Hybrid Trotter

#1
Summary

This oddity is a spin off from a failed project to build a quadruped to run in the garden. Loosely styled on Spot from Boston Dynamics it used 55g servos. Unfortunately the quoted torque was not available in practice and the bot would not be able to carry the weight of 12 servos. Rather than consign the entire construction to the bin, I backtracked to an earlier test mode, which used ‘trainer’ wheels. Like those on a kiddy bike, the wheels add stability and aid the development of the leg action.

https://www.youtube.com/watch?v=IeNIzslJXOk

Cleaned up and with the addition of a ‘steering’ servo, the bot has a character of its own. I would recommend this hybrid approach to anyone wanting to start a quadruped project because success is more likely and it will help to understand the real development issues. The unpowered cart is handy for containing the batteries and electronics.

Leg Design and Kinematics

The hydraulic Spot design is emulated in wood and uses chunky servos mounted one above other in a block. The upper limb is directly attached to the lower servo using a cut-down horn bolted inside the hollow box. A thrust rod passes through the box section from the upper servo horn to the lower limb. It took a number of experiments to arrive at a suitable arrangement of overhang and offset.

There is no ‘foot’ as such so the two-part leg is simple to analyse using trigonometry on two right angle triangles. I use Excel to set up these functions one step at a time to aid in error checking. Then to produce a table of 60 degrees of movement is just a few mouse clicks. Unfortunately the servos do not move exactly one degree per increment and they will each be slightly different. Since a calibration is required anyway it might as well be for the whole leg!

The original project of four leg modules used a rig to hold them above a floor switch covered in squared paper. The thrust rod was uncoupled so the lower limb could drag across the ‘floor’. Nudging the servo values the foot was moved across the ‘floor’ one centimetre at a time. These rotational values were then loaded into routines to measure the lower limb values, lifting the limb then lowering it until the floor switch was activated.

The leg assembly is shortest when the foot is vertically under the ‘hip’ and must lengthen as it rotates forward or backwards to keep the bot at the same height. This is very non-linear but (by design) the thrust rod doesn’t move much through most of the rotation.

Once the hybrid was made, the leg modules had to be recalibrated to work at a different height. Now the wheels show another advantage – no rig or floor switch is needed! Simply place the hybrid on squared paper or a cutting board. Lift one leg and set the other leg to be at its most forward position (this is the beginning of the power stroke). Note the servo values for each one-centimetre wheel movement. The lower limb will also need adjustment to keep the height through the range. The bot has now been calibrated under load so the values should not need further adjustment.

Gait Study

Spot can change from walking to trotting and these quadruped gaits put different strains on a servo. A ‘creep’ gait is slow and keeps three feet on the ground at all times. Its stability is ensured if the centre of mass stays inside the tripod. However, the servo must be in power stroke for 75% of the cycle and has only 25% of the time to ‘dismount’ and return to the beginning. This is twice as fast as the trot gait where 50% is power stroke.

There is a brilliant set of gif files by Erica Liszewski showing different gaits. Microsoft’s gif editor will break these down to help in coding and understanding. The complex animal structures show such elegance when compared to stiff and simple robots. It is the dismount that can be the robot’s downfall when working on rough surfaces such as grass. A good leg design will allow the foot to be raised without snagging as the leg continues to move forward.

55g Servos

There is a lot of anecdotal chat regarding the Tower Pro MG995: favoured for steering four wheeled vehicles but too slow for expensive flying models, they exhibit wild overshoot, dead spots and uncertain current peaks. They also squeeze in motor, metal gear train, positional feedback and control electronics into a small space. A simple ‘closed system’ with one input - but no output to tell you it cannot do what is expected of it.

Spot has its lower limbs close to horizontal at times but this requires maximum torque. Quoted as anything between 6 and 10kg.cm depending on website this should mean up to 1kg if a 10cm limb is held horizontal. I set the upper limb just so on the test rig and used unwrapped servos as weights to hang from the ‘knee’ 10cm away. Two servos would rotate the limb maybe 5 degrees, three would move it 20 degrees. Clearly the static torque is just not there. The hybrid bot has limbs set much nearer to vertical in order to function.

There is, however, destructive power in a system of metal cored motor and metal gear train when on the move. With a tendancy to overshoot the momentum can crack a glued wood joint or bend a thrust rod. Those tyraps are not just for show and I had to use sections from a discarded umbrella.

Lipo Investment

Rechargeable AA cells will drive one or two servos but under strain the current rises too much. This bot required a change to Lithium technology.

There are some frightening videos concerning fake chargers and Lipo safe bags. To avoid this problem, and as I wanted a mains charger, I ordered from Hobby King in China. Air freight HK to Stanstead and the subsequent Customs and Fedex handling fee doubled the listed prices. However the SKYRC imax B6AC v2 charger (with authentic hologram) is a beautiful piece of kit. It did arrive with a US mains cable but fortunately I had a UK cable to hand.

The hybrid uses a 1000mAh 3C lipo capable of supplying 20A and feeding two 6V 5A SBECs; all by Turnigy. One switched mode regulator on its own runs very hot so the servos must need at least one amp each. They can run hot too!

It will run for a while with a 2C lipo which might not need protection when the voltage falls as the SBECs should give up first. I use a lipo monitor with alarm and once the battery supply falls below 10.5V it discharges rapidly until the screamer goes off and the bot must be shut down.

The Head

This is mounted on the horn of the single steering servo set between the two leg module blocks. The blocks pivot at the bottom so the feet can move inwards. There are springs between the two ‘push plates’ and the servo horn can push either leg but not both at once. Between the plates the horn can move rhythmically without affecting the direction. Greater rotation will move the rooted foot slowly to make the turn.

The ‘eyes’ are dimensioned for an ultrasound sensor but that is not used here. Peel and Stick Holographic
 

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#4
Thanks Erco

Because you are the collector of all things robotic here I had to publish this one for you. My wife picked up on the time question - apparently too much on the wrong application! However, I now have the bug and there is a lot of homework still to do on axial flux motors, harmonic drives and the rest before the next attempt.

Interesting recent forum discussions on Arduino. I have just loaded the correct Chinese drivers for a Nano v3 with CH340 chip and made the LED flash! I bought some back in March for £2.19 and with no other outlay am now a beginner again.

PS: your rabbit needs wheels

Regards
Keith
 
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