Electronics

To use a biological metaphor, the motor controller corresponds to a spinal cord and peripheral nervous system. It receives movement instructions from the host computer through the RS-232 serial cable, and can return position, velocity, and other data back to the host. Decisions about where, when, how much, and how fast to move are made in the host. The desired actions are then executed by the controller. The controller actually does quite a bit of real time processing.


As with many components, this one started by cutting metal and drilling holes.	Controller shown with its Linux/Java companion

Controller in operation, showing line voltage,
camera power, video in and out, motor leads,
and RS-232 wiring

In order to reduce the possibility of tangling as the camera arm moves up and down, the camera power and video line are carried in a zip cord style video cable. The two wires move together as one. The video line goes to a video monitor. To keep the cables neat, and to keep strain off the connectors, a feed-thru is supplied on the controller chassis. The top RCA connector goes to the video monitor.

Control information from the host computer comes in on a standard RS-232 serial cable. RS-232 can carry a conversation between two devices. It was originally designed to connect modems to dumb terminals. Later, its use spread to connecting modems, printers, and other devices to PC's, but always as a line with only two endpoints. Other methods of carrying serial communications were invented which can carry data between multiple devices at more or less the same time. One of these is RS-485. In order to "bus" multiple PicServo controller boards onto one serial port of a PC, JRKerr provides a Serial Port Converter Board (Z232-485). In addition to doing the serial port protocol conversion, the converter board also provides the power supply for the logic section of of all the PicServo controller boards through the daisy-chained ribbon cables. In this case it takes 12 VDC from the Logic and Camera Supply and converts it to 5 volts for the controller chain. It is important to note that the power supply for actually running the motors themselves is a separate matter.

The "spinal chord" of the controller unit is the PicServo controller board, by JRKerr.com . In terms of price and performance, the PicServo occupies a previously unfilled niche in the motion control marketplace. It allows the small user to implement a full-fledged closed-loop servo control system for less than $200 per axis, plus motor. The PicServo implements the "PID" control algorithm. PID stands for Proportional Integral Derivative. All controllers need some error between the setpoint (where do I want to be?) and actual (where am I now?) values for position and velocity, and some idea of how the error is changing with time, to decide how much power to give the motor. Proportional Integral Derivative controllers do a lot of math to reach their output values, and the user has to enter some parameters into the controller to work with the PID algorithm. These values are referred to a "weights" and tell the controller how much emphasis to put on respectively, the P, I, and D values when doing the calculation for the next value of output. Selecting the right values for the P, I, and D weights is called "tuning the controller", and if you're lucky will only have to be done once for a given combination of motor and load. Each PicServo board performs its calculation loop and updates its output around 1,900 times per second. New commands can be sent to a controller board at a maximum of 1,000 times per second. Commands to make new position or velocity moves come from the host computer. In this case, the software which ultimately decides how the motors move is a Java application running on a Linux box. To program move commands into the PicServo, library commands are called in Java with parameters that tell the controller how far and how fast to move. We are developing our own high level API (Application Programmer's Interface) to make access to the PicServo from Java as easy as possible.

As is the case with most servo motor systems, feedback to the controller is provided through optical encoders. Feedback is what allows the controller to make continual adjustments to the power output, in order that the motor is always pulling accurately towards its command position, velocity, or acceleration. A disc with usually between 50 and 1000 lines cut through its edge and housed in the back end of the motor, slices a tiny light beam shining on a detector as the motor shaft turns. This setup is known as a photointerrupter, shaft encoder, incremental encoder, or in the specific case where there are two light detectors, the arrangement is called a quadrature encoder. More lines on the encoder disk gives more resolution, and higher cost. Quadrature encoders do two tricks in addition to ticking off position counts. They can tell the controller what direction the motor is turning, and they multiply the number of encoder counts per revolution of the motor shaft by four. So an encoder with 100 lines cut into its edge gives a resolution of 400 counts per revolution when read in quadrature. The PicServo controller is designed to attach to motors equipped with quadrature encoders. You may notice that the cable going from the motor to the controller board is really two cables tied together. Both are shielded to suppress electrical noise. One cable is controlled current to power the motor. The other is the encoder cable. As a rule, signal wiring should be kept separate from power wiring.

This controller unit contains few active components.

a 24 volt, 8 amp switching supply to run the motors
a 12 volt 1 amp switching supply to power the PicServo logic and the video camera
a serial converter board
fuses (one for each controller board plus one main fuse)
a terminal strip to make interconnections on
three PicServo control boards with heat sinks (one heat sink is larger than the other two)
a monitor LED for each power supply
a power switch

User input to the host computer is through three pushbuttons. Their functions are turn left, turn right, and go. Movement control and response is similar to the game Asteroids. The pushbuttons are read by an EZIO board and converted to RS-232 which connects to a serial port on the computer, separate from the port serving the PICServo chain. The EZIO has 10 digital inputs 10 digital outputs, two PWM outputs, and eight 8-bit analog inputs. The digital lines are bit or byte addressable.

Control buttons used during development, and EZIO interface board