How to “Build” a Robotic Systemand The Neuroscience We can do

With Robotics

Joseph T. Francis

Robotic Systems

• Build a simple robotic system for a rat 1-D

• What we need to build a 2-D system for motor control experiments.

• Describe a system made for Haptics, 3-D phantom interface.

What do we need

• A control system

• Something to control, we are going to use torque motors

• What signal do we control?

• Encoders

• Current control?

• Position control?

• Computer system and language to use.

EncodersOur robots don’t have eyes, only proprioception

• Give us position of the motor axis.

• Two types that are used

1. Absolute: Each tick has a “name” Don’t need to recalibrate each time you start.

2. Incremental: All ticks have the same name we only know how many we have moved.

What Type of controller?

• In our application we will use current control = torque control. We can also do software position control.

• In industry you often would like hardware position control for automation.

Internal models

• Brains behind a control system

• Desired trajectory

What signal do we control?For Force Field and Haptics

• We will use the encoders to determine the distance from a point (spring force field)

• The velocity of the Endpoint (hand or paw) to generate viscous force fields.

• Haptic Simulation: Position to determine when we have touched an object in our haptic simulation

Computer system and language to use

• Matlab, labview are nice and cute, but they take control away from you!

• Now Matlab has a real time kernel (xPC Target)

• C++ and MFC. Almost any piece of hardware you get will have a C++ .dll That comes with it.

How Real will our “Real Time” need to be???

• The answer to this question can make your life easy, or put you in a home.

• If less than 100Hz no problem use windows. If it must be faster than a couple hundred Hz with no exceptions Windows CAN NOT BE USED. Try linex, unix, etc...

Hints for Windows

• Use a multi-threaded architecture

• Use the Set thread priority function

• Write your own CWinApp::Run() and use the QueryPerformanceFrequancy, and QueryPerformanceCounter, Not Timers!

1-D System For Rats 1k$

• Must be Very low weight, and almost no inertia, Thus we use a Brush motor.

• C++ and the .dll from Maxon to control torque, National inst board and .dll to control Water reward system.

2-D System 75-80k$

• Motors that are strong enough to produce a large amount of force for pulse perturbations

• Stall torque, max torque, and time constants of the motor

Safety

• You must keep the system from injuring your subjects!

• Hardware kill of system

• Software boundaries as safe guards Max Vel and Position.

3-D Haptic System 20-70k

• Must have a fast control Loop 1000Hz for dealing with edges

• Saving data during the control loop is a real problem when using the “Ghost” software that comes with a phantom system