Servo - June 2008

Loki Crosses the Pond — Part 1

4.5000

on-board. Specifications for the servos are:

0.4622

1.4567

0.4622

Ø0.1160

0.5000

1.0000

Hitec HS-645MG: 107 oz-in at 4.8V, 133 oz-in at 6.0V

Hitec HS-475HB: 61 oz-in at 4.8V, 76 oz-in at 6.0V Futaba S3004: 44 oz-in at 4.8V, 56 oz-in at 6.0V

Sensors

2.0961

Drawing to make IR sensor bracket from aluminum plate.

IR Sensor Bar Auminum Plate

Drawing to make IR sensor bracket from PCB stock.

2.5000

1.4567

0.5000

Make 2

2.2500

17.1090°

0.5000

0.1741

0.2140

2.0961

3.7752

Ø0.1160

IR Sensor Bar

Loki presently has three sensors. Two IR distance sensors (also called proximity sensors or rangefinders) and an ultrasonic sensor. The sensors allow Loki to avoid obstacles. I started out with the Devantech ultrasonic sensor, but then realized I’d need two sensors to determine which way to turn. As a result, I added the two Sharp GP2D12 IR sensors that I had on hand.

The ultrasonic sensor gives good range information from practically touching (a little over 1”) out to several feet, although we only use it out to something less then 3’. Mounting this sensor on a small R/C servo would allow it to rotate and collect better data. This sensor is read by the I²C peripheral on the controller board in a similar fashion to reading I²C EEPROMS.

The IR sensors are a little smaller and cheaper than the ultrasonic sensor. The max range is about 30”. They are inaccurate (way!) at distances less than about 4”. These sensors output a DC voltage between 0.45V and 2.45V and are to be read by two A/D channels of the controller board. Either a look-up table or a scaling algorithm must be used to scale the voltages read. A good white paper is mentioned in the references.

PCB Stock

allowed me to use some old Futaba S3004 R/C servos I had laying around until I could determine what size servos I needed.

NOTE: Hitec HS-475HB servos or better are needed to move Loki with the batteries on board. The Futaba S3004 servos were NOT able to move Loki properly with the batteries

Servos

WARNING! Servos can rapidly jump when first turned on! Positioning the feet as mentioned will minimize any undesired jerking motion of the feet upon application of servo power. KEEP YOUR FINGERS CLEAR of Loki’s feet when starting up!