

PROJECT  DESCRIPTION:

         A4988 is a complete microstepping motor driver with built-in translator for easy operation. The product can be full, half, 1/4, 1/8 and 1/16 step mode to operate bipolar stepper motor output drive capacity of up to 35 V and ± 2 A. A4988 includes a fixed off-time current regulator, the regulator can slow or mixed decay mode. The converter is the key to the easy implementation of the A4988. In inputting one pulse on the STEP input drives the motor one microstep. No phase sequence tables, high frequency control lines, or complex interfaces to programming. A4988 interface is very suitable for complex microprocessor is unavailable or is overburdened.

        Micro-step operation, the chopping control in the A4988 automatically selects the current decay mode (Slow or Mixed). In the mixed decay mode, the device is initially set to fast decay in the part of the fixed off-time, then a slow decay in the rest of the downtime. Mixed-decay current control scheme results in reduced audible motor noise, increased step accuracy, and reduced power consumption. Internal synchronous rectification control circuitry to improve the pulse-width modulation (PWM) power consumption when operating. Internal circuit protection includes: thermal shutdown with hysteresis, undervoltage lockout (UVLO) and crossover current protection. Special power-Sort.

    A4988 surface mount QFN package (ES), the size of 5 mm x 5 mm nominal overall package height of 0.90 mm, and with exposed pad for enhanced thermal dissipation. The package is Pb (suffixes-T), with 100% matte tin leadframe plating.

         A stepper motor is one kind of electric motor used in the robotics industry. Stepper motors move a known interval for each pulse of power. These pulses of power are provided by a stepper motor driver and is referred to as a step. As each step moves the motor a known distance it makes them handy devices for repeatable positioning. There is a good article on Wikipedia explaining the technology behind stepper motors. The mechanical operation of a stepper motor is shown in this article.

Properties

Bipolar

The two coils of a bipolar motor.

Bipolar refers to the internals of the motor, and each type has a different stepper driver circuit board to control them. In theory a RepRap could use a unipolar motor, but in practice most are bipolar. They are also the type of motors we are using in the RepRap Project's Mendel and Darwin designs.

Bipolar motors are the strongest type of stepper motor. You identify them by counting the leads - there should be four or eight. They have two coils inside, and stepping the motor round is achieved by energising the coils and changing the direction of the current within those coils. This requires more complex electronics than a unipolar motor, so we use a special driver chip to take care of all that for us. Some designs (the eight-wire ones) split each coil in the middle so you can wire the motor either as bipolar (short the middles) or unipolar (short the middles and treat the link as the centre tap - see below). 

Unipolar

The two coils of a unipolar motor, each with a centre tap.

Unipolar motors also have two coils, but each one has a centre tap. They are readily recognizable because they have 5, 6 or even 8 leads. It is possible to drive 6 or 8 lead unipolar motors as bipolar motors if you ignore the centre tap wires. A 5 lead motor has both centre taps connected, so re-wiring them to a 4 lead version requires at least opening the motor, if it can be done at all.

The main beauty of unipolar motors is that you can step them without having to reverse the direction of current in any coil, which makes the electronics simpler. Some early RepRap prototypes used this trick. Because the centre tap is used to energise only half of each coil at a time, unipolar motors generally have less torque than bipolar motors. 

Holding torque

Stepper motors do not offer as much torque or holding force as comparable DC servo motors or DC gear motors. Their advantage over these motors is one of positional control. Whereas DC motors require a closed loop feedback mechanism, as well as support circuitry to drive them, a stepper motor has positional control by its nature of rotation via fractional increments.

The Mendel officially requires approximately 13.7 N·cm torque (19.4 ozf·in) of holding torque (or more) for each of the X, Y and Z axis motors to avoid issues, although one stepper with less has been used successfully (see below). Recent designs for extruders (ExtruderController) almost exclusively require stepper motors as well, but no torque requirements have been given in those designs. If in doubt, higher is better.

For Wade's Geared Extruder (most widely used one as of 2012) it is suggested to use motor that is capable of creating a holding torque of at least 40 N·cm.

If you need to convert between different units for the torque you can use the torque unit converter.

In September 2013, Willy did a comparison of a number of commonly used NEMA17 steppers and their torque at various speeds.

Size

The physical size of stepper motors are usually described via a US-based NEMA standard, which describes the bolt-up pattern and shaft diameter. In addition to the NEMA size rating, stepper motors are also rated by the depth of the motor in mm. Typically, the power of a motor is proportional to the physical size of the motor.

If using the smaller NEMA 14 motors, aim for the high torque option. NEMA 14s are neater, lighter and smaller, but can be hard to obtain with the appropriate holding torque. NEMA 17s are quite easy to get in the specification that Mendel needs, but are bulkier and less neat. NEMA 14s are running near the edge of their envelope: they will get warm. NEMA 17s are well inside what they can do, and will run much cooler.

FEATURES:

·         Low RDS (ON) outputs

·         Automatic current decay mode detection / selection

·         Mixed with slow current decay modes

·         Synchronous rectification for low power dissipation

·         Internal UVLO

·         Crossover-current protection

·         3.3 and 5 V compatible logic supply

·         Thermal shutdown circuitry

·         Ground Fault Circuit

·         Load short-circuit protection

·         Of five optional step mode: full, 1/2, 1/4, 1/8 and 1/16

Applications

The DRV8825 provides an integrated motor driver solution for:

  • 3d printers
  •  Scanners
  • Automated equipment applications. 

This is the Bill of Materials for a standard Stepstick, ie one limited to 1A.

Item

Package

Value

Value

Tolerance

Voltage

Position

Note

Capacitor

0402

0.1uf

 

10%

16V

C1, C2, C5, C6

16V capacitor for 12V maximum voltage; use higher voltage capacitors for higher voltage applications, max 35V

Capacitor

0402

0.22uf

 

10%

16V

C4, C7

16V capacitor for 12V maximum voltage; use higher voltage capacitors for higher voltage applications, max 35V

Capacitor

1206

4.7uf

 

10%

16V

C3

16V capacitor for 12V maximum voltage; use higher voltage capacitors for higher voltage applications, max 35V

Motor driver chip

QNF

       

IC1

Allegro A4988

Resistor

0805

0.2ohm

0.25W

1%

 

S1,S2

 

Resistor

0402

10k

 

10%

 

R4

 

Resistor

0402

20k

 

10%

 

R1

 

Resistor

0402

100k

 

10%

 

R2, R3

 

Trimpot

3mm

10k

     

T1
 

 

 


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