KINEMATICS FUNDAMENTALS 5 STEPPER MOTORS are designed to position an output device.Unlike ser- vomotors,these run open loop,meaning they receive no feedback as to whether the out- put device has responded as requested.Thus they can get out of phase with the desired program.They will,however,happily sit energized for an indefinite period,holding the output in one position.Their internal construction consists of a number of magnetic strips arranged around the circumference of both the rotor and stator.When energized, the rotor will move one step,to the next magnet,for each pulse received.Thus,these are intermittent motion devices and do not provide continuous rotary motion like oth- er motors.The number of magnetic strips determines their resolution (typically a few degrees per step).They are relatively small compared to AC/DC motors and have low torque capacity.They are moderately expensive and require special controllers. Air and Hydraulic Motors These have more limited application than electric motors,simply because they require the availability of a compressed air or hydraulic source.Both of these devices are less energy efficient than the direct electrical to mechanical conversion of electric motors, because of the losses associated with the conversion of the energy first from chemical or electrical to fluid pressure and then to mechanical form.Every energy conversion involves some losses.Air motors find widest application in factories and shops,where high-pressure compressed air is available for other reasons.A common example is the air impact wrench used in automotive repair shops.Although individual air motors and air cylinders are relatively inexpensive,these pneumatic systems are quite expensive when the cost of all the ancillary equipment is included.Hydraulic motors are most often found within machines or systems such as construction equipment (cranes),air- craft,and ships,where high-pressure hydraulic fluid is provided for many purposes. Hydraulic systems are very expensive when the cost of all the ancillary equipment is included. Air and Hydraulic Cylinders These are linear actuators (piston in cylinder)which provide a limited stroke,straight- line output from a pressurized fluid flow input of either compressed air or hydraulic flu- id (usually oil).They are the method of choice if you need a linear motion as the input. However,they share the same high cost,low efficiency,and complication factors as list- ed under their air and hydraulic motor equivalents above. Another problem is that of control.Most motors,left to their own devices,will tend to run at a constant speed.A linear actuator,when subjected to a constant pressure fluid source,typical of most compressors,will respond with more nearly constant accelera- tion,which means its velocity will increase linearly with time.This can result in severe impact loads on the driven mechanism when the actuator comes to the end of its stroke at maximum velocity.Servovalve control of the fluid flow,to slow the actuator at the end of its stroke,is possible but is quite expensive. The most common application of fluid power cylinders is in farm and construction equipment such as tractors and bulldozers,where open loop (non servo)hydraulic cyl- inders actuate the bucket or blade through linkages.The cylinder and its piston become two of the links (slider and track)in a slider-crank mechanism.See Figure I-lb (p.7).STEPPER MOTORS are designed to position an output device. Unlike ser￾vomotors, these run open loop, meaning they receive no feedback as to whether the out￾put device has responded as requested. Thus they can get out of phase with the desired program. They will, however, happily sit energized for an indefinite period, holding the output in one position. Their internal construction consists of a number of magnetic strips arranged around the circumference of both the rotor and stator. When energized, the rotor will move one step, to the next magnet, for each pulse received. Thus, these are intermittent motion devices and do not provide continuous rotary motion like oth￾er motors. The number of magnetic strips determines their resolution (typically a few degrees per step). They are relatively small compared to AC/DC motors and have low torque capacity. They are moderately expensive and require special controllers. Air and Hydraulic Motors These have more limited application than electric motors, simply because they require the availability of a compressed air or hydraulic source. Both of these devices are less energy efficient than the direct electrical to mechanical conversion of electric motors, because of the losses associated with the conversion of the energy first from chemical or electrical to fluid pressure and then to mechanical form. Every energy conversion involves some losses. Air motors find widest application in factories and shops, where high-pressure compressed air is available for other reasons. A common example is the air impact wrench used in automotive repair shops. Although individual air motors and air cylinders are relatively inexpensive, these pneumatic systems are quite expensive when the cost of all the ancillary equipment is included. Hydraulic motors are most often found within machines or systems such as construction equipment (cranes), air￾craft, and ships, where high-pressure hydraulic fluid is provided for many purposes. Hydraulic systems are very expensive when the cost of all the ancillary equipment is included. Air and Hydraulic Cylinders These are linear actuators (piston in cylinder) which provide a limited stroke, straight￾line output from a pressurized fluid flow input of either compressed air or hydraulic flu￾id (usually oil). They are the method of choice if you need a linear motion as the input. However, they share the same high cost, low efficiency, and complication factors as list￾ed under their air and hydraulic motor equivalents above. Another problem is that of control. Most motors, left to their own devices, will tend to run at a constant speed. A linear actuator, when subjected to a constant pressure fluid source, typical of most compressors, will respond with more nearly constant accelera￾tion, which means its velocity will increase linearly with time. This can result in severe impact loads on the driven mechanism when the actuator comes to the end of its stroke at maximum velocity. Servovalve control of the fluid flow, to slow the actuator at the end of its stroke, is possible but is quite expensive. The most common application of fluid power cylinders is in farm and construction equipment such as tractors and bulldozers, where open loop (non servo) hydraulic cyl￾inders actuate the bucket or blade through linkages. The cylinder and its piston become two of the links (slider and track) in a slider-crank mechanism. See Figure I-lb (p. 7)