Drive Systems II

Hydraulic and pneumatic drive systems use devices such as linear pistons and rotary vane actuators to accomplish the motion of the joint. Pneumatic drive is typically reserved for smaller robots used in simple material transfer applications. Both electric drive and hydraulic drive are used on more sophisticated industrial robots.

Hydraulic Drives
Hydraulic drives are electric pump connected to a reservoir tank and a hydraulic actuator.

precise motion control over a wide range of speeds and loads, robust,
 and greater strength.
expensive, high maintenance, not energy efficient, noisy, not suited for clean-air environments.

Pneumatic Drives
Pneumatic drives: air-driven actuators.
Advantages: economical, easy installation, less costly than hydraulic drives, good speed and accuracy.
Disadvantages: precision is less than electric drives (air is compressible), air needs conditioning, noisy, vibration.

Electric Drives
They are readily adaptable to computer control, the predominant technology used today for robot controllers. Electric drive robots are relatively accurate compared to hydraulically powered robots.

Types:  ac servomotors, dc servomotors, stepper motors.
Advantages: quiet, less floor space, electric power readily available, clean-air environments, precision.

Stepper Motors
Stepper motor, unless a step is missed, steps a known angle each time it is moved. Angular position is always known and no feedback is necessary. Stepper motors come in many different forms and principles of operations.

Stepper Motors Operation
Stepper has multiple windings in its stator and a permanent magnet as its rotor. When each of the coils of the stator is energized, the rotor will rotate to align itself with the stator magnetic field. Steppers rotate only when the magnetic field is rotated through its different windings. Each rotation is equal to the step angle (1.8 – 7.5). With the opposite on-off sequence, the rotor will rotate in the opposite direction.

Stepper Speed-Torque Characteristic
Steppers develop maximum torque (holding) at zero angular velocity.
As the speed of motor increases, the torque it develops reduces significantly. Steppers cannot rotate fast. If the signals coming are too fast, the rotor will miss steps.

L297 / L298 Stepper Motor Driver
n This Step motor controller uses the L297 and L298N driver combination; it can be used as stand alone or controlled by microcontroller. It is designed to accept step pulses at up to 25,000 per second. All eight inputs are pulled up to +5V by RP1 (4.7K). The output driver is capable of driving up to 2 Amp into each phase of a two-phase bipolar step motor.

L293 Dual Stepper Motor Driver
The circuit consists of three ICs, a PIC16F84 and either two L293D H-bridge drivers for bipolar steppers or two ULN2803 for unipolar steppers.
* A 4 MHz resonator, a 10K pull-up resistor, and some connectors.
* A pack of 6 x 1.2V batteries, supplying 7.2V, is linearly regulated to 5V to supply the logic voltage and the raw unregulated power is applied to the 5V steppers.

L6203 – a full bridge driver
The L6203 is a full bridge driver, which can handle the high peak current up to 5A and supply voltage up to 48V. The chip can run the motor at 4A continuous with proper heat sinking.

4424 Driver
Direct motor driving with this chip is only possible for motors that draw less than 50 mA under load. TTL/CMOS compatible 4424 MOSFET driver chips protect the logic chips, isolate electrical noise, and prevent potential short-circuits inherently possible in a discrete H-bridge.
Schottky diodes to protect against overvoltage or undervoltage from the motor. Capacitors to reduce electrical noise and provide spike power to the driver chips. Pull-up resistors that prevent unwanted motor movement while the microcontroller powers up or powers down.