A robot moves in space to perform tasks and hence it needs actuators to move the links and sensors to know where each joint is. Sensors inform the controller by how much each joint has moved and thus enables the controller to enforce a particular velocity or position during motion. As we have seen in the previous section each joint has an independent control structure in which there is an actuator, reduction gear mechanism and a sensor at each joint.
Sensors can be divided into two parts:
1. Internal sensors These are responsible for the internal working of the robot and are mainly used for closing the loop in feedback control e.g., position sensors.
A robot cannot function properly without these if it is using a closed loop feedback control system. The main internal sensors are position and velocity sensors.
2. External sensors These are responsible for interaction with the environment. A robot can use external sensors like touch sensor for interaction with the environment.
In case any of these sensors fail the robot can still function but its ability to interact with the external world is reduced. External sensors are of many different types depending on the kind of interaction with the environment. The main external sensors are force/torque sensors, vision, touch, pressure sensors, etc.
We will discuss about internal sensors like potentiometers, resolvers and encoders in this chapter. External sensors will be discussed in later chapters.
3.4.1 Position and velocity sensors
Potentiometers Potentiometers are analog devices whose output voltage is proportional to the position of a wiper. Fig. 3.11 illustrates a typical pot. A voltage is applied across the resistive element. The voltage between the wiper and ground is proportional to the ratio of the resistance on one side of the wiper to the total resistance of the resistive element. Essentially the pot acts as a voltage divider network. That is, the voltage across the resistive element is divided into two parts by a wiper. Measuring this voltage gives the position of the wiper. The function of the potentiometer can be represented by the following function:
Vo(t) = Kpq(t) (3.28)
where Vo(t) is the output voltage, Kp is the voltage constant of the pot in volts per radian (or volts per inch in the case of a linear pot) and q(t) is the position of the pot in radians (or inches). Since a pot requires an excitation voltage, in order to calculate Vo, we can use
Vo = Vex q q
act tot
(3.29) where Vex is the excitation voltage, qtot is the total travel available of the wiper, and qact is the actual position of the wiper.
Potentiometers can be single turn in which the rotating wiper can move only by 360° or they can also be multi turn in which the rotating wiper can move by several 360° turns. Potentiometers suffer from disadvantages like non-linearity and low life due to the continuous friction between the wiper and the variations in the resistive element. In addition, the variation in wiper contact between the coil and the wiper can lead to noise in position measurement.
Vin
Vout
Rotating wiper
Resistive element
Fig. 3.11 Potentiometer.
Example 3.5 Find the output voltage of a potentiometer with the follow ing characteristics. Also determine the Kp. The excitation voltage = 12 V; total wiper travel = 320°; wiper position = 64°.
Solution The Kp = Vex/qtot which is 12 V/320° = 0.0375 V/deg. The output voltage is
(64°)(0.0375 V/deg) = 2.4 V.
Resolvers A resolver is another type of analog device whose output is proportional a resolver has a single winding on its rotor and a pair of windings on its stator. The stator windings are 90° apart as shown in Fig. 3.12. If the rotor is excited with a signal of the type A sin(wt) the voltage across the two pairs of stator terminals will be
Vs1(t) = A sin (wt) sin q (3.30)
and
Vs2(t) = A sin (wt) sin q (3.31)
where q is the angle of the rotor with respect to the stator. This signal may be used directly, or it may be converted into a digital representation using a device known as a ‘resolver-to-digital’ converter. Since a resolver is essen tially a rotating transformer, it is important to remember that an ac signal must be used for excitation. If a dc signal were used there would be no output signal
Example 3.6 At time t the excitation voltage to a resolver is 24 V. The shaft angle is 90°. What is the output signal from the resolver?
Fig. 3.12 Resolver. (Courtesy: Litton Systems, Incorporated, Clifton Precision Division)
Solution
Vs1 = (24 V) (sin 90°) = 24 V Vs2 = (24 V) (cos 90°) = 0 V.
Example 3.7 At time t the excitation voltage to a resolver is 24 V and Vs1 = 17 V and Vs2 = – 17 V. What is the angle?
Solution
arcsin 17 24 ÊËÁ ˆ
¯˜ = 45° or 135°
arccosÊ- ËÁ ˆ
¯˜
17
24 = 135° or 225°
The shaft angle must be 135°.
Encoders As microprocessors have become cheaper and with a move towards digital electronics, the encoder is virtually used everywhere for position measurement.
Almost all industrial robots, NC machines, etc., use encoders to measure the position and velocity of motion. Encoders are available as two basic types: incremental and absolute. There are various categories of encoding devices, but we will limit our discussion to those that are most commonly used in robots, i.e., optical encoders. A simple incremental encoder is illustrated in Fig. 3.13.
Fig. 3.13 Incremental encoder.
An incremental encoder consists of a disk marked with alternating transparent and opaque stripes aligned radially. A phototransmitter (a light source) is located on one side of the disk and a photo receiver is on the other Fig. 3.14. As the disk rotates, the light beam is alternately completed and broken. The output from the photoreceiver is a pulse train whose frequency is proportional to the speed of rotation of the disk. In a typical encoder, there are two sets of phototransmitters and receivers aligned 90°
out of phase. This phasing provides direction information, that is, if signal A leads to signal B by 90° the encoder disk is rotating in one direction, if B leads A then it is going in the other direction. By counting the pulses and by adding or subtracting based on the sign, it is possible to use the encoder to provide position information with respect to a known starting location. Normally, two incremental encoders are used in parallel so that the resolution of measurement is increased. These two signals are passed through an XOR gate. It can be seen that the resolution of the resulting
Fig. 3.14 Photo transmitter and receiver place on a incremental encoder.
signal is now increased two times, as we now have two pulses in place of only one pulse from each encoder. Most modern position control systems have two or more encoders in parallel to increase the resolution of the systems. The rate at which the pulses are generated by the encoder can also be counted to get an estimate of the velocity of the rotating shaft. Hence, an encoder can also be used as a velocity sensor.
In some cases, it is desirable to know the, position of an object in absolute terms, that is, not with respect to a starting position. For this an absolute encoder could be used. Absolute encoders employ the same basic construction as incremental encoders except that there are more tracks of stripes and a corresponding number of receivers and transmitters. Usually the stripes are arranged to provide a binary second four, the third eight and so on. In this way the angle can be read directly from the encoder without any counting being necessary. Figure 3.15 illustrates an absolute encoder. The resolution of an absolute encoder is dependent on the number of tracks and is given by
Fig. 3.15 Absolute optical encoder.
resolution = 2n (3.32)
where n is the number of tracks on the disk.
Example 3.8 What is the resolution, in degrees, of an encoder with 10 tracks?
The number of increments per revolution is 210 = 1024 increments/rev
The angular width of each control increment is therefore 360
210
∞ = 360 1024
∞ = 0.3515°
The output of an absolute encoder or of an incremental encoder and counter combination is represented by
out(t) = Keq(t) (3.33)
where out (t) is a number, Ke is the number of pulses per radian and q is the shaft angle, expressed in radians.
Example 3.9 What is the output value of an absolute encoder if the shaft angle is 1 rad. and the encoder has 8 tracks?
The resolution is 256 parts/rev. There are 27p rad/rev. Therefore, the output is 256
2p = 41